What Is an MCL? The Legal Limit for Drinking Water

The MCL is the number to compare your results with. If your utility’s water is at or under it for every contaminant, the utility is meeting the law. If it’s over, that’s a violation, and the utility has to tell you and fix it.

Is the MCL the same as a safe level?

Not exactly. The EPA sets two numbers for each regulated contaminant, as it explains in how it regulates drinking water:

• The health goal (MCLG) is the level below which there is no known or expected risk to health, with a margin of safety. It considers health only, and it isn’t enforceable.
• The legal limit (MCL) is set as close to that goal as feasible, taking into account the best available treatment and cost. It is enforceable.

Sometimes the two match: nitrate’s goal and limit are both 10 ppm. Sometimes the goal is zero and the limit isn’t: arsenic’s MCL is 10 ppb, per the EPA’s table of drinking water regulations. A result under the MCL is legal and meets the standard. When the goal is zero, it means the EPA found no level known to carry no risk, not that a trace under the limit will make you sick.

What’s the difference between an MCL and a secondary standard?

Secondary standards (sometimes written SMCLs) cover 15 contaminants that affect taste, odor, color or staining, such as iron at 0.3 mg/L or total dissolved solids at 500 mg/L. The EPA doesn’t enforce them and doesn’t consider those contaminants a health risk at those levels; they’re guidelines, though a state can make them binding. Going over one can make water look or taste bad without making it unsafe. If your water stains fixtures or tastes off, see our hard water guide.

How do I check my water against the MCL?

On city water, open your water quality report and compare each “highest level detected” with the MCL in the same row and unit. For some contaminants, compliance is judged on a running annual average, not a single sample, so one high result in the range isn’t necessarily a violation.

A few things work differently:

• Lead and copper have no MCL. They’re regulated by a treatment technique with an action level.
• PFAS: the 2024 rule set an MCL of 4.0 ppt each for PFOA and PFOS, with compliance due in 2029. In May 2026 the EPA proposed keeping those limits but letting systems ask for two more years. As of October 10, 2026, that’s a proposal, not final. Our PFAS guide has the full picture.
• Private wells aren’t covered by MCLs. Testing is up to you, as our well water testing guide explains.