How common are substances that cause cancer in the environment?

Very Common: Cancer-causing substances include uv radiation from the sun, radon, chemicals from agricultural that get into the water supply, certain types of plastics like bpa, and many others. A report from the president's cancer panel gives more details. Go to: http://deainfo.Nci.Nih.Gov/advisory/pcp/annualreports/pcp08-09rpt/pcp_report_08-09_508.Pdf.

They're everywhere: Your own body contains potassium, which has a low-level radioactivity that contributes to the mutations of your genes, the common denominator for everything that really causes cancer. Cosmic rays, ultraviolet light, several of the products of the bacteria in our guts, chemicals in lemons, pepper, orange juice -- you can't avoid them, so stop troubling yourself and keep up surveillance.