The EPA sets two numbers for most drinking water contaminants. Most people only see one of them.

The MCL is the legal limit. The MCLG is the health goal. They sound like the same thing. For some contaminants, they’re close. For many of the most dangerous ones, they’re miles apart, and that gap is intentional.

What the MCL Actually Is

MCL stands for Maximum Contaminant Level. It’s the highest concentration of a contaminant that’s legally allowed in drinking water at the point of delivery to customers.

Water utilities must meet the MCL. If they don’t, they’re in violation of the Safe Drinking Water Act and must notify customers and take corrective action. The MCL is what shows up in your Consumer Confidence Report as the legal standard your utility is measured against.

The key phrase in how the EPA defines it: the MCL is set “as close to the MCLG as is feasible.” Feasible means technically achievable using the best available treatment technology at a cost that utilities can reasonably meet. That phrase does a lot of work.

What the MCLG Actually Is

MCLG stands for Maximum Contaminant Level Goal. It’s the concentration at which no known or anticipated adverse health effects occur, with an adequate margin of safety factored in.

The MCLG is set purely on health science. No cost considerations. No treatment feasibility. Just: at what level does this contaminant stop being a health problem?

For non-carcinogens, the MCLG is usually set above zero, there’s some level below which the risk is considered negligible. Nitrate has an MCLG of 10 ppm, for example, which matches the MCL.

For carcinogens, the answer is almost always zero. The EPA’s position for known or probable human carcinogens is that there’s no concentration at which exposure carries zero risk. So the MCLG goes to zero.

The Gap, and What It Means

Here’s where it gets concrete. A few examples:

Arsenic has an MCLG of 0 and an MCL of 10 ppb. The EPA classifies arsenic as a known human carcinogen based on bladder and lung cancer data. Getting drinking water arsenic to zero isn’t achievable with current treatment at reasonable cost. So the MCL was set at 10 ppb, a number the EPA estimated would reduce cancer risk substantially while remaining technically achievable.

PFOA and PFOS have MCLGs of 0 under the EPA’s April 2024 final rule. The MCLs are set at 4 parts per trillion, the lowest level that can be reliably measured and confirmed with current laboratory methods. The MCL literally couldn’t go lower given today’s detection technology.

Benzene has an MCLG of 0 and an MCL of 5 ppb. Same pattern: known carcinogen, no safe level established, MCL set at the feasibility floor.

Lead is different, and worth its own explanation.

Lead Has No MCL

Lead doesn’t follow the MCL/MCLG pattern exactly. The MCLG for lead is 0, the EPA recognizes no safe level of lead exposure, particularly for children. But instead of an MCL, lead is regulated through an “action level.”

The action level is 15 ppb. If more than 10% of tap samples across a utility exceed 15 ppb, the utility must take corrective action, corrosion control treatment, public notification, pipe replacement programs.

What this doesn’t mean: water at 14 ppb is “safe.” The action level is a regulatory trigger, not a safety threshold. The MCLG is still zero. Lead at 14 ppb is below the action level, but it isn’t below the health goal, because the health goal is zero.

This distinction matters especially in older housing stock. The CDC says no safe blood lead level in children has been identified. The regulatory structure for lead in water reflects treatment-side limitations, not a scientific finding that 14 ppb is harmless.

Why Congress Designed It This Way

The Safe Drinking Water Act, passed in 1974 and amended several times since, explicitly requires the EPA to balance health goals against feasibility. The MCLG is the health ideal. The MCL is the regulated reality.

This wasn’t an oversight. Congress recognized that requiring perfect purity in drinking water, zero detectable carcinogens, would be technically impossible with any known treatment technology. Setting MCLs at zero would have made compliance impossible and potentially shut down water systems.

The result is a two-number system. One number tells you what’s ideal. The other tells you what’s legally required. Both are worth knowing.

Why EWG and EPA Numbers Differ

If you’ve used EWG’s Tap Water Database, you’ve probably seen the phrase “above EWG health guideline” applied to contaminants that your utility reported below the MCL. This confuses a lot of people who think EWG is being alarmist.

EWG’s health guidelines are based on risk assessments that target a specific cancer risk level, typically one additional cancer case per million people exposed over a lifetime. That’s a stricter standard than the EPA’s MCL because it doesn’t factor in treatment cost or feasibility. It’s closer to the MCLG philosophy than the MCL approach.

Both are legitimate ways to look at the data. The EPA MCL answers: is this utility meeting its legal obligation? EWG’s guideline answers: at what level would this contaminant pose a practically negligible cancer risk? Understanding the difference lets you use both correctly instead of dismissing one as too strict or too lenient.

What This Means When You Read Your Water Report

When you see a contaminant listed in your CCR at a level below the MCL, your utility is meeting its legal requirement. That’s meaningful. It means the water has been treated and meets federal standards.

It doesn’t mean the contaminant poses zero risk. For carcinogens with an MCLG of zero, arsenic, PFAS compounds, benzene, any exposure carries some theoretical risk. The question is whether the level is practically significant given your overall exposure and health context. That’s a conversation worth having with accurate information, not one that gets answered by the MCL alone.

The direct takeaway: know the MCLG for any contaminant that appears in your water report. If it’s zero, your utility’s measured level is below the legal limit but above the health goal. For most people at typical utility concentrations, the practical risk is low. But it’s worth knowing what the numbers actually represent before you decide whether to take additional steps like point-of-use filtration.

For contaminants that matter most, read the dedicated pages: PFAS, lead, and arsenic all have detailed breakdowns of what the research says at real-world exposure levels.

Sources: EPA: How EPA Regulates Drinking Water Contaminants | EPA National Primary Drinking Water Regulations | EWG Health Standards