Diet associated cancers include colon, rectum, breast and stomach cancer. For example, high fat consumption is considered a risk factor for colon and possibly breast cancer. By comparison, the role of chemical pollutants is unclear. Scientists have estimated that chemicals in the environment account for about two percent of all cancer deaths. To determine the potential of chemical pollutants to cause cancer, scientists rely primarily on laboratory studies in which animals are exposed to chemicals, and when available, population studies involving human exposure to chemicals.
Scientists assess carcinogenicity by weighing the available data from both animal and human studies.
So far, out of thousands of chemicals tested, scientists have identified only a few hundred that cause cancer in rodents and around 50 that definitely cause cancer in humans. The number of confirmed human carcinogens is lower because, for the majority of chemicals, there is often little or no information available on cancer incidence rates in humans. Moreover, effects that occur in laboratory animals will not necessarily occur in humans. It is generally assumed, but has never been proven, that there is some chance of developing cancer at any level of exposure. In other words, contact between a single human cell and a carcinogen molecule may be enough to cause cancer.