Uncertainty in the science, continued.

The next "extrapolation" we make is from effects observed from animal testing to expected human effects. This is not an "extrapolation" in the scientific or mathematical sense of that word. It is an assumption based on the fact that many biological systems are the same among different species, and that the closer (philogenetically) the species are, the more biological systems are the same, and thus the effects observed would also be the same. While this is logical and often true, it is not a scientific law or principle. For example, of chemicals tested for cancer in both rats and mice, only about half were carcinogenic in both species. On the other hand, of the chemicals that are known to cause cancer in humans, almost all of them cause cancer in laboratory animals. While there is little scientific certainty in the "animal to human extrapolation," it is the best we can do.

A larger problem lurks in the nature of the animal testing. In the Course Documents folder are two articles by Ames and Gold, two Berkeley scientists who are pioneers in molecular biology and cancer research and who are part of a research center funded by the National Institute of Health, NIEHS Center for Environmental Health Sciences. Read the article, Paracelsus to parascience the environmental cancer distraction, carefully save it for homework. It is a very readable article. The second article is more complex and is not required reading, but goes into some important details that the first article does not.

This may be a good time to present the EPA's old cancer ranking scheme:

Group A: Human carcinogen: Compounds for which there is sufficient evidence in epidemiological studies to support causal association between exposure and cancer.

Group B: Probable human carcinogen: Compounds for which there is:

Group B1, limited evidence in epidemiological studies or

Group B2, sufficient evidence from animal studies to support causal association between exposure and cancer.

Group C: Possible human carcinogen: Compounds for which there is limited or equivocal evidence from animal studies and inadequate or no data in humans to support causal association between exposure and cancer.

Group D: Not classified: Compounds for which there is inadequate or no human or animal evidence of carcinogenicity.

Group E: No evidence of carcinogenicity for humans: Compounds for which there is no evidence of carcinogenicity in at least two adequate animal tests in different species or in adequate epidemiological and animal studies.

Today, under the EPA’s 2005 guidelines for carcinogen risk assessment, a narrative approach, rather than categories, is used to characterize carcinogenicity. Five standard weight-of-evidence descriptors (Carcinogenic to Humans, Likely to Be Carcinogenic to Humans, Suggestive Evidence of Carcinogenic Potential, Inadequate Information to Assess Carcinogenic Potential, and Not Likely to Be Carcinogenic to Humans) are used as part of the narrative.

This is a "weight-of-evidence" ranking. It says nothing about the doses involved, just that there was sufficient scientific testing done and the effects were consistent enough such that the chemical could be described by one of those groups.

Here is the IARC classification scheme (scroll down and click on the group you are interested in.) and lists of chemicals. International Agency for Research on Cancer (IARC, "eye ark") is the gold standard in cancer pronouncements. Note their classification scheme and how many chemicals are listed as "Group 1" which is analogous to EPA's Group A. How many are "mixtures" and how many are "exposure circumstances." Bookmark for homework.

End of Sub-module 10B

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