Mammalian Toxicology, Session 4

Discussion Questions on History, Biochemical Kinetics, Chronic Toxicity & Carcinogenesis, Metabolism, Risk Assessment, and Toxicity Protection


Each of the project ideas submitted in response to the instructions for the projects was examined.  Problems with preservation of links generated during database searches are common.  These documents may need to be downloaded initially as files and then transferred as supplementary files linked to the master via hyperlinks.  Incorporation of hyperlinks to references may be accomplished either by inclusion of the links in footnotes, or by common hyperlink references for the footnote and the bibliographic citation.  Note that the wording on the presentation page does not need to match the text in the hyperlink.

Of more toxicological, rather than technical, substance is the need to address each project authors' ideas about synthesis and integration of the information gathered regarding compounds or case studies with respect to use of toxicological approaches, risk assessment, and whether any resolution of the problems involved were actually found.  This needs to appear as part of the composition.  Thus, though the HTML document may include some lists or tables, it should go beyond this to make significant comments about the information that has been gathered.  There needs to be an analysis done and presented that covers the chosen project topic.

Discussion Questions:

Topic: Question 1. Questions arising from the historical background of toxicology.

Posted: by Kenneth Campbell

1. The idea of causality arose during the middle ages and Renaissance. This builds on careful observation and testing. How might this give rise to the notions of occupational hazards or the nonmystical explanation for oncogenesis?

What about toxicity? As a toxin can have a lethal or a sublethal effect, is one word adequate to define it? It would seem there is a need to define varying types and levels of toxicity or toxic response.

Lethality: mortality, death (induced reasonably rapidly as an obvious consequence of exposure)

Morbidity: a decrease in functionality that may ultimately lead to a decrease in lifespan

Reduced reproductive capacity: decrease in reproductive output (this has a genetic carry-over to the next generation because there is a resulting change in the potential gene pool)

Decrease in specific functions or acuities (sensory: sight, hearing, taste, smell, touch; specific metabolic functions - food allergies; appearance - hair growth, acne): these decrease quality of life but are often nonlethal, do not decrease lifespan and do not alter the gene

Reduced aesthetics or other quality of life issues: these are often issues that have little or no medical impacts except at the psychological or emotional levels

Reduced quality of the environment: this may impact any or all of the above items

Note that these range from severe immediate impacts on the individual, through prolonged or delayed impacts on the individual or the species, to subtle effects that may only alter survival or functions of individuals or species via alterations of behavior or mental vitality. Evaluating risks of toxic exposure then may encompass analysis of biological risks and exposure tests, potential medical or economic risks of exposure to or lack of exposure to the toxicant of interest, and possible impacts on the integrity or improvement of environmental quality. The overall picture may involve a variety of competing interests or benefits and definitely involves a cost/benefit analysis. Even at the individual level this is reflected in the statements that exist on drug and household chemical packaging. For medications there are not only directions for proper use, but indications of possible deleterious side effects. These are often unrelated to the drug target tissue of interest, e.g., antihistamines not only decrease swelling in nasal mucus membranes, they often suppress mental alertness and/or increase blood pressure. The detailed analysis of positive and negative drug impacts is subsumed in the science of pharmacology but has obvious overlap and relation to toxicology both in its tenets and methodologies but also in the subjects of study and the analyses used to evaluate study results.

Posted: Feb-12-03 by Student 1

1. The idea of studying causality through careful observation and testing might give rise to the notion of occupational hazards because certain occupations expose workers to unique risks of exposure. If particular subsets of workers are all experiencing a similar disease or condition, the logical way to explain the symptoms is to carefully observe the working conditions. This would entail sampling the environment in search of the causative agent, or the "toxicon". Potential candidates could then be used in animal or in vitro tests to determine response to various dose levels, site of action and what types of symptoms are observed in the test subjects. They could also undergo biochemical analysis to determine structure/activity relationships. The information gathered could then be used to determine the causative agent and could also be used to infer other potential hazards. This type of testing could then also be used to test other potentially hazardous working environments before workers exhibit symptoms. This would allow for preventative measures to be taken against toxic exposure.

In the case of non-mystical explanation of oncogenesis, the same process applies. In the onset of unexplained cancer, one can carefully observe the afflicted individual's potential exposure to a potential carcinogen. Similar tests to those described above could then be performed to identify the culprit.

Posted: Feb-12-03 at 3:38 PM by Student 2

The process of simple observation from occupational or incidental exposures may be suitable for noncarcinogenic effects resulting from acute exposures; however, for chronic adverse effects or carcinogenesis, it is not as easy a matter as to pinpoint the potential culprit. Liver cancer, or heart disease, for example, may result from a lifetime of cumulative effects of numerous causative agents, which may or may not be obvious based on the individual's exposure.

Posted: Feb-13-03 at 10:34 PM by Student 2

"Watch and learn" tactics. In the past, people have noted adverse effects associated with exposure to particular substances. Likewise, it was noted that certain occupations had correspondingly higher incidences of diseases (e.g., "Mad Hatters" syndrome, resulting from mercury toxicity (neurotoxicity). Mercury was commonly used by hat makers…..). As for oncogenesis, the same premise holds: exposure to a particular substance eventually initiated the onset of cancer (e.g., the elevated incidence of lung cancer among smokers). One problem with this approach, however, is that a particular type of cancer (or, say, a nonspecific noncancer effect) may correlate with a substance's use, but may not necessarily be directly related to exposure. Simple observation does not take into account confounding factors such as the incidence of background cancer rates, or the numerous other entities in our lives that may also attribute to adverse effects (e.g., diet, acute exposures, etc.)

Posted: Feb-15-03 at 11:30 AM by Student 3

During the Renaissance, the idea of scientific experiments and replicability arose. It became understood that an event can be caused by something non-immediate. The spread of literacy and printed works made it easier for scientists and others to share observations and note patterns, such as the high occurrence of scrotal cancer in chimney sweepers. Likewise it was known that mining and covering building domes with gold leaf (I believe the gold was in a mercury solution) were dangerous occupations, because many of these processionals died relatively quickly of similar diseases, such as lung problems. While the exact mechanisms of lung injury were not known, the cause-effect relationship was established. Similarly, while initially oncogenesis was attributed to mystical causes, the observation of patterns of lifestyle and environment among victims contributed to the uncovering of certain cause-effect relationships, like the above-mentioned scrotal cancer of chimney sweepers and, more recently, lung cancer and emphysema among heavy smokers.

Posted: Feb-15-03 at 2:59 PM by Student 4

While scientific advancements of the cause and effect relationship of toxicants on the human system did occurred during the Middle Ages, one could say that these were great scientific alchemy experiments than the precursors of government regulatory bodies like OSHA, EPA, and the NIH. One pioneer of the Middle Ages, Catherine de Medici, tested toxic concoctions on men. Now, her horrific crimes can be interpreted to modern toxicology terms such as onset of action, specificity of action, and clinical significant. The fate for her evil ways was execution, but one could argue that her scientific studies are exactly what occurs during Phase I, II, and III clinical trials of new chemical entities therapies. Overall humans have tried to learn from there mistakes and this what has initiated governmental control Back in the 1900's one pharmaceutical company killed 108 children with their cough drop medicine. They had used ethylene gylcol instead of propylene glycol in their formulation. Their medicine was termed the Elixr of death and hence the FDA was then established soon afterwards.

With regards to the explanation oncogenesis, the formation of tumors can be environmentally stimulated as well as genetic inherited. Indeed OSHA is there to protect workers from safety and toxic hazards, but if they were really that concerned why does mining still exist. An occupation were many malignancies have been well document. Maybe humans have not really progressed as much as we have thought. And as far as governmental control, the pharmaceutical company that made the Elixir of death is still in business.

Posted: Feb-15-03 at 4:19 PM by Student 4

While scientific advancements of the cause and effect relationship of toxicants on the human system did occurred during the Middle Ages, one could say that these were great scientific alchemy experiments than the precursors of government regulatory bodies like OSHA, EPA, and the NIH. One pioneer of the Middle Ages, Catherine de Medici, tested toxic concoctions on men. Now, her horrific crimes can be interpreted to modern toxicology terms such as onset of action, specificity of action, and clinical significant. The fate for her evil ways was execution, but one could argue that her scientific studies are exactly what occurs during Phase I, II, and III clinical trials of new chemical entities therapies. Overall humans have tried to learn from there mistakes and this what has initiated governmental control Back in the 1900's one pharmaceutical company killed 108 children with their cough drop medicine. They had used ethylene glycol instead of propylene glycol in their formulation. Their medicine was termed the Elixir of death and hence the FDA was then established soon afterwards.

With regards to the explanation oncogenesis, the formation of tumors can be environmentally stimulated as well as genetic inherited. Indeed OSHA is there to protect workers from safety and toxic hazards, but if they were really that concerned why does mining still exist. An occupation were many malignancies have been well document. Maybe humans have not really progressed as much as we have thought. And as far as governmental control, the pharmaceutical company that made the Elixir of death is still in business.

Posted: Feb-15-03 at 4:38 PM by Student 5

During the Middle Ages and the Renaissance, the beginnings of careful observations and testing began. Looking at the cause and affects of a toxicant/toxin began. For example, Catherine de Medici looked at toxic effects on the poor by noting the rapidity of the toxic response (onset of action), the effectiveness of the compound (potency), the degree of response of the parts of the body (specificity, site of action) and the complaints of the victims (clinical signs and symptoms). As cruel as these experiments were, they were really an early version of the Phase I, II, and III testing. During the Renaissance, Paracelsus was the first to look at a toxicant as a chemical entity rather than a mixture of some sort. His 3 concepts were:

1.) Experimentation is Essential in examination of responses to chemicals.
2.) One needs to make a distinction between therapeutic and toxic properties of chemicals.
3.) These properties above are sometimes distinguishable by dose but not always.
4.) One can ascertain a degree of specificity of chemicals and their therapeutic or toxic side affects.

This can really be seen as the beginning of the dose-response relationship. Paracelsus also looked at the etiology of diseases associated with miners and how to treat/prevent them. Also the hazards of metal working began with the works of Ellenborg warning of toxicity of mercury and lead in goldsmith work. All these lead to looking at the occupational hazards of toxicants and looking at nonmystical explanations for oncogenesis.

Posted: Feb-16-03 at 8:18 PM by Student 6

If one considers simple hunter-gathering an occupation, causality via observation and testing likely developed prior to the middle ages and Renaissance, during early hunting and foraging expeditions. Early man likely encountered poisonous plant, animal and insect species. Tribal leaders likely recorded the species of plant, animal and insect that caused a toxic response in a member; thus establishing early causality. Over time I imagine that unexplained and/or mysterious death previously attributed to mystical causes was later attributed to an exposure to a naturally occurring toxin/species delivering said toxin. Once studied and understood, it is evident from early manuscripts that early man was able to apply this toxicological knowledge to hunting and warfare.

This early study was later refined by persons like Maimonides and Hippocrates who furthered the study of toxins and early toxicants. They and their peers authored papers on poisoning and antidotes. Their work helped to define how most poisons/toxins of the time manifested themselves and establish the notion of bioavailability.

As this study was furthered, individuals like Paracelsus and Ramazzini documented disease trends amongst asbestos workers, miners (radon, dusts), metallurgists, printers, potters, weavers and end users of products and/or resources. Like the early hunter-gatherers and their peers furthering the study of poisons and antidotes, I image that individuals like Ellenbog, Paracelsus, and Ramazzini desired to further establish the causal relationships between occupation and disease. Once established and like Maimonides and Hippocrates, Ellenbog, Paracelsus, and Ramazzini worked to develop treatment and prevention strategies.

Scientists like Percival Pott were able to build upon the foundation that Paracelsus and Ramazzini had established. Armed with a broader depth and understanding of biological and chemical science, scientists like Pott were able to establish that specific occupational exposures led to the development of organ and/or tissue specific disease and/or manifestations.

In summary, it is clear that man's pursuit to understand causality has led to our present understanding of occupational disease and oncogenesis. Perhaps as we further broaden our base of knowledge, man will not only be able to prevent the incidence of occupational disease but also repair the damage that an accidental exposure has caused.

Posted: Feb-18-03 at 3:15 PM by Student 7

The idea of causality seems to have developed around the introduction of the physician-alchemist, Paracelsus. He promoted the idea of a dose-response relationship. Looking at the relation of cause (i.e. "toxicon" exposure) and effect (i.e. disease, death) parallels Paracelsus' studies of how chemical entities can elicit various responses in individuals- both neg. and positive.

The idea/doctrine that "all things" have a cause definitely give rise to the notions of occupational hazards and nonmystical explanations for oncogenesis. For example, exposure to mercury and lead (cause; high dose)leads to disease (effect; response). Also, in the field of oncology, the role of soot in scrotal cancer demonstrated that the cause of disease was nonmystical (moving away from antiquity beliefs). As the book noted (the big picture), the idea of causality has improved medical practices, given a greater understanding to the broader fields of biology and chemistry, developed the beginnings of the scientific method (Campbell), and aided in prevention as well.

Posted: Feb-18-03 at 10:25 PM by Student 8

I would just point out again the Age of Enlightenment and the Industrial Revolution as major breakpoints in human history as far as toxicology is concerned. Throughout most of the history, including the Middle Ages, concern about the toxic effects of chemicals has been focused on poisons which act quickly and result in death. Socrates ingested hemlock to commit suicide. Infamous Toffana made many young married women wealthy widows by her arsenic containing cosmetics. Since exposure to these chemicals was not common and the risks were well known there was little public concern about these poisons.

Gradually we became more concerned with poisons, especially those that cause adverse effects after long periods of exposure. Industrial revolution has led to new and increased uses of known chemicals and the synthesis and widespread use of newly developed chemicals which has started a greater awareness of possible adverse health effects of chemicals other than lethality. More recently, increase in the average human life span due to cures and treatments for infectious diseases has made chronic non-infectious diseases more common and opened up a whole new area of research for toxicology.

Referenced from:

Posted: February 19, 2002 by Student 9

1) The idea of casuality would potentially give rise to the notions of occupational hazards and/or the nonmystical explanations for oncogenesis by one's observing and posing a plausible causal relationship between the environment (work or otherwise) and an adverse (cancer promoting ..etc) reaction in the exposed individual. Instead of attributing ailments to superstitious beliefs, they had begun to establish the fundamental beginnings of the scientific method.

Posted: February 19,2002 by Student 10

1. Occupational hazards associated with metal working were recognized during the 15th century. Paracelsus published a major piece of work addressing the etiology of miner's disease, along with treatment and prevention strategies. Occupational diseases and causuality increased during the Industrial Revolution. The recognition of the basis behind scrotal cancer among chimney sweeps was the first reported example of polyaromatic hydrocarbon carcinogenity. These findings led to improved medical practices and prevention. Through careful observation, diseases that were prevelant among a certain group of workers or people led to testing and experimenting on metals, plants, and minerals that they might be exposed to or using in their daily activity. This led to the understanding and further development of determination of how workers were becoming ill and the specific cause of these diseases.

Posted: February 19,2002 by Student 11

1. The occupational hazards and causalities were very commonly occuring in olden days.

People were perhaps, exposed to many chemicals and other hazardous material in their work environment due to ignorance or they simply did not have any other choice. They had to work in the environment they were working in bceause there were not many jobs and because they had to provoide living for themselves and their families. After the industrial revolution, people started becoming educated and taking more precautions, which made the working environments safer. With the advanced type of technology, and medical techniques, conditions are much better now than they were during industrial revolution. Learning, understanding and taking precautions while working, definately helped prevent occupational hazards.

Topic: Question 2. Questions arising out of the historical background of toxicology.

Posted: Mar-04-02 at 3:43 PM by Kenneth Campbell

2. What information already discussed is subsumed in the notion of biochemical kinetics?

Posted: Feb-12-03 at 12:55 PM by Student 1

1. The similarities between enzyme kinetics and toxic effects are apparent when comparing dose-response curves to the plot of the Michalis-Menton equation also called the rate equation. The concentration of substrate on the X-axis is analogous to the dose and the velocity of the enzymatic reaction on the Y-axis is analogous to the response (efficacy). The median lethal dose (LD50) is analogous to the Km., the substrate concentration at which the reaction velocity is half-maximal, ½ Vmax. A small Km means that the enzyme achieves maximal catalytic efficiency at low substrate concentration. A small LD50 means that 50% lethality is reached at a low dosage of the toxicant. This also applies to the effective dose (ED) and toxic dose (TD). The mechanisms that facilitate the toxicants delivery to its target site could potentially influence the LD50 or ED50. If the facilitators decrease the dosage needed to reach the half-mark, they would be akin to catalysts of an enzymatic reaction.

Posted: Feb-13-03 at 0:50 AM by Student 8

Toxicants affect their targets (enzymes) by binding to them in the same ways their normal substrates do: noncovalently and covalently. Examples: TCDD binding to aryl hydrocarbon receptor, and acridine yellow to DNA in a noncovalent manner; metal ions and free radicals are able to form covalent bonds with biomolecules.

Just like their normal substrates, toxicants can stimulate their target molecules and hence cause their dysfunction by activating or inhibiting them at inappropriate times. Well known are microtubule and actin impairing drugs such as colchicin and phalloidin or morphine which activates opiate receptors.

Toxicants can also destroy their targets upon the chemical attack by means of cross linking and fragmentation. Some chemicals are just big enough to bind to the cell surface proteins inducing antibody production and evoking an immune response (e.g., drug induced lupus, agranulocytosis, and hepatitis like syndrome).

Posted: Feb-13-03 at 10:32 PM by Student 2

Chemical absorption, transformation, elimination, etc. is variable not only among substances, but among individual receptors (as in a human receptor) as well as on a cellular or subcellular level. In other words, toxicity is a function of chemical application, uptake, and transfer to a target, each of which in turn are dependent on the organism's distinct physiology and biochemistry.

Posted: Feb-15-03 at 11:45 AM by Student 4

The dose-response curves of toxic substances on living organisms are similar in shape to virtually any plot of some effect of a chemical (in varying amounts) on a living organism, such as an enzyme and its product's concentration or even ingestion of glucose and and its level in the bloodstream. This is because the general mechanisms of any substance's function are similar: There is a minimal dosage at which an effect is found, when the substance's amount is sufficient to trigger a reaction, a subsequent somewhat linear relationship between dose and effect (though the slope will differ), and a later reduction in the rate of effect increase. This last may be due, depending on the type of effect observed, to a saturation of receptors for the chemical, or possibly to the death of the organism and shutdown of biological processes in the case of a highly toxic substance.

Posted: Feb-15-03 at 4:51 PM by Student 3

Biochemical kinetics refers to the rate at which a reaction takes place (Purich and Allsion). In mammalian systems, kinetic mechanisms play a vital function in metabolic pathways, the mechanistic action of enzymes, and the processing of genetic material. The biochemical kinetics covered in class was focused on a sigmoidal curve. While simple in structure, this shape pertains to a wide variety of biological and chemical systems; titration curves, isoelectric points of proteins, and curves exhibited by regulatory enzymes. This sigmoidal shape also exists in toxicology and is seen with the quantal-dose response relationship curve. The quantal dose response is used extensively in toxicology. First, the LD50 is determined where 50% of animals tested die from a certain dose. Second, a range of doses are used in a larger number of animals. From this a sigmoidal curve can be obtained. As with all biochemical kinetics, a saturation point exists where a stimulus will no longer result in an effect. For enzyme kinetics, only so many active sites exist on the enzyme for the substrate to bind. For a cumulative mortality curve, only so much dose can be given before death occurs.

Posted: Feb-17-03 at 0:31 AM by Student 6

Our discussion of biochemical kinetics touched upon the following:

  1. the rate and/or response to a given toxin and/or toxicant could be graphically depicted as a dose-response curve;
  2. the dose-response curve is most often sigmoidal;
  3. curves may be individual or quantal;
  4. the basal, middle and maximal points of the curve may correspond to a wide variety of endpoints;
  5. cellular, tissue, organ, individual and population variability may be attributed to dosage, duration and frequency of dosage, route of exposure, chemical and/or physical interaction, genetics, environment and a wide variety of other factors
  6. a greater level of confidence exists around the linear portion of the curve;
  7. the lack of confidence at the asymptotic portions of the curve is due to factors including hypersusceptibility, the inability to define sub-threshold responces, and resistance.
Posted: February 19,2002 by Student 9

2) The notion of biochemcial kinetics was encompassed in our discussion of dose-response relationships as well as routes of entry of toxins/toxicants. For instance genetic backgrounds of the specimen in question ( recipient of toxin/toxicant) will have an effect on the biochemical result depending on its underlying genetic makeup. Such variations include:susceptibility to toxins/toxicants as well as the ability to excrete/eliminate toxins/toxicants once they infiltrate through the biological system. The particular route of entry may be more significant dependent upon the genetic background of the individual organism.

Posted: February 19,2002 by Student 10

2. Dosage and form of delivery and the aspect of bioavailability are parts of biochemical kinetics. Many toxicant problems occur when naturally occuring toxicants are concentrated and exposed to other people. Exposure at low levels over a long enough time for the toxicants to be deactivated either by metabolism or adsorption to surgaces reduces the risk of a toxic response. However, if the same total dose is provided over a shorter period of time, the toxic response will be more observable due to the inactivation and reduction of metabolic and adsorption capacities. The forms of delivery are also important factors in biochemical kinetics. Lipophilic materials will not be adsorbed on hydrophobic surfaces like charcoal or clay and the toxicant will be unavailable to biological systems, unless extreme pH or mechanical agitations allow the toxicant to pass through the digestive system. Dilution of lipophilic materials in oils will lead to slow release of the material. Bioaccumulation following administration is also related to bioavailability and biochemical kinetics. Lipophilic toxicants accumulate and partition into lipid droplets and may be prevented from biological breakdown and against biological response until they reach high levels that the adipose cells within an animal cannot accumulate anymore or the degradative systems become saturated so levels of the toxicant in acqeous compartments increase and evoke toxic responses.

Posted: February 19,2002 by Student 11

2. Bioavailability is an important factor when talking about biochemical kinetics. The amount of dose absorbed by the systemic circulation is referred to as bioavailabilty. The concentration of  <>the toxin is as important as the dose of the toxin when dealing with toxins. The form of delivery also plays a role because some ways are better than others to make sure that the drug enters the systemic circulation.

Topic: Question 6. Chronic Toxicity & Carcinogenicity

Posted: Mar-04-02 at 3:59 PM by Kenneth Campbell

6. Note the emphasis of chronic toxicity testing and mutagenesis testing on carcinogenesis. Is this emphasis currently appropriate, or are there other possible chronic outcomes to be concerned with?

Posted: Feb-12-03 at 10:27 PM by Student 8

Certainly, as scientific methods and techniques get more advanced it will become much easier and cost-effective to assess various other effects of toxicants beside carcinogenesis. Why is this important? For instance, a particular signaling protein affected by a toxicant can cause an increase in cellular proliferation (then cancer) but this protein might also have other functions either in the same organ or in different organs. Toxicant might also affect more than one protein with similar binding clefts (e.g. ATP is a substrate for various enzymes). Activation of one tyrosine kinase receptor by a toxicant can have multiple effects via diverging signaling cascades. Genes with distinct functions could be affected by mutagenesis (see chapter 3 for many specific examples).Continuous displacement of homeostasis can therefore lead to various other outcomes that might not be readily observed under submaximal doses but require further investigation (as mentioned in Session 2 notes: "neural degeneration, arthritic conditions, vascular rigidity, loss of muscle mass, loss of bone mass, decline in immune function"). Animals that undergo toxicity testing for carcinogenesis should be used for further microscopic and biochemical examination. Are there any increases/decreases in protein expression in experimental group compared to controls, any changes in tissue/organ architecture? Do animals show any of the effects mentioned above? Recent advances in proteomics such as DNA, mRNA, and protein chips will allow faster analysis.

Posted: Feb-13-03 at 5:52 PM by Student 1

Although it remains important that the emphasis of chronic toxicology testing is on carcinogenesis and mutagenesis, other possible outcomes are similarly significant. One such example is chronic exposure to mycotoxins. Mycotoxins are secondary products of Fungi found primarily in food or animal feed due mainly to poor storage environments. Due to their lipophilic nature, these toxins are generally stored in fat cells. The various species produce a variety of mycotoxins of which a few have been studied extensively for toxicity in wide ranges of animal species (mainly farm and laboratory animals). Symptoms exhibited by mice and rats chronically exposed to mycotoxins included, along with carcinogenesis, such deleterious effects as extensive liver damage, gastric ulcers, and thymic depression. It has been shown that the mycotoxin, aflatoxin, is cytotoxic due to its ability to induce lipid peroxidation leading to oxidative damage in certain cell types. Other mycotoxins damage cells through action on cellular respiration indicated competitive inhibition of ATPase succinate dehydrogenase, and cytochrome C oxidase. Some examples inhibit protein and DNA synthesis, compromise immunity, and/or disrupt the endocrine system (above issues reviewed in Hussein and Brasel 2001) These effects could damage any number of essential functions for survival. The reason that it is important to seriously consider these effects alongside carcinogeneisis and mutagenesis is that they may be just as deadly. In the case of mycotoxins, chronic exposure to humans may come in the form of consuming milk or meat from intoxicated farm animals, consuming contaminated food directly, and inhaling air from contaminated buildings. Lately, the issue of infected buildings has repeatedly been in the news, and the report often includes contamination of a school with mold and many sick children.

Hussein, HS and Brasel JM, Toxicity, metabolism, and impact of mycotoxins on humans and animals. Toxicology. 167 (2001) 101-134.

Posted: Feb-13-03 at 10:36 PM by Student 2

Carcinogenesis is certainly emphasized as a selective endpoint for toxicity testing, although noncarcinogenic effects that may potentially impact one's life are by no means considered a weaker endpoint. Not all substances are carcinogenic; therefore, long-term, low-dose exposures to such substances obviously should be evaluated with regard to noncarcinogenic, nonlethal effects. (Furthermore, some substances shown to be carcinogenic in laboratory animals have not been proven to cause cancer in humans).

Posted: Feb-15-03 at 11:14 AM by Student 12

Chronic toxicity test usually requires at least 1,000 animals and 18 months to complete. It should have the same route of exposure that would occur in humans and needs a close watch every day. I think the current emphasis of chronic toxicity testing is pretty appropriate to check on carcinogenesis, but the test is expensive and there is no standard measure for chronic toxicity, unlike LD50 for acute toxicity. Humans are generally more vulnerable than the tested animals, and different species have different way of responding to toxic substances. Today the estimation of human risk is still one of the difficult problems, since quantitative risk analysis is especially difficult to make relationships based on animal data, while qualitative risk analysis is relatively easier to develop by finding some negative toxic effects in the tested animals although the applicable toxicity use level is quite different in each country.

Posted: Feb-16-03 at 10:09 PM by Student 6

Chronic testing looks at periods from 6 months - two years. These tests look at exposure over most of the animal's lifetime. These tests require a minimum of 1000 animals. These animals have to free of any tumors. These studies look at MYD (maximum tolerable dose). These tests are fairly appropriate but can be costly and errors do occur. Humans have different responses than lab animals and generally are more vulnerable. Estimating responses to toxicants in humans is still difficult today since what might be toxic in lab animals isn't necessarily going to be toxic in humans and vice versa.

Posted: Feb-18-03 at 12:30 PM by Student 5

Since cancer, speaking extremely generally, is caused by (1) a mutation of some sort in DNA which then causes (2) uncontrollable mitotic division of the originally affected cell or tissue, with the potential to (3) metastasize, there are several routes in which a toxin can either induce or enhance cancerous development. General toxicity testing is. of course, appropriate, just to see if any physiological response is elicited, and mutagenesis testing checks for the first part of my definition. However, a mutation may or may not lead to cancer, and it is, in my opinion, more important to check for the ability of a toxin or toxicant to break the normal regulation of a cell cycle. The toxin/toxicant may not be mutagenic itself, but may cause the disruption of the cell cycle, possibly in addition to damage done by some other agent previously, which damaged it and yet did not cause the cancer cascade. There is also the possibility of a toxin/toxicant damaging cell-to-cell connections and initiating metastasis of the cancerous cells.

Posted: February 20, 2002 by Student 10

6. Chronic toxicity tests are performed to assess the cumulative toxicity of chemicals, which often include a consideration of the carcinogenic potential of chemicals so that a separate study that solely addresses carcinogenicity does not have to be performed. Chronic toxicity assays are regulated with guidelines that require that both benign and malignant tumors be in the evaluation. While this type of testing may be validated with discoveries of potential cancer causing agents, there are some problems. All known chemical carcinogens in humans have been found to be carcinogenic in some species but not in all laboratory animals. However, all chemicals that are carcinogenic in animals are carcinogenic in humans. Species variation and genetic variation within a species being tested may account for some discrepancies when chronic testing for carcinogens is applied. Information about the mechanism of action of the carcinogen may indicate that a positive response in animals is not relevant to humans and may be irrelevant for human risk assessment. In this case, many question the purpose of chronically testing animals that may not provide results that actually relate to humans.

Another issue with chronic toxicity testing is that they are used mainly for carcinogenic studies when the possibility of other toxic effects and disease may occur. These other effects include, neurodegenerative disorders, problems with other organs, and toxic effects to the digestive and reproductive systems. Also, many chronic studies expose agents in one or two forms of delivery but do not test the effects of exposing the toxin through another method of administration. This may also provide results that differ from carcinogenic effects but instead lead to various other diseases and malfunctions.

Posted: February 20, 2002 by Student 9

6) The emphasis on chronic toxicity with respect to carcinogenesis is presumably due to the many oncogenic promoting agents in the environment as well as the relatively easiness in measuring them. As our society becomes more sophisticated in terms of understanding, technology and preventative strategies, it is certain that the range of chronic toxicity testing will expand.

Posted: February 20, 2002 by Student 11

6. Chronic toxicity is performed to find out the possible toxic effect s of a certain chemical. It is also done to check if the drug has any carcinogenic potentials. The exposure time is long (more than 3 months) for chronic toxicity; exactly how long depends on how long the drug will be used for in humans. High dose is required for mutagenesis testing and chronic testing requires the use of a drug for a long period of time. These two factors (high dose and long time exposure) can be deleterious for the subjects. Therefore, in order to perform chronic toxicity, it is very important to keep maximal tolerance dose (MTD) concept in mind. MTD is a maximum dose that can be used in testing subjects without harming them before they go through the rest successfully. Also, it is important to use neat animals; animals whose medical history has been clear of any tumors.

Topic: Question 7. Toxin Metabolism Considerations

Posted: Mar-04-02 at 4:00 PM by Kenneth Campbell

7. Emphasis in the discussion of biochemical and physiological means of toxin metabolism and clearance is on the adult animal. Are there other important considerations or routes to be considered in growing offspring or during the reproductive process?

Posted: Feb-12-03 at 11:45 PM by Student 8

For a developing fetus the major route of exposure is the placenta. Along with the nutrients, various toxins and toxicants that enter the mother via standard routes move across this barrier against a concentration gradient. Though this barrier could consist of multiple layers, their number is not of great importance for the rate of diffusion of chemicals. It is rather the solubility of toxicants that is important so if a toxicant was able to rapidly diffuse across other body membranes it will do so across the placenta. Luckily, 50-50 equilibrium is not always established between the mother and the fetus because fetal tissue is usually less developed and less able to concentrate the toxicant (exception: brain).

In addition to this, developing offspring can encounter toxicants postnatally (through lactation) as well as prior to conception (through seminal fluid). For this purpose many developmental and reproductive toxicity tests are available. Segment I tests evaluate the effect of chemicals on general fertility and reproductive performance; segment II tests administer chemicals during early pregnancy; segment III tests are done in late pregnancy and continue into lactation.

Posted: Feb-13-03 at 6:35 PM by Student 1

In addition to the placental barrier, another important route to consider is the blood-testis barrier. This barrier prevents free exchange of chemicals/drugs between the blood and the fluid inside the seminiferous tubules. The barrier is composed of tightly fit epithelial cells, which may have sections with leaky junctions. Lesser-developed barriers in immature testes have more of this leakiness, thus leading to a greater chance for exposure. If a toxin does pass through this barrier, several mechanisms for metabolism exist in the testes. These include mixed-function oxidases, epoxide-degrading enzymes, gonadal cytochrome P450 and AHH present in microsomes. This metabolism, however, can have adverse effects on spermatogenesis and/or steriodogenesis. Metabolites of toxicants alter testicular structures resulting in deleterious effects to the germ cells. Examples of this include the oxidization product of n-hexane, 2,5-HD, which alters the microtubules in Sertoli cells ultimately preventing the paracrine support for the germ cells. Other toxic metabolites cause abnormal sperm, testicular lesions, decreased sperm mobility and damage to the Sertoli and Leydig cell functions (see C&D 689-690). All of these factors can be deleterious to the possibility of reproduction.

Posted: Feb-13-03 at 10:57 PM by Student 2

Differences in a child's behavior and physiology are major factors that need to be considered when addressing toxicity. Children often have a much higher exposure potential (to environmental toxicants, in particular) than adults: they play in the dirt outside or on the carpet; they consume more food and water and have a higher inhalation rate per body weight; they orally explore their surroundings; they drink breast milk. Likewise, their uptake, metabolism, and excretion mechanisms vary from those of an adult. Because of their enhanced exposures, children are generally more susceptible to chemical insult than adults.

Posted: Feb-15-03 at 11:33 AM by Student 12

Some routes of exposure which negatively effects on reproductive process are inhalation, ingestion of food and water containing toxin/toxicant, and skin. As an example, lead can be found in many kinds of food or in leaded paint. Lead is a tetratogen that can cause fetal malformation, a mutagen that can impair fertility and affect both sperm and eggs. Through inhalation lead is almost completely absorbed after entering lower respiratory tract, and it affects major organs in the body, especially children's nervous system. Its absorption rate through ingestion for young children is 4 to 5 times higher than for adults, causing lead poisoning and permanent neurological, cognitive impairments.


Posted: Feb-16-03 at 6:24 PM by Student 6

In the growing offspring or developing fetus, a number of considerations need to be made. The growing offspring is more susceptible to toxicants due to its behavior as a young, i.e. it eats more, plays around and explores. Also, their methods of clearance, metabolism, detoxification, and excretion are not as developed as adults along with their immune systems. During development, the fetus absorbs nutrients from the mother from the placenta, so whatever toxicants the mother may be able to clear, the fetus may not be able to and absorbs it. Examples of this can include leaded paint, where in small amounts to adults is not toxic while can cause numerous developmental problems during development.

Posted: Feb-18-03 at 7:10 PM by Student 5

First of all, embryonic and fetal development in mammals makes use of the placenta, which is permeable to many toxins and toxicants present in the mother's circulation, as well as viruses and other pathogens. If the pregnancy happens at a difficult time for the organism, fat reserves may be mobilized, which may contain a significant amount of toxic substances that are then passed to the fetus. Fetuses and newborns often have underdeveloped metabolic pathways for clearing toxins and toxicants crossing the placental barrier or passed through mother's milk. Once infants are weaned at least partially, any other food is also a consideration, as it may contain substances harmful to the underdeveloped systems, which might go unnoticed in adult toxicity testing. The same goes for other routes of exposure: anything inhalable or in dermal contact with the baby which may be considered safe for adults may not be safe for the newborns. In addition, once children are mobile, they tend to ingest and come in contact with a wider variety of materials not intended for consumption or inhalation.

Teratogenic effects are also a consideration. When a toxicity study is performed, it may or may not be done on pregnant or impregnating animals; if it is not, there is no way to definitely rule out possible effects on the reproductive system even if the adult test subject exhibits no adverse effects whatsoever. It is also possible that a long-term effect exists, as a result of a slow metabolic pathway to the ultimate toxicant. For example, a rat injected with a potential toxicant is mated, say, three days after the injection, and the offspring exhibit no ill effect. However, a mating in two weeks will show severe teratogenic effects originally attributable to the injected substance.

Posted: February 20, 2002 by Student 10

7. Growing offspring and embryos developing during the reproductive process are more susceptible to the effects of toxins than the adult animal. Biochemical and physiological means of toxin metabolism and clearance are usually focused on the adult animal but many studies have been performed that provide data on the vulnerability of growing offspring as well as the knowledge that most chemicals are not denied entrance into a number of compartments or secretions of the reproductive tract. This includes the placenta being unable to restrict and prevent most chemicals from crossing the placenta. Xenobiotic and certain drugs can be detected in uterine secretions, in the milk of lactating mothers, and in seminal fluid. There also appear to be no specialized barriers to prevent chemicals or drugs from acting on the ovary. Some important considerations and routes of delivery to be looked at in growing offspring and during the reproductive process are that tissues and bones have not fully developed and therefore do not have the ability to withstand toxins as in adults and to metabolize and secrete to eliminate them before harmful effects can take place. Growing offspring do not have fully developed digestive or reproductive systems and are more susceptible due to protective mechanisms not being intact and fully developed.

Posted: February 20, 2002 by Student 9

7) YES! Teratogenesis (i.e. abnormal development) is evidenced by mutations, chromosomal breaks, altered mitosis, altered nucleic function, osmolar imbalance..etc..etc.. All of which may be triggered by toxic agents delivered through the conceptus at the cellular level, the insult may occur through a direct effect on the embryo/fetus, indirectly through toxicity of the agent to the mother and/or the placenta or through a combination of direct and indirect routes of entry ( C & D, page 365).

Also - a developing child will be differentially effected as a young child's nervous system is still developing until approximately age 10. For instance, organophosphate insecticides, which are the commonly used product since the removal of organochlorines, have been found to differentially effect adults and children. Some of the typical cholinergic signs of bradycardia, muscular fasiculations, lacrimation and sweating were less common in children but seizures, lethargy and coma were most prevalent in children.

Because the toxic agent is acting on two different systems (i.e. developed vs undeveloped NS, a different result would be expected).

Posted: February 20, 2002 by Student 11

7. It is important to understand the fetal metabolic pathways because offspring can be exposed to chemical and physical agents that could result in adverse effect. This exposure can occur at the time of conception, during pre and post natal development (until puberty is reached). yes, important considerations and routes should be considered in growing children because the offsprings are still developing, and are not as developed as adults, and therefore, their body works little differently than that of adults.

Topic: Question 10. Risk Strategies & Levels of Toxicity

Posted: Mar-04-02 at 4:04 PM by Kenneth Campbell

10. What distinctions might be made between lethality, morbidity, reproductive capacity, quality of life, and quality of environment impacts vis a vis risk assessment strategies?

Posted: Feb-12-03 at 12:11 PM by Student 1

10. The distinctions between these categories of various levels of risks and the risk assessment strategies are that of result of exposure to a particular toxic substance. Assessing the risk of a lethal substance would include animal testing and determination of LD50 under a varying set of conditions, such as length of exposure, site of exposure, number of test subjects, etc. Assessing the risk of a morbid agent would require more specific testing. The observations would now have to include clinical observation of symptoms and postmortem examination to determine the site of action and characterization of the specific effect of the chemical. These observations may be the occurrence of tumors, damage to particular organs, or interruption of a biological function. In the case of assessing the risk to reproductive capacity, the tests must focus specifically on the endocrine and reproductive systems of the organisms. The tests to determine the reproductive toxicity of an agent are performed on rats through six stages described by the International Commission on Harmony, from premating to weaning and sexual maturity of the offspring (Klaasen 2001). Quality of life risk assessment would need to test the ability of the agent to affect a condition of life that does not cause death or disease such are skin and eye irritants. Quality of environment impacts risk assessment must test all of these conditions as well as the effect of the agent on the natural habitat. This assessment might include the effect of an air pollutant on the human population (lethality, morbidity, reproductive capacity, quality of life) and on the wildlife (LD50 of certain populations) and on the plant life (observation of decrease in growth).

Posted: Feb-14-03 at 9:58 PM by Student 2

On an individual basis, the first three terms reflect varying degrees of toxicity: lethality, obviously, is the most severe; morbidity may ultimately decrease lifespan; reproductive capacity in and of itself may not adversely affect an individual, although could negatively impact a population through decreased fecundity.

Quality of life impacts may not necessarily result in harmful effects to an organism; however, that quality is degraded from optimal conditions. Nuisance odors- chemicals present at concentrations above the odor recognition threshold, but below deleterious levels-are a good example of this. In terms of risk characterization (in Massachusetts, anyway), the presence of nuisance odors indicates a condition of significant risk to public welfare exists (even if the contaminants may not present a risk to human health).

Quality of the environment, be it on a localized or global scale, ultimately determines the level of impact. Risk assessment is a tool by which we relate the quality of the environment to public health or the environment. The underlying premise is that there is an "acceptable" level of risk--such as allowing residual contamination to remain in the soil, using a drug for therapeutic effects despite the potential for side effects, or using household cleaners or pesticides that may be lethal if used incorrectly.

Posted: Feb-16-03 at 12:03 PM by Student 4

Risk assessment can be classified as a quantitative estimate of the potential effects on human health and the environmental significance of various types of chemical exposure (Klaassen). In identifying a hazardous agent, tests are performed to determine if the agent will cause an adverse effect through structure activity analysis, in vitro testing, animal bioassays, and epidemiology studies. Additional testing is performed to determine the dose-response relationship and the duration of exposure to the hazardous agent in question. These assessments form a Venn diagram that make up the three categories of toxicological studies; mechanistic, regulatory, and descriptive. While lethality, morbidity, reproductive capacity, quality of life, and quality of the environment all play significant roles in the study of toxicology, further distinctions can be made by the following; lethality is an acute test first performed on a NCE with a fixed dose on a small population where an LD50 is determined. Morbidity can be considered a long term exposure study where a population is subjective to an agent for greater than 3 months where alterations on biological functions are addressed. Reproduction takes into consideration the effects of the reproductive system and the potential of the hazard to disrupt embryonic development. And furthermore, quality of life and quality of the environment assess the effects of a hazard on a large population and in nature where risk managers summarize the potential risks.

Posted: Feb-17-03 at 1:29 AM by Student 6

Looking at individuals, lethality which leads to death, morbidity which leads to a decrease in lifespan, and reproductive capacity all are the most serious. The last one could end up affecting a population as a whole in the long run. Quality of life toxicity issues wont necessary affect the organism in harmful ways, but may cause degradation in the quality of life for the organism. For example a annoying smell may occur in the organism or a scar which may be unpleasant but won't probably lead to death. But, the presence of an odor in an area may lead to fears of something harmful even though there is no harmful toxicant. Quality of environment is looking at the safety of the environment as a whole. It is important for risk assessments because it allows us to keep the public informed to what is a safe level of risk for products that are toxic such as household products. Also in what are side effects for medicines.

Posted: Feb-17-03 at 7:09 PM by Student 5

A risk assessment is an expert's opinion of the likelihood of a deleterious effect. Depending on what that effect might be, lethality(death), morbidity(disease), quality of life (level of comfort for the living organism), etc., the assessment will differ for the same toxicant. There may be a high risk of reduction in the quality of environment (how conducive the surroundings are to life) but not necessarily lethality. For example, an oil slick is a high risk for the beach by your house, but it carries a very low risk of death for you personally. Similarly, reproductive capacity of pond-living organisms may be severely reduced by a toxicant dumped into their pond, but have no effect on their own quality of life. Assessment strategies would thus have to be tailored to estimate the risk to a specific effect (i.e. if it is necessary to carry out an assessment of morbidity of tadpoles in Lake Erie, one should probably observe behavior and collect some specimens and inspect their tissues, rather than spend time observing a reproductive cycle and counting young.)

Posted: Feb-18-03 at 4:23 PM by Student 8

Risk assessment strategies require the following components:

-hazard identification-an evaluation of the adverse health effects the agent is capable of causing: damage to liver, nervous system, carcinogenesis

-dose response assessment-includes prediction of exposure levels at which risk is likely to be negligible or nonexistent

-exposure assessment - a determination of how much of an agent people might be exposed to under various conditions such as use of a drug or a consumer product, environmental exposure at a hazardous waste site.

-risk characterization- explicit description of the assumptions and uncertainties that go into the risk assessment, and the overall confidence in the results of the analysis.

It seems like risk assessment strategies will only characterize lethality of a substance with a high degree of certainty (even then there is genetic variability) but that for other levels of toxicity it will be extremely hard to predict the levels of risk. Even for a simpler task such as cancer assessment, mathematical models are needed to connect the results of experiments with a limited number of animals to large scale predictions (more than 1 additional cancer in 1 million exposed people is unacceptable risk, but you cannot test 1 million animals). So morbidity, reproductive capacity, quality of life and environment differ from lethality in the certainty of risk predicted by risk assessment strategies.

Posted: Feb-20-03 at 10:29 PM by Student 3

The variety of adverse effects of chemicals on humans clearly impacts risk assessment strategies. Bioassays and LD50 aid in developing these strategies. When assessing risk of toxins, lethality (ability to cause death), morbidity (ability to cause disease), reproductive capacity, quality of life, and quality of environment do have distinct impacts on determining risk. In this case, since the question is quite vague, it is probably more helpful to look at an example: mold spores.

(Radiation is actually a more clear cut example, however, I am assuming that we aren't considering that a toxin.)

Lethality: Clearly the most significant factor does not play much of a role with mold spore exposure. Put another way, when assessing risk, this "toxin" does not cause death.

Morbidity: Exposure to mold (esp. indoor constant exposure) can cause a wide array of health problems. Very large doses of mold spores (inhaling or ingesting) can result in poisoning by mycotoxins. Exposure can cause allergic illnesses, asthma, and infection (lung or respiratory tract). In this case, morbidity has a very high and quite distinct effect on risk assessment.

Reproductive capacity: Mold spores are not usually a health risk factor on repro.

Quality of life: Mold spores indoors can not only cause a health risk, but can make living uncomfortable by adhering to ceilings, etc. in the home.

Quality of environment: Varies depending on indoors vs. outdoor exposure- indoor causing more direct inhalation and infringing on safety of the individuals exposed.

Posted: February 20, 2002 by Student 10

10. Risk assessment strategies are definitely impacted by differences between lethality, morbidity, reproducing capacity, and quality of life and environment. In determination of the risk of a toxin, acute testing would be performed on toxins that my be considered lethal. Morbidity involves a compromise of function, including, disease rates, genetic defects, and toxicant effects, all of which may not be lethal. In testing whole populations, reproductive capacity and morbidity may lead to lethality, which is very damaging and would be considered in those risk assessments. However, when looking at individuals rather than a species as a population, morbidity , reproductive capacity, as well as, quality of life and environment may not be extremely toxic in the sense that one can still survive. Toxic effects exist at various levels and when assessing risk of a toxin, it is possible that they will not induce effects that generate lethality of morbidity but will alter quality of life and/or environment. In these cases, the strategy about how assess risk changes and preventing death is no longer a goal, but instead preserving a quality of life and general well being.

Posted: February 20,2002 by Student 9

10) Lethality: death

Morbidity: diseased state, decreased life span

Reproductive capacity: ability to reproduce

Quality of Life: nonmedical threats that compromise one's ability to enjoy life

Quality of environment: degree of safety, comfort and security in surroundings

In terms of these factors with respect to risk assessment strategies, they are ordered in terms of importance with the greatest stressor on lethality and in descending significance thereafter.  Obviously, lethality would have the greatest influence on risk assessment. Although there may appear to be an element of subjectivity (i.e. reproductive ability vs quality of environment..etc..etc), overall they are ranked in accordance with what the majority of individuals would assess in terms of personal importance.

Posted: February 20, 2002 by Student 11:

10. Lethality is related to death, morbidity is related to compromise in functionality. In individual lethality, it is not necessary that it means inability to reproduce. When talking about population lethality, the morbidity might be lethal. The toxins might change one's quality of life or living standards but it is not necessarily lethal. The quality of environment will effect each individual differently.

Topic: Question 16. Oral Toxicity Paradox

Posted: Mar-04-02 at 6:54 PM by Kenneth Campbell

16. Oral toxicity studies have defined a paradox of this route of exposure. What is the paradox and how might it be explained? A somewhat similar paradox can arise if dietary insoluble fiber is high and exposure is again oral. Why might this occur? Are either of these situations of possible use in treating acute poisonings? In chronic intoxications?

The paradox of toxic intake by the oral route: that lower dose concentrations (and larger volumes) can sometimes be more toxic than higher dose concentrations [same amount of toxicant in either case, just different volumes].

Note the volume sensitive sphincter separating the stomach and the small intestine which has a very large absorptive layer. If the volume of the dose is increased this may lead to a faster emptying of the stomach, a lesser time for the HCl in the stomach to potentially inactivate the drug or toxicant, and more rapid exposure of the intestinal lining to a toxicant containing solution. In this situation even a lower total dose of compound may be more effective than a small concentrated dose.

Is there a secondary paradox? What if dietary insoluble fiber is present in the stomach at the same time as the dose?

What is insoluble fiber?

It tends to be macromolecular and will include polymers that are nondigestible for the species in question, e.g. cellulose (in nonherbivores), hemicellulose, pectin, silicates, and some minerals.

An increase in insoluble fiber will increase the transit rate through the gut, i.e., it will decrease the time needed to transit the gut. This effect is often protective as it reduces time of exposure to a toxicant. The effect is also enhanced by the presence of clean insoluble fiber as opposed to previously contaminated fiber. This is because clean fiber can efficiently adsorb toxicants from solution and effectively sequester them from absorption by the intestinal lining.

But contaminated fiber can also release toxicants for absorption. So there is an equilibrium between toxicants free in solution and those particulates or colloids, including insoluble fiber, within the digestive tract. Insoluble fiber can act as a sink or source for toxicants depending on its prior exposure to toxicant.

Posted: Feb-13-03 at 8:58 PM by Student 1

The paradox of oral toxicity studies is that the absorption of a substance depends on a number of variable factors. Organic acids and bases are absorbed in the section of the GI tract where they exist in the nonionized form. The acidic environment of the stomach versus the neutral environment of the intestine can markedly affect the ability of a toxin to be absorbed. Differences in surface area of the stomach, small intestine and large intestine also influence absorbency, larger surface areas generally allowing for more opportunity for absorption. Another factor is that a number of toxicants are biotransformed in the GI tract into substances with either increased or decreased toxicity. The time a toxic agent spends in the GI tract can also influence absorption, the longer time spent, the more opportunity. Here is where dietary insoluble fiber can come into play. If the level of insoluble fiber is high, then movement through the GI tract is faster, thus decreasing the opportunity of a toxin to be absorbed. These situations can be used to treat acute poisonings by capitalizing on the absorptive properties of poison. If the nature of the poison is known, administrating ions that prevent the absorption could be used as a treatment. Another tactic may be to administer a substance that would decrease the solubility of the substance. If the poison cannot be dissolved, it won't be absorbed. The property of dietary insoluble fiber could be used to flush out the poison from the GI tract by essentially pushing it through before it could be absorbed at high enough concentrations to exhibit a toxic effect.

Posted: Feb-14-03 at 10:00 PM by Student 2

The paradox is that the oral route of exposure is probably the most common route (e.g., accidental or intentional ingestion), and intuitively seems like it would be the most direct route to toxicity, but toxicity generally will not occur through oral exposure until the toxicant is absorbed into the body (with the exception of caustic materials). In principle, this is not very different from dermal exposures-a chemical would have to penetrate the skin and be absorbed into the bloodstream in order for toxicity to result. Thus if absorption can be inhibited within the gastrointestinal (g.i.) tract, toxicity may be reduced or eliminated. Numerous factors influence a toxicant's absorption in the g.i. tract: pH, surface area, active and passive transport, pinocytosis, etc.

Regarding insoluble fiber, ingestion of fiber increases the rate at which a bolus is passed through the gut, thereby reducing the amount of time available for exposure, and by extension, reducing absorption. Therefore, if a toxicant is delivered to the gut coincidentally with another substance that either increases g.i. elimination rates or inhibits absorption of that toxicant, toxicity will be reduced. For acute poisonings, this would be (and is) an effective antidote. For chronic exposures, it may be also be beneficial, as in the case of insoluble fiber. One problem that I can foresee, however, is that chronic administration of certain substances that may regulate absorption of a toxicant may also decrease absorption of beneficial compounds (Olestra, anyone?).

Posted: Feb-17-03 at 7:03 PM by Student 8

I am not sure about other substances that regulate absorption but in the case of dietary fiber continuos exposure is extremely beneficial. In the diets of many third world cultures between 150 and 175 grams of fiber is consumed daily (only 30 for US). Accordingly, they are not plagued with most of the "industrialized" disorders which include colon and breast cancer, heart disease, diabetes, arthritis, chronic fatigue syndrome, weakened immune systems, personality disorders, and all of the hormone-based female disorders. Most experts now agree that these nondigestible carbohydrates promote not only laxation, but blood cholesterol and glucose attenuation as well. The latter two functions are promoted by soluble fiber. Soluble fiber reduces cholesterol by reducing its absorption in the intestine and by complexing with bile acids (made from cholesterol in the liver) and preventing their reabsorption (so that liver must use additional cholesterol for making more bile acids). Soluble fibers also help normalize blood glucose levels by slowing the rate at which food leaves the stomach and by delaying the absorption of glucose following a meal. They also increase insulin sensitivity.

Insoluble fiber, as mentioned before, helps maintain bowel regularity by increasing the bulk of the feces and decreasing the transit time of fecal matter through the intestines. Bowel regularity is associated with a decreased risk for colon cancer and hemorrhoids. Additionally, fibers have been shown to have a gentle brooming effect on the inner walls of the intestines, performing what might be called "daily house cleaning." They have a binding effect on toxins in the colon, as well as harmful estrogen metabolites, excess dietary fat and cholesterol, so they are able to assist in the rapid excretion of these materials, thereby blocking their re-absorption into the bloodstream. Research has shown that heavy metals, such as lead and mercury, are excreted harmlessly and much more efficiently when pectin (a type of fiber) is included in the diet. Apple pectin, rice bran, wheat bran, alfalfa fiber and burdock root fiber, along with other sources of dietary fiber, have been shown to protect the body, and especially the gut, against the toxic effects of several common food additives, including amaranth, Tween 60, sodium cyclamate, tartrazine, and Sunset Yellow.

Taken all of the above into account, chronic exposures to many toxins/toxicants CAN be treated by continuous, moderate intake of fiber. Only large amounts of fiber taken continuously were found to partially block the digestion, absorption or reabsorption of a wide variety of drugs and fat soluble vitamins. Taking twice as much fiber as is recommended (~ 50 grams daily) will improve rather than compromise our health.

References for the above question were:

Posted: Feb-17-03 at 8:57 PM by Student 2

I couldn't agree more in the case of insoluble fiber-but I was suggesting, rather, that perhaps other substances (not noting any specifically) that may inhibit uptake of toxicants into the gi tract to treat chronic exposures may possibly have a negative effect on absorption of essential nutrients. I gave the example of Olestra, a synthetic fat that is not absorbed (and therefore is 'calorie-free'). However, it was noted that certain nutrients were also not absorbed. In the long-term, reduced absorption of essential nutrients will be detrimental to an organism's health.

Posted: Feb-15-03 at 8:06 PM by Student 12

Oral toxicity may involve transmucosal absorption and also chemical modification in the stomach acid, which degrades and hydrolyze the compound to make it easily degraded or absorbed. The paradox is that the body of animals, including humans, does not really resist toxic materials until they enter into the bloodstream. Most of animals have "oral tolerance"-- the immune system of the animals does not recognize potential immunogens if they are exposed orally in their childhood. Through an oral route those organisms invade the animal's intestinal tract and grow inside white blood cells without being killed.

Insoluble fiber, such as cellulose, hemicellulose and lignin, can be found in food like wheat bran and is useful in weight control because it gives us a fullness feeling. It is not broken down by digestive enzymes and could be useful for treating acute poisoning because it slows digestion, reduces metal absorption, and speeds up bowel elimination processes. Intoxication occurs when waste in the bowel is trapped for a longer while, causing toxins to go back into the colon. The toxins then eventually go into the bloodstream through the blood capillaries along the bowel wall and harm other organs and cells. Insoluble fiber can help foods keep moving the digestive tract, reducing potential carcinogens in the colon.

Posted: Feb-17-03 at 7:17 PM by Student 8

Other answers have described in detail the paradox of the oral toxicity and pointed at intestine as the main site for entry of xenobiotics. Under normal conditions, xenobiotics are poorly absorbed within the mouth and esophagus, due mainly to the very short time that a substance resides within these parts of the GI tract. There are some exceptions though. Nicotine readily penetrates the mouth mucosa and nitroglycerin is placed under the tongue for immediate absorption and treatment of heart conditions. The sublingual mucosa under the tongue and in some other areas of the mouth is thin and highly vascularized so that some substances will be rapidly absorbed. Very little absorption takes place in the colon and rectum. As a general rule, if a xenobiotic has not been absorbed after passing through the stomach or small intestine, very little further absorption will occur. However, there are some exceptions, as some medicines may be administered as rectal suppositories with significant absorption. An example, Anusol is used for treatment of local inflammation which is partially absorbed (about 25%).


Posted: Feb-18-03 at 9:56 PM by Student 3

A paradox in the po route of exposure lies in the fact that a chemical passes through several environments (the mouth, esophagus, stomach, small, then large intestine, rectum), most of which are lined with mucosal layers and are functionally geared to eliminate the contents. The stomach and small intestine also radically change the pH of their contents, likely biotransforming the initial toxicant. Thus, most poisons do not do much damage in the alimentary canal, but only once they are absorbed from there into the bloodstream, even if they are broad cellular mechanism disruptors such as cyanide, which blocks the final step in the electron transport chain in aerobic respiration. Insoluble fiber promotes a speedy passage through the intestines and has, as someone mentioned, a "gentle brushing effect" on the walls, thereby removing mucosal cells that already contain the poison and would allow it to diffuse deeper and closer to the bloodstream. Though one might intuitively think that fiber "stops up" the flow as it does in a sink, it in fact promotes elimination. The faster the flow, the less material will get absorbed in the alimentary lining.

This idea has an obvious application in acute poisoning cases: after such an event, a good quick treatment might be a fast-acting laxative, such as syrup of ipecac. Vomiting may or may not be a good idea; although elimination from the body might be faster, the ingested contents may no longer be in the stomach, and the esophagus does not have much lining, so it may be damaged by a corrosive poison twice (up and down). Drinking large amounts of water would probably not be a good idea, since water is mostly not eliminated through the bowels, but enters circulation and goes through the liver and kidneys at the very least. I don't see how it can help clear the GI tract, but I could be wrong. Anyone?

In chronic (6 months to 2 years) intoxication cases, the technique is not likely to help. I suppose it's possible that if the poison is extremely slow to absorb, intake of insoluble fiber would help clear the outer mucosal layers where much of the toxicant would still linger.

Posted: February 28, 2002 by Student 10

16. One of the main sites of absorption is the GI tract. Many environmental toxicants enter the food chain and are absorbed together with food from the GI tract. Unless a noxious agent has caustic or irritating properties, poisons in the GI tract usually do not produce systemic injury to an individual until they are absorbed. Absorptions of toxicants can take place along the entire GI tract, even mouth and rectum. There are factors though that alter the GI absorption of toxicants and serve as a paradox in these studies. The mass action law, surface area, and blood flow rate have to be taken into consideration. Some studies have shown that oral toxicity of some chemicals is increased by diluting the dose. This can be explained by more rapid stomach emptying induced by increased dosage volume, which in turn leads to more rapid absorption in the duodenum because of the larger surface area there. There is also a species-dependent absorption different of toxicants due to the gastrointestinal flora. It dietary insoluble fiber is high and exposure to a toxicant is oral, some of the same problems might arise. The toxicant may not produce any deleterious effects until once it has absorbed into blood at the time of elimination. Acute poisonings might be treated by this situation if the insoluble fiber can absorb the toxicant and prevent absorption into the bloodstream during elimination. Chronic intoxications might be more difficult to treat because continuous exposure to the toxicant will ultimately be hazardous to the liver and be unable to be eliminated before deleterious effects take place.

Posted: February 28, 2002 by Student 11

16. The paradox is the change in level of toxicant's absorption into the blood stream and the interference of toxicant with other minerals (if talking about insoluble fibers). The toxins that pass through the GI tract (main path taken when dealing with oral toxicity), do not cause any toxicity until they are absorbed into the blood stream, which might take some time. A somewhat similar paradox can arise if dietary insoluble fiber is high and exposure is oral. Insoluble dietary fibers include cellulose, hemi-cellulose etc. It is mostly found in whole grain products such as wheat bread and is known to induce bowel movement. Insoluble fibers speed up the passage of food via the GI tract, promote bowel movement, slow down the breakdown of starch and delay the absorption of glucose. Fiber is not toxic; it can however, interfere with other mineral absorption, if the intake go above the recommended level (20-35 g/day). TOO much fiber intake can also result in too much calories intake and therefore, can cause intestinal discomfort and excess gas formation ( nutrition/nutrition basics/necessarynutrients/no nutrients.jsp).  The acute poisoning can be treated because the exposure of the toxicant is for a short period of time and the toxicant might not be absorbed by the blood stream just yet. But the chronic poisoning might be hard to treat because of the longer exposure time and the toxicant will be able to absorb into the blood stream and cause deleterious effects.

Posted: February 28, 2002 by Student 9

16) A paradox in the oral route of exposure is that of the alimentary canal. It is the route that serves both digestion and respiration…that in and of itself is a blatant paradox. In terms of toxicity however, the coupling of food which is required for maintenance and sustenance of the organism, may also be threatened by inadvertently taking in toxins/toxicants into the GI tract. Many environmental toxicants enter the food chain and are absorbed with food from the GI tract ( C & D page 111). Although the GI tract is within the body, its content may be considered "exterior" to the body thereby not typically producing injury following the entrance of noxious agents until they are actually absorbed. Absorption can take place "anywhere" along the GI tract (i.e., mouth to rectum) depending on the chemical composition of the toxic agent in question.

For example, if it is an organic acid/base, it is usually absorbed by simple diffusion at the point where it manifests its most nonionized form. An additional paradox exists when thinking about digestion/respiration as routes of oral entry. For the absorption of toxic gases in the lungs differs from intestinal and per cutaneous absorption of compounds in that dissociation of acids and bases and the lipid solubility of molecules are less important factors in pulmonary absorption because diffusion through cell membranes is not the rate limiting step as it was in the GI tract ( C & D 115).

Different species have different types of digestive systems. Some are more capable/efficient at breaking down food products than others. Others may be less efficient and less capable of breaking down certain food products. Those with efficient systems may quickly rid themselves of a food product while others may store these products in their less than optimal systems.

With respect to question #16 and the case of insoluble fiber:

With the increase of insoluble fiber, a decrease in gut transit time would be expected because fiber is able to bind to steroid compounds in the lumen facilitating their excretion. The decrease in gut transit time would seem to lessen intestinal absorption of compounds in the gut. Such reduced absorption, limits the availability of a compound to travel through enterohepatic circulation and results in the increase of the original compound deposited in solid waste products and decreases the amount in urine and infiltrating metabolites. Because less of the xenobiotic compound is traveling through enterohepatic circulation, less hepatic filtering may contribute to a reduction in the polarity of metabolites excreted. By tracking such steroidal components may prove to be beneficial markers in tracking carcinogenic properties. Whether acute or chronic depends on the individual digestive system and the alteration of the ingested compound in question. For instance some fibers contain estrogenic or anti-estrogenic effects..Consumption of certain fiber substances may reduce the risk in developing certain types of cancers by reducing the risk of tumor growth which is often fostered by steroid compounds.

Topic: Question 22. Time, dose, shielding.

Posted: Mar-07-02 at 3:15 AM by Kenneth Campbell

22. In radiation protection there are three cardinal rules for minimizing radiation exposure:

    1. minimize time of exposure to the source

    2. maximize distance from the source to minimize dose delivered

    3. optimize the amount and type of shielding used to minimize dose delivered.

Can these be translated into considerations in toxicant exposure? What are the analogs of time, dose, and shielding for chemical exposures? Are there any differences among routes of exposure or nature of toxicant that need to be considered?

Such questions raise the points of health concerns during radioisotope use, i.e., radiation health physics (RHP) concerns. RHP is concerned with minimizing the exposure of subjects to the electromagnetic and particulate emissions from radiation sources. Note that both of these types of radiation primarily impact targets or subjects by delivering energy: in the case of x-rays (arising from electron transitions in decaying radioisotopes, often a fall from a high energy outer orbital into a low energy inner orbital accompanied by the release of a corresponding photon of high energy electromagnetic radiation) or gamma rays (arising from transitions within the unstable nucleus, e.g., conversion of a neutron to a proton and a beta particle (energetic electron) often with the loss of a photon of high energy electromagnetic radiation) the energy delivered is in the form of high energy photons that can cause chemical bond alterations including breakage and/or molecular motions disruptive of normal molecular function (e.g., in biological systems). Particulate emissions like beta particles, positrons (positively charged electrons), alpha particles (2 neutrons + 2 protons), or neutrons can impart kinetic energy to target molecules causing either movement or bond disruptions. If particle bombardment is intense enough, the larger particles like alphas or neutrons, especially, can also cause nuclear alterations that can generate secondary radioactive atoms, e.g., chlorine. Thus, in either case energy is being imparted to the target and may result in alterations, some deleterious, to the target. Particles, however, in colliding or glancing off the electron shells of the atoms of the target, rapidly lose energy and, with the exception of the neutron, do not penetrate efficiently within a target medium. Neutrons can penetrate because they lack a net charge, they have no intrinsic tendency to approach or be repelled by the electrons, or protons, of target molecules, they are unaffected by the atomic magnetic fields and do not affect those particles or fields unless they undergo a direct collision with a nucleus or electron. (Photons penetrate better than charged particles but have an intrinsic electronic and magnetic character that causes them to interact with electrons and atomic magnetic fields; when the allowed energy transition levels of target molecules match those of the photon moving through the target medium, those photons are efficiently absorbed.)

Recapping the electromagnetic spectrum: long wavelengths mean fewer photons per unit time through a linear distance, lower frequency, lower energy (wavelength is inversely proportional to frequency. Radiowaves with wavelengths of >um (up to m or km) occupy the long end of the spectrum, followed by microwaves (um lengths), infrared (IR, heat, ~ 800 nm - a few um), visible (300-800 nm), ultraviolet (UV, 10's- 300 nm), x- and gamma-rays (angstrom - nm), cosmic rays, and intranuclear forces (atomic/subatomic dimensions).

Note that since radiation is propagated from the source in all directions the flux of that radiation across any plane placed near the source will be inversely proportional to the square of the distance from the source (distribution in two dimensions, not just one). Thus, several of the rules of RHP have to do with maximizing the distance or effective distance from the source.

The three cardinal rules of RHP are:

    1. Minimize time of exposure

    2. Maximize distance from source

    3. Optimize shielding (protective clothes, gloves, shields, behaviors)

Note that 3. is actually a variant of 2. in that it addresses "effective" distance from the source. Thus, if a radiation absorbing shield is interposed between the source and the subject, we are essentially increasing the effective distance between the source and the subject. Note also the indication of optimizing shielding. Although it might be intuitive to use a high density shield which normally corresponds to a shield made of a material of high atomic number, we should be aware that such materials provide larger atomic targets for some forms of radiation, particularly highly energetic particles. When such highly energetic particles, e.g., the beta particle from 32P decay, encounter large atoms (e.g., in lead or leaded glass shields) they often cause electron orbital alterations that result in the production of secondary particle (Compton electrons) or x-ray emission (Bremstrallung). As the electromagnetic radiation is more penetrating than most particles, the secondary emissions may prove more damaging than the primary radiation. Thus, in such cases, low to moderate sized atomic nuclei are used to build composites that can then be used as effective shielding, e.g., high density plastic (carbon/oxygen/hydrogen) or aluminum.

Now note that these same rules for limiting radiation exposure have close or exact analogs in rules to limit chemical or toxicant exposures:

    1. Minimize time of exposure. (This limits the dose and/or the time for the toxicant to bind to its target molecules or to act on its target molecules.)

    2. Maximize distance from source. (This minimizes the dose or encountered concentration.)
    3. Optimize shielding. (This limits the routes of exposure and/or optimizes the actions of endogenous barriers, e.g., the immune system -- vaccines, or repair processes.)

So how do we go about preventing or treating toxicity?

For the individual:

    1. Avoid exposure

    2. Minimize dosage

    3. Maximize elimination (e.g., modify urine pH, often alkalinize)

    4. Block circulation and absorption of toxicant (chelators, antibodies, charcoal, clays)

    5. Apply antagonists (for specific molecular targets)

    6. Promote repair
(Note that antibodies may be introduced by passive, e.g., antivenins, or active vaccinations.)

For the environment:
    1. Modify chemical use and disposal

    2. Institute regulations

    3. Minimize releases or exposures

    4. Isolate polluted/contaminated areas/populations

    5. Institute remediation procedures

Note that Education plays a big role in avoiding both individual and environmental exposures as well as in minimizing dosages. Foreknowlege allows comparative risk evaluations to be made. Regulations help codify the knowledge base and provide guidelines for toxicant handling procedures. They externalize comparative risk evaluations. Education also allow formulation of appropriate response information and the description of pertinent treatments to handle detoxication.

What about treatments in the clinical sense?

Procedure usually involves the following steps:

    1. Take a complete history of the intoxication (acute, chronic, contributing conditions)

    2. Evaluate the subject's condition, identify the poison and dose, if possible

    3. From databases or references determine the expected pharmacokinetics for clearance (How long will elimination take and what effects can be expected in the dose range involved?)

    4. If the intoxication is acute and recent, act to decrease the dosage.

    5. If minimal risks are involved, provide medical support with minimal interventions.

    6. If there is a potential for extensive morbidity or mortality undertake active interventions:

        o For oral exposure: dilute the stomach contents, pump out the stomach, give one or more cycles of activated charcoal, provide antidotes or antagonists as appropriate.
        o For blood exposure: provide chelators or appropriate antibodies, or perform dialysis or transfusion as required.

        o Since dermal and respiratory exposures normally pass through the blood the only palliative measures that apply beyond the treatments suggested for blood exposures are removing the subject from the exposure source and doing a thorough washing with appropriate detergents of the exposed dermal surfaces.

Posted: Feb-12-03 at 12:12 PM by Student 1

These three cardinal rules can definitely be translated into considerations in toxicant exposure. The analog of time in chemical exposure is the duration and frequency of exposure. This includes the concepts of acute, subchronic and chronic exposure. The analog of dose is the applied amount of the toxic agent (mg agent/ kg test subject) that an individual is exposed to or more specifically the concentration of the agent at the target site. The analogs for shielding for chemical exposure include mechanisms for preventing absorption, presystematic elimination by biotransformation at exposure sites (lungs, stomach, skin), and prevention of agent's function through inhibitory binding. Shielding could also come in the form of protective gear specific to the route of exposure for the particular toxicant. The nature of the toxicant needs to be considered if the effect of the toxicant is increased by certain conditions. If the delivery of the toxicant were facilitated within the system, then the dosage amounts would be increased. Also if the toxicant were held in an inactive state after exposure until particular conditions released it, then the timing of exposure would also be altered.

Posted: Feb-13-03 at 6:53 PM by Student 8

Depending on the route of exposure biological responses to a toxicant will differ/its toxicity changes. A chemical may be severely toxic by inhalation but not toxic by oral or dermal exposure. Differences in toxicity arise due to several factors:

-Rate of absorption- this will depend on the nature of the toxicant. Main sites of absorption are GI tract, lungs, and skin. Lungs are generally more penetrable for volatile compounds such as gases, vapors and aerosols. Skin is impermeable to many chemicals but not to nerve gases, carbon tetrachloride etc. GI tract absorbs a range of molecules:lipophilic molecules by simple difffusion and hydrophilic molecules by active transport or by passing through aqueous pores at the tight junctions.

-Availability of sites- because of a large surface area (size of a tennis court ) lungs are generally the most available site of adsorption)

-Presence of degradative enzymes-liver detoxifies many chemicals so LD50 for a given toxicant decreases if given via portal circulation (goes to liver) than via inhalation (systemic circulation). Degradative enzymes are also present in stomach and intestine. For instance snake venom is much less toxic if given orally rather than IV because it is broken down by digestive enzymes in the GI tract.

-Extreme conditions- newborns have a higher pH in their GI tract which allows for conversion of nitrates to nitrites and leads to a condition called methemoglobinemia (defect in oxygen transport)

-Rate of transit through the body- the slower the movements of the GI tract the more absorption.

Posted: Feb-13-03 at 10:40 PM by Student 2

In terms of toxicant exposure, these same three rules likewise hold:

  1. Minimize the time of exposure to the source. If you reduce your exposure frequency and duration, you reduce your risk of harm.
  2. Maximize the distance from the source to minimize the dose delivered. In other words, avoid exposure, if possible. No exposure is equivalent to no toxicity.
  3. Optimize the amount and type of shielding used to minimize dose delivered. We can reduce exposures further by wearing protective equipment (e.g., gloves, respirators, goggles) or by other measures such as washing exposed surfaces following contact.
When working with radioactive materials, dermal penetration of rays is probably one of the greater concerns; however, a radioactive compound, as with any compound, can enter the body through various routes of exposure-ingestion, inhalation, via injection, etc.

Posted: Feb-15-03 at 4:14 PM by Student 4

As ionizing radiation passes through a substance it will lose energy, due to the fact it produces an electron and a positively charged atom residue. When humans are exposed to radiation for certain periods of time, DNA synthesis becomes disrupted as the radiation breaks apart the precursors of DNA replication (purines and pyrimidines molecules). The following cardinal rules for minimizing exposure to radiation can be translated into considerations to toxic exposure because they somewhat pertain to an individual dose response curve: (1) Deficiency and Toxicity will occur with no protection from the radiation source, and (2) the Region of Homeostasis will not be perturbed with protection from the radiation source.

The analogies or similarities that exist would be time is to the frequency of exposure to radiation, the dose would be considered the intensity (delivered dose) of the radiant rays from the distance you stood from the energy source (closer you are the greater the intensity), and shielding exposures would be the protective garments worn to protect the skin and mucosa from absorption and distribution to the target site (DNA). Differences need to be consider among the nature of the toxicant rather than the routes of exposure. There are four main types of radiation, due to: alpha particles, electrons [beta particles], gamma rays, and X-rays (Klaassen). While the frequency and dose of radiation given to take pictures of teeth and broken bones is suitable to patients for X-rays, it would be determinantal to patients if gamma rays were used.

Posted: Feb-16-03 at 3:37 PM by Student 3

The radiation exposure rules can definitely be translated into toxicant exposure rules. For chemical exposure, time of radiation exposure is analogous to the amount of chemical one is being exposed to, since the more time one spends exposed to radiation, the more particles have the potential to hit the body. The dose is analogous to the toxicity of the chemical, the relative speed and amount of damage done at target sites, or possibly route of exposure, since that will control how much of the chemical will get inside the body. Shielding can be compared to either the effectiveness of detoxification processes in the body or the physical barriers preventing the toxicant reaching the target site.

Since toxins and toxicants are so different from each other in their mechanisms of toxicity, any of the three above variables may be the crucial step in preventing the most damage. For example, for highly toxic materials that do not easily detoxify in the body, 'shielding' takes on a much greater importance. For chemicals of relatively low toxicity, limiting the 'time' or concentration factor may prevent major damage.

Posted: Feb-16-03 at 5:02 PM by Student 6

Yes, these can be translated into consideration in toxicant exposure.

1.) Minimize time of exposure to the source-if you can reduce the amount of time you are exposed to a toxicant you can possibly avoid toxicant effects since your body may be able to eliminate the toxicant in small doses.
2.) Maximize distance from source, if you can avoid the toxicant than no toxic effect will be produced.
3.) Optimize the amount of shielding to minimize dose-using goggles, face masks, gloves, etc. can minimize the amount of toxicant received.

Yes there are differences among routes of exposure or the nature of the toxicant that need to be considered. In radiation there are basically just one way of exposure, via x-ray exposure through the skin. In toxicant exposure there are a number of different routes that need to be considered, i.e. Subcutaneous, intravenous, inhalation, etc. Toxicants can be a number of different forms also, i.e. solid, powder, liquid, gas.

Posted: Mar-11-02 at 12:07 PM by Student 10

The three cardinal rules for minimizing radiation exposure can be translated in toxicant exposure. Minimization of time for toxicant exposure reduces the time available to act on the toxicant's targets and primary molecules. The ability of the toxicant to bind and act on targets is minimized by minimizing time, as in radiation exposure as well. Maximizing distance of a toxicant relates to dose/potency/concentration. By maximizing distance from source of toxicant the dose/potency/concentration are minimized. Intensity of toxicant is inversely proportional to the distance squared. Toxicants can be minimized by optimizing shielding. This includes limiting the routes of exposure available to the toxicant. Vaccines may also act as an internal shield, which will optimize endogenous barriers or repair processes (Campbell, Lecture 3/7/02). There are also differences among route of exposure or nature of toxicant that need to be considered in minimizing toxicant exposure because limiting access to one route of exposure that is known to be a risk for toxicants may make another route of exposure more accessible to the toxicant after chronic response.

Posted: Mar-14-02 at 6:19 AM by Student 9

Analogs in chemical exposure:

1) (minimizing time) Decrease in time allotted for binding on targets, we had recently discussed the GI tract and how toxic effects occur only after they are absorbed, by limiting the time factor, one decreases the degree of absorption, thereby lessening the toxicity effect.

2) (maximizing distance) Since the intensity of the toxicant is inversely proportional to the distance squared. The increase in distance of the toxicant from its target cell will certainly have a compromised effect in terms of potency and concentration.

3) (optimizing shielding) This may be accomplished by limiting the amount/dose, limiting the exposure routes, dilution may be helpful in some cases and interaction with other chemicals may decrease the effect.

Posted: Mar-12-02 at 1:57 PM by Student 11

Yes, these rules can be translated into considerations in toxicant exposure. Minimizing time means lowering the time of toxicant to interact with its target molecule. Dose/Potency/ Concentration can be a analog of dose and limiting routes of exposures via vaccines; optimize endogenous barrier of repair processes can be analog of shielding for chemical exposure. Well just like with any other toxicant; you want to choose a route of exposure that will not take a long time to eliminate that toxicant. You also want to choose a toxicant that can be easily discarded by the body. Here, we are talking about radiation, therefore, we have to be little more careful than if we are talking about some kind of drug because radiation can cause mutations rapidly and therefore, can cause developmental defects. Also, depending on the nature of the toxicant, you want to choose the route of exposure that is limited and will easily eliminate the toxicant. For example, if the toxicant can be degraded in liver, you want to choose the route of exposure that is limited to liver only.

Notes: (This repeats an earlier description.)

The major project for this course is a project chosen by the student in collaboration with the instructor.  The project will be submitted as an HTML compatible document (as generated by a word processor) that explores a gap or questionable aspect of current toxicological practice or a topic that will not be covered fully within the context of the course, e.g., appropriateness for classifying toxicants as initiators, promotors, or progressors of carcinogenesis; current knowledge of the impact of phytoestrogens on apoptosis within the rat endometrium; the molecular relatedness of P450 enzymes involved in Type I metabolism across multiple species as ascertained using online molecular databases; or, comparisons of several related case studies of toxicant exposures.  This document can use figures, models, and tables as well as a narrative argument to make or illustrate points.  It should incorporate standard references for all texts and journal articles cited as well as complete URLs for Internet or Web resources.  No materials should be copied or reproduced without alteration in production of this document.  The penultimate version of the project should be submitted electronically one month prior to the end of the term.  This will be critiqued by the instructor and at least two student peers.  The final version of the project will be due at the time of the final and must be submitted electronically.  The documents may be added to the course Website at the end of the term including appropriate information on their authorship. If it deals with case studies, it should contain an evaluation and discussion of at least two related studies dealing with a substantive topic in mammalian toxicology.  Although there are no set limits on the size or extent of the document, it is probably realistic to approximate 10-20 total pages including 40 or so references and URLs.  The document should be hypertext linked so the studies or documents being addressed can be accessed. The project constitutes 40% of the grade for the course and should be approached accordingly. 

Possible Case Study Topics

1. DDT and persistent pesticide bans
2. Dredging of the upper Hudson for PCBs
3. Arsenic wells in northeast and eastern India and Bangladesh
4. Zero tolerance for Alar in apples
5. Labeling and restriction of milk from cows injected with or transgenic for bovine GH
6. Oral contraceptive contamination of municipal sewage effluents in Great Britain

These should take into account the following items:

A. Mammalian associations
B. How does toxicology play a role in these cases?
C. What are the assumptions, stated or unstated, that support the contrasting arguements in these debates?
D. Are risk assessments being used?  If so, are these realistic and well grounded?
E. What particular elements of toxicity testing and modeling are being employed? Give examples.

Additional project ideas, case study possibilities.  What about Hanford radiation ground water contamination case?  Suggestion was, in such situations, to contrast this with other similar risk assessment cases, e.g., storage site contaminations or studies on possible contamination in South Carolina or the new site in Nevada.  Alternatively, look at contaminations at Three Mile Island, PA versus Chernobyl, USSR perhaps focussing on only a single isotope like 131I.  The intent of contrasts for environmental cases is to see if the assessment strategies were similar and if the public and private responses to the cases were justified and/or adequate.  Other examples might be the groundwater contaminations in Woburn, MA, those on Cape Cod near Otis Airforce Base, those at Love Canal, NY, Bopal, India or those associated with the tanning industries or gasoline storage.  Sources to include in case studies would be books, journal articles, websites, newspaper articles, regulatory documents or laws, maps, and images resulting from the cases.

What about food safety history?  Upton Sinclair’s The Jungle which depicted conditions of workers in the Chicago meat packing industry in the early 1900's initiated a public response that resulted in Congressional action and the passage of the earliest pure food legislation.  This eventuated in the creation of the Food and Drug Administration and became the model for other regulatory agencies charged with limiting public risks.  Examination of recent cases or present day conditions in the poultry industry, meatpacking, and fish processing would seem very suitable as case studies as would an historical approach to development of any of several of the existing agencies charged with risk assessment, limitation, and reduction.  New opportunities to look at food safety might also involve the handling and management of transgenic organisms such as grains or animal products.  Do genetically modified foods/crops require FDA approval, for example?

Assignments Due:

Chosen Project Topics/Case Studies:

By March 9 each group should have settled on a topic and a set of no more than two related case studies.

Penultimate Project Submissions:

By mid-April a penultimate (next to final) version of the project should be posted to the course site for review by all course participants.

Final Project Submissions:

The final presentation should be posted to the Prometheus site in the appropriate Discussion area by the second week of May.