Low levels of rodent carcinogens of natural origin are ubiquitous in the environment. It is thus impossible to obtain conditions totally free of exposure to rodent carcinogens or to background radiation. Regulating trivial risks or exposure to substances erroneously inferred to cause cancer at low doses can harm health by diverting resources from programs that could be effective in protecting the health of the public. Epidemiological evidence in humans is sufficient to identify several broad categories of cancer causation for which the evidence is strong and plausible. Since many of these are avoidable, it is possible to reduce incidence rates of many types of cancer.
Tobacco is the most important global cause of cancer and is preventable.
Decreases in physical activity and increases in smoking, obesity, and recreational sun exposure have contributed importantly to increases in some cancers in the modern industrial world, whereas improvements in hygiene have reduced other cancers related to infection. Chronic infections contribute to about one-third of the world's cancer.