Environmental factors may play a role in more than half of all cancers, with smoking leading the way as the most important contributor to cancer death.

Environmental factors can cause cancer in the same ways as other factors. The carcinogen enters the body, mainly through breathing but also by being ingested (for example, eating contaminated food) or absorbed through the skin. Having entered the body, the substance either can stay in the lungs, as asbestos does, or become absorbed into the body. Once absorbed, the carcinogen travels through the body in the blood, and can undergo chemical changes that make it more or less toxic. Eventually, the carcinogen or an activated form finds its way into individual cells and can cause mutations that lead to cancer.

The cancer risk posed by most environmental factors becomes greater with increased exposure, either in one large toxic dose or in small-dose exposures over a long period of time. Since it is not possible or ethical to willingly expose people to environmental carcinogens, information about their effects is gathered in four ways:

• Epidemiological studies. Cancer incidence within large population groups is measured by comparing subgroups with different exposure levels to the environmental carcinogen. For example, in any population group, when people who smoke are compared to people who don't smoke, the smokers have a higher rate of lung cancer.
• Natural experiments. If people are exposed accidentally to harmful levels of an environmental factor, they later can be compared to the general population to see if the exposure caused an increased cancer risk. For example, scientists have determined that children who lived near the site of the Chernobyl nuclear accident have an increased risk of thyroid cancer due to exposure to iodine radioactive fallout.
• Animal studies. By exposing animals (usually rats or mice) to dosages of a suspected carcinogen over time and measuring the effects, scientists make assumptions about what effect that carcinogen might have on humans. Although extrapolations based on animal data are subject to uncertainty, this method is very helpful in assessing human risk for factors that cannot be tested in any other way.
• Laboratory tests. A number of tests involving bacteria or cells are used to test if a suspected carcinogen can change DNA.

Considering these methods of measuring risk and the amount of potential carcinogen present in a situation, scientists can estimate the risk of cancer from that substance. For example, animal studies have shown that certain pesticides may promote tumor growth in the mammary tissue of rodents. But the amounts of those pesticides in foods are so small that they are believed to contribute negligibly, if at all, to human breast cancer. This assumption has been supported by studies that compared the amount of pesticide residue in a woman's blood to her risk of developing breast cancer. Breast cancer was found to be unrelated to the level of pesticide exposure.