A bit of water jargon that you may come across from time to time is the term “MCL,” which stands for “maximum contaminant level.” This abbreviation may appear in your water provider’s water quality report or Consumer Confidence Report.

The U.S. Environmental Protection Agency (EPA) sets the standards for maximum levels of contaminants that it considers unsafe after a series of steps (explained below). Any contaminant’s MCL is the “highest quantity of the contaminant that should be in drinking water.” It is usually communicated as a concentration of micrograms or milligrams per liter of water. Most MCLs are set much lower than the amount that could affect human health.

EPA arrives at any given MCL after it goes through a series of steps, including:

• “Before enforcing MCLs, EPA carries out thorough research into a contaminant of concern, examining data to determine the potential health risks and dangers of the contaminant. 
• “Next, the EPA creates a maximum contaminant level goal (MCLG) for a contaminant. This non-enforceable public health goal describes the measurement of a contaminant that could be present without causing adverse health effects. 
• “The EPA then enforces a maximum contaminant level for the same contaminant. The MCL is very similar, if not higher than, the MCLG. There are several factors affecting the EPA’s decision on an MCL, including how easy the contaminant is to measure in small quantities, the potential for the contaminant to occur in public water systems, and whether the public health benefits of a lower MCL are outweighed by the costs of treatment. 
• “Some contaminants have a treatment technique (TT) rather than an MCL. The EPA enforces TTs to set out the procedure that public drinking water suppliers must follow to reduce or remove a specific contaminant.”

It is important to note that EPA does not regulate all contaminants since some of them are not harmful in drinking water in trace amounts and others are used to improve water’s taste, appearance or color.

To learn more about how water utilities treat and mitigate other common water contaminants, check out DrinkTaps’ “What’s in My Water Page.”