Background
Many beneficial natural and man-made substances can damage our bodies if the exposure is great enough. For the most part, we know what those substances are, and we avoid them. But can little doses of something gradually cause changes that after twenty or thirty years cause damage such as cancer? If direct evidence is lacking, one way science tries to find out is to test substances on laboratory animals, usually rodents. A typical method is to give rodents very large doses of a substance over various periods of time. It is far from a perfect answer to the question. Rodents and people are different in many important ways, and giving them large doses over various periods of time is not the ideal way to test for effects on humans of small doses over long periods of time. But it does offer clues, and can point out areas for further research.
In 1982, results of a rodent study conducted by the National Toxicology Program (NTP), a research program that combines the scientific resources of a number of government agencies, indicated that rats and mice fed very high doses of a phthalate called DEHP for their entire lifetimes - two years - eventually developed tumors. This led the NTP, which classifies substances according to their tendency to be cancer causing in animals, to place DEHP in its second category, as a compound "reasonably anticipated" to be carcinogenic to humans. (The top category is "known carcinogen," which is applied to compounds that have been proven to cause cancer in humans.) An arm of the World Health Organization called the International Agency for Research on Cancer (IARC), which is the international authority on cancer, also labeled DEHP as a "possible carcinogen," based on the animal evidence. It noted there were no data showing any carcinogenicity in humans.
When a substance shows a tendency to cause tumors in animals, a critical question to ask is: are the results relevant to humans? This often leads to more specialized studies to determine the mode of action. Understanding the mode of action is critically important to projecting the substance's possible effect on other animals, including humans. While DEHP does indeed cause tumors in rodents, the evidence is not relevant to humans because the mode of action of DEHP in rodents is very different. The story is the same for another plasticizer used in vinyl called DINP.
In 2000, based on its judgment that the rodent results were not relevant to humans, IARC changed its ranking of DEHP to "not classifiable" as a human carcinogen. Regulatory agencies in Europe and Canada have reached the same conclusion. Similarly, a panel of experts convened by the U.S. Consumer Product Safety Commission examined the scientific evidence for DINP, and concluded that human exposures to DINP are not plausibly associated with a significant increase in cancer risk.
Last Updated: August 22, 2006
