Cancer scares created by environmental activist groups frequently focus on man-made chemicals. However, such chemicals play a very small role in human cancer rates and are far outweighed by the risks posed by natural chemicals, according to Misconceptions about the Causes of Cancer, a book published by the Fraser Institute and authored by leading cancer researchers Lois Swirsky Gold, Thomas H. Slone, Neela B. Manley, and Bruce N. Ames.
In the following excerpts from the book, the authors explain why animal cancer tests tend to overstate human cancer fears, especially with respect to man-made chemicals.
High Test Rates
“Approximately half of all chemicals that have been tested in standard animal cancer tests, whether natural or synthetic, are rodent carcinogens. Why do so many test positive? A reasonable explanation is that the design of these experiments produces effects that would not occur at lower doses. In standard cancer tests, rodents are given chronic, near-toxic doses. Evidence is accumulating that cell division caused by the high dose itself, rather than the chemical per se, is increasing the carcinogenic effects and, therefore, the positivity rate.” (page 31)
“At the low levels to which humans are usually exposed, such increased cell division does not occur. Therefore, the very low levels of chemicals to which humans are exposed through water pollution or synthetic pesticide residues may pose no, or only minimal, cancer risks because these effects do not occur at low doses.” (page 32)
“Historically, standard practice in regulatory risk assessment for chemicals that induce tumors in high-dose rodent bioassays has been to extrapolate risk to low dose in humans by multiplying rodent potency by human exposure, i.e. by assuming linearity in the dose response. However, the true human risk of cancer at low dose is highly uncertain and could be zero. Several mechanisms have now been identified that indicate that carcinogenic effects at the high doses of rodent tests would not be relevant to the low doses of most human exposures.” (page 34)
Rodents as Indicators
“Since the results of high-dose rodent tests are routinely used to identify a chemical as a possible cancer hazard to humans, it is important that we try to understand how representative the 50% positivity rate might be of all untested chemicals. If half of all chemicals (both natural and synthetic) to which humans are exposed would be positive if tested, then the utility of a rodent bioassay to identify a chemical as a ‘potential human carcinogen’ is questionable. To determine the true proportion of rodent carcinogens among chemicals would require a comparison of a random group of synthetic chemicals to a random group of natural chemicals. Such an analysis has not been done.” (page 36)
“The use of bioassay results in risk assessment requires a qualitative species extrapolation from rats or mice to humans. The accuracy of this extrapolation is generally unverifiable, since data on humans are limited. Ultimately one wants to know whether the large number (many hundreds) of chemicals that have been shown to be carcinogenic in experimental animals would also be carcinogenic in humans.” (page 38)
“Evidence about interspecies extrapolation can be obtained by investigating whether chemicals that are carcinogenic in rats are also carcinogenic in mice, and vise versa. If mice and rats are similar with respect to carcinogenesis, this provides some evidence in favor of interspecies extrapolations; conversely, if mice and rats are different, this casts doubt on the validity of extrapolations from mice to humans.” (page 39)
“One measure of interspecies agreement is concordance, the percentage of chemicals that are classified the same way as to carcinogenicity in mice and rats (i.e. results are concordant if a chemical is a carcinogen in either both species or neither, and results are discordant if a chemical is a carcinogen in one species but not in the other). Observed concordance in bioassays is about 75%, which may seem low since the experimental conditions are identical and the species are similar.” (page 39)
For more information …
The full text of Misconceptions About the Causes of Cancer is available in Adobe Acrobat’s PDF format on the Fraser Institute’s Web site at http://www.fraserinstitute.ca/shared/readmore.asp?sNav=pb&id=477. The book can also be ordered from the Fraser Institute for $19.95 (CAN) plus $10.00 shipping and handling. Call 604/688-0221 or email your order to [email protected].
