Rodents (mice and rats) are commonly used in studies of environmental causes of cancer. They have a relatively short lifespan (2 to 3 years), and their bodies' responses to known cancer causing chemicals are similar to a human response. Dietary studies in rodents are more difficult, however, due to differences in the digestive systems of rodents and humans.
In animal studies, the chemical exposures are usually at much higher levels than would be seen with human exposure. If an extremely high level of exposure does not lead to cancer, researchers reason that the chemical most likely does not cause cancer at lower levels either.