Water quality standard set by the Environmental Protection Agency (EPA) that determines the maximum concentration of a hazardous substance allowed in public drinking water systems. Contaminant levels usually are measured where water enters a public water system user's outlet and where water first enters a distribution system when turbidity is a problem.

When creating MCLs, the EPA first finds the maximum contaminant level that has no adverse health effect. That level becomes a non-binding guidance figure called a maximum contaminant level goal (MCLG). After considering treatment technology and measurement capabilities, the EPA then sets an MCL as close as practicable to the MCLG. Contaminants that cause bad water odor or flavor, or cosmetic problems like tooth discoloration are not considered health problems, and therefore the EPA does not create MCL levels for them.