How Does EPA Set Drinking Water Standards?
EPA has issued drinking water standards, or Maximum Contaminant Levels (MCLs) for more than 80 contaminants. (See Appendix A.) The standards limit the amount of each substance allowed to be present in drinking water. A process called risk assessment is used to set drinking water quality standards. When assessing the cancer and non-cancer risks from exposure to a chemical in drinking water, the first step is to measure how much of the chemical could be in the water. Next, scientists estimate how much of the chemical the average person is likely to drink. This amount is called the exposure. In developing drinking water standards, EPA assumes that the average adult drinks 2 liters of water each day throughout a 70-year life span. Risks are estimated separately for cancer and non-cancer effects. For cancer effects, a risk assessment estimates a measure of the chances that someone may get cancer because they have been exposed to a drinking water contaminant. EPA generally sets MCLs at level
