The Safe Drinking Water Act requires EPA to identify pollutants in drinking water that may adversely affect human health. These are listed in the table above. Where feasible, the agency is required to specify a Maximum Contaminant Level (MCL) for each pollutant. These MCLs are referred to as the National Primary Drinking Water Standards (NPDWS). Some of the contaminants listed above do not have established MCLs because it is not possible to determine their levels in drinking water. For these substances, EPA requires the use of specific treatment techniques (depending on the contaminant), identified by the initials "TT" in the MCL column.

Public water systems are required to meet these standards.

The MCLs for the listed contaminants are set as close to the Maximum Contaminant Level Goal (MCLG) as cost-effective treatment technologies allow. The MCLGs are almost entirely a health-based determination (for example, the MCLG for substances suspected to cause cancer is zero).

Note: Individual states may enforce their own drinking water standards as long as they are no less stringent than the NPDWS regulations. Each state may regulate additional contaminants in drinking water or may establish more stringent MCLs for the contaminants listed in the table above. For a list of the drinking water contaminants regulated by your state, contact your state agency. EPA has a list on the
Safe Drinking Water Hotline at (800) 426-4791