HGL vs MCLG vs MCL: What Drinking Water Standards Mean

If you’ve looked at the results of your Tap Score water test, you may have seen terms like HGL, MCL, and MCLG.

All three are benchmarks used to evaluate drinking water quality and health risk, but they serve different purposes. The following short guide will help you understand how they relate, and, in turn, help you better interpret your results to make informed decisions about your water.

Table of Contents:

• Tap Score HGL
• EPA MCLG
• EPA MCL
• How They Compare
• Common Questions

Tap Score’s Health Guidance Level (HGL)

Developed by the science team at SimpleLab and used by Tap Score as a composite health benchmark, HGL is not a federal regulation. Instead, it is:

• Based on the most health-protective values from trusted public health agencies (e.g., EPA MCLGs, California Public Health Goals, WHO guidelines)
• Indicates the concentration above which long-term exposure may pose health risks

Levels at or below the HGL are considered protective over a lifetime of daily consumption.

Why HGL matters:

HGLs are designed specifically for interpreting your personal water test results, not regulating utilities. That means they often reflect more precautionary, health-based thresholds.

EPA’s Maximum Contaminant Level Goal (MCLG)

The Maximum Contaminant Level Goal is a federal non-enforceable public health goal set by the EPA. MCLGs:

• Represent a level at which no known or expected health risk occurs, including a margin of safety
• Are often set at zero for carcinogens (like benzene)
• Are based on a lifetime “no-effect” dose for non-cancer effects
• Are the basis for many of Tap Score’s HGLs

Bottom line:

MCLGs are purely science- and health-based targets, without considering cost or feasibility.

EPA’s Maximum Contaminant Level (MCL)

The Maximum Contaminant Level is a federal, legally enforceable standard under the Safe Drinking Water Act. The MCL:

• Defines the maximum allowed concentration of a contaminant in public drinking water systems
• Is set as close as possible to the MCLG, but may be higher due to:
• Treatment technology limits
• Detection capabilities
• Economic and operational feasibility

Why this matters:

MCLs reflect what utilities can realistically achieve, not necessarily the ideal health target.

How HGL, MCL, and MCLG Compare

• MCLG: Non-enforceable health goal that targets a level that is risk-free
• MCL: Legal regulatory limit for public water systems, typically higher than the MCLG due to practical constraints
• HGL: Consumer-focused interpretive benchmark, often as strict or stricter than MCLGs

What this means in practice: You might see a contaminant below the MCL (regulatory limit) but above the HGL (health benchmark). That does not mean your water is “non-compliant,” but it may still merit attention to protect long-term health.

What This Means for Your Water

Most water quality messaging focuses on whether water is “in compliance” with regulatory standards. But if you’re testing your own water — especially from a private well or older plumbing — the better question is:

“Is my water optimized for long-term health?”

That’s exactly what Tap Score’s HGL helps answer.

Common Questions

Why are MCLs sometimes higher than MCLGs?

MCLs can be higher than MCLGs because regulators must balance health protection with feasibility. If removing a contaminant to the MCLG level isn’t currently practical or affordable at scale, the MCL will be set higher.

Is water above the HGL unsafe?

Not necessarily. HGLs are based on lifetime exposure, so exceeding them suggests potential long-term risk, not immediate danger.

Why does Tap Score use HGL instead of just MCL?

Because MCLs are designed for regulation, not personalized health interpretation. HGLs help individuals understand what their results mean for their own health, not just compliance.

Do private wells follow MCL standards?

No — MCLs apply only to public water systems. If you use a private well, you are responsible for monitoring and managing water quality, which makes health-based benchmarks like HGL especially useful.

I haven’t tested my water—should I?

If you don’t know what’s in your water, you’re relying entirely on system-level averages (or nothing at all).

Testing is the only way to:

• Identify your specific contaminants
• Compare them to health-based benchmarks
• Take targeted action (if needed) 

Ready to Understand Your Water?

If you already have test results, you’ll notice that Tap Score translates them into clear, health-based insights using HGLs. If you haven’t tested your water yet, browse our collection of award-winning water test kits.