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Maximum Contaminant Levels are standards that are set by the United States Environmental Protection Agency (EPA) for drinking water quality. A Maximum Contaminant Level (MCL) is the legal threshold limit on the amount of a hazardous substance that is allowed in drinking water under the Safe Drinking Water Act. The limit is usually expressed as a concentration in milligrams or micrograms per liter of water.

To set a Maximum Contaminant Level for a contaminant, EPA first determines how much of the contaminant may be present with no adverse health effects. This level is called the Maximum Contaminant Level Goal (MCLG). MCLGs are non-enforceable public health goals. The legally enforced MCL is then set as close as possible to the MCLG. The MCL for a contaminant may be higher than the MCLG because of difficulties in measuring small quantities of a contaminant, a lack of available treatment technologies, or if EPA determines that the costs of treatment would outweigh the public health benefits of a lower MCL. In the last case, EPA will set the MCL to balance the cost of treatment with the public health benefits.

For some contaminants, EPA establishes a Treatment Technique (TT) instead of an MCL. TTs are enforceable procedures that drinking water systems must follow in treating their water for a contaminant.

MCLs and TTs are known jointly as National Primary Drinking Water Regulations (NPDWRs), or primary standards.

Some contaminants may cause aesthetic problems with drinking water, such as the presence of unpleasant tastes or odors, or cosmetic problems, such as tooth discoloration. Since these contaminants do not cause health problems, there are no legally enforceable limits on their presence in drinking water. However, EPA recommends maximum levels of these contaminants in drinking water. These recommendations are called National Secondary Drinking Water Regulations (NSDWRs), or secondary standards.

• Safe Drinking Water Act
• Drinking water

• EPA's list of drinking water contaminants and standards
• EPA's standard setting process