This fact sheet provides a brief background on the United States Environmental Protection Agency's drinking water standards, sources of contaminants, and cancer risks associated with exposure to certain drinking water contaminants. Contaminants discussed include pesticides, volatile organic compounds (solvents), chlorination disinfection by-products, heavy metals and radioactive chemicals (radon and radium). Precautions are given to help reduce water contamination, and recommendations are given on when private wells should be tested for contaminants.