What Your Air & Water Score Actually Means

Two EPA records, folded into one number

The score you get for a ZIP code is not a single measurement. It’s two independent EPA records — one on outdoor air quality, one on drinking water compliance — each converted to a 0–100 scale and then blended. Nothing about the number is modelled or estimated: every input traces to a specific EPA record for the county your ZIP sits in, and the ZIP score tool shows the full breakdown under every result. This guide is the plain-language version of that breakdown.

The air half: what a 90th-percentile AQI means for you

The Air Quality Index (AQI) is EPA’s standard 0–500 scale for how polluted the air was on a given day, built from measured ozone and particulate matter. Rather than scoring a county on its single worst day or a plain average, the air score is anchored on the 90th-percentile AQI — the level the county stayed at or below on 90% of measured days. That resists the one smoke-filled day or freak ozone spike that a maximum would be hostage to, while still reflecting a genuinely bad stretch, not just an average that a handful of clean days could quietly rescue.

Here’s what the underlying AQI bands actually mean day to day:

• 0–50 (Good): Air quality is satisfactory for essentially everyone.
• 51–100 (Moderate): Acceptable for most people. A small number who are unusually sensitive to air pollution may notice mild effects.
• 101–150 (Unhealthy for sensitive groups): The general public is unlikely to be affected, but people with asthma, other lung disease, heart disease, older adults, and children are more likely to have symptoms on these days.
• 151–200 (Unhealthy): Everyone may begin to notice health effects; sensitive groups may see more serious ones.
• 201–300 (Very unhealthy) and 301+ (Hazardous): Health alert conditions — the entire population is more likely to be affected, with serious risk for sensitive groups.

If you or someone in your household has asthma, COPD, or another respiratory condition, a county that spends much of the year in the “Unhealthy for sensitive groups” band or worse is worth reading as a practical planning signal, not an abstract number — that’s roughly the point on the scale where symptom flare-ups become more likely on an ordinary day, not just a bad one.

The air formula turns that 90th-percentile figure directly into a 0–100 score: an AQI of 50 (top of Good) scores 75, an AQI of 100 (top of Moderate) scores 50, an AQI of 150 scores 25, and 200 or worse scores 0. A ZIP with no air score at all means exactly that — its county has no EPA air monitor, which is true for roughly half of US counties. See what a state air ranking actually ranks for more on that coverage gap.

The water half: not every violation is the same violation

The water score starts at 100 and subtracts a penalty built from the drinking-water violations on record for the community water system(s) serving the county, pulled from EPA’s SDWIS/ECHO system. The penalty deliberately does not treat every violation the same way — a violation still open counts far more than one the utility already fixed, and a health-based violation (a limit tied directly to a contaminant that can harm health) counts far more than a monitoring or reporting violation (the utility missed a required test or a paperwork deadline, with no measured contamination implied). The guide on health-based vs. monitoring violations goes into that distinction in full.

Where a county is served by more than one utility, the penalty is averaged weighted by how many people each system serves — so a small rural system with one violation doesn’t swing the score as hard as a city utility would, and a single bad system is capped so it can’t single-handedly zero out an otherwise sound county.

Why the composite sometimes refuses to be one number

Blending two scores into one is only useful when the two halves roughly agree. Some places don’t: a county can score in the twenties for air and the high nineties for water, and averaging those into a single “Good” would hide the half a reader actually needs to know about. When the air and water scores diverge by 30 points or more, the tool stops blending and tells you plainly to read the two scores separately instead of trusting the composite. That refusal is itself information — it means the number one score away isn’t telling the whole story.

Reading the verdict bands

The composite (or the single available half, where only one exists) maps to a plain label: Strong (80+), Good (60–79), Mixed (40–59), Weak (20–39), and Poor(under 20). Treat the label the way you’d treat a credit score band, not a pass/fail grade — it’s a summary of where a place sits on a continuous scale, not a certification that a “Good” ZIP is problem-free or that a “Mixed” one is dangerous. Read the breakdown underneath the label before drawing a conclusion, especially near a band boundary.

What the score can’t tell you

• It’s county-level, not address-level. Air quality varies within a county — a block next to a highway is not the same as a block five miles away — and the water score describes the utility serving your area, not a test of the water actually coming out of your tap.
• A clean water record is a compliance record, not a water test. It means no violations were reported under the Safe Drinking Water Act in the last five years. It says nothing about lead service lines on the customer’s side of the meter or a private well.
• Missing data is shown as missing, never guessed. Where a county has no air monitor or no water system on record, that half of the score is left blank rather than filled in from a neighbouring county.

With that in mind, the score is exactly what it claims to be: a fast, sourced read on two EPA records for wherever you’re looking, with every assumption behind it printed on the page and adjustable if you disagree with our weighting.

This guide is for informational purposes only. It explains how to read public EPA records and what this site's tools compute from them — it is not a health assessment, a water test, or a substitute for medical or professional advice about a specific address or a specific person's health.

Last reviewed: September 2026 · Against EPA AirData's AQI category definitions and the ZIP score tool's own formulas (lib/zip-score.js).