What "Cleanest Air by State" Actually Ranks (and the Gap Every Ranking Like This Has)

What the ranking measures

The state air ranking uses the same formula as the ZIP score tool’s air half — a 90th-percentile AQI, turned into a 0–100 score — applied per state instead of per county. But a state doesn’t have one air reading; it has however many counties EPA has a monitor in, and the state score is a population-weighted average across only those counties. That single fact is the one worth holding onto before trusting any rank on the page.

The coverage gap this ranking inherits

EPA’s air monitor network covers roughly half of US counties — about 982 of just over 3,100 — concentrated where people actually live. That means a state only appears on this ranking once it has at least one monitored county. A state whose population is spread across small, sparsely monitored, or entirely unmonitored counties either won’t appear at all, or will appear ranked from a much thinner slice of evidence than a state with dozens of monitors spread across its major metros. The ranking never fills that gap with a guess — an unmonitored county is simply absent from the average, not scored as though it matched its neighbors.

Why population weighting, and what it doesn’t fix

Each monitored county’s 90th-percentile AQI is averaged into the state figure weighted by that county’s own population — so a lightly populated rural county with one monitor can’t single-handedly drag down (or flatter) the score for a state where most monitored residents live somewhere else entirely. That’s a real improvement over a plain unweighted average of counties. But it’s still not the same claim as “every resident of this state breathes this air” — it’s a weighted average across the counties that happen to have a monitor, which may or may not resemble the state as a whole.

How to sanity-check a state’s rank before you trust it

Every row on the ranking expands to show exactly which counties fed into it and how many days each one measured. Before citing a state’s position:

• Check the monitored-county count. A state built from a dozen monitored counties is a sturdier estimate than one built from a single small county standing in for the whole state.
• Check whether your own county is even in the average. If it’s not, the state’s rank tells you nothing directly about where you live — look up your specific ZIP code instead.
• Treat a close rank as a tie. Two states a point or two apart on a population-weighted average of measured days are not meaningfully different from each other.

What a state rank can’t tell you

A state can rank well overall while a specific monitored county inside it — or an unmonitored county nearby that never entered the average at all — reads worse than the state figure suggests. Air quality is genuinely local: a county next to heavy industry or a major highway corridor can differ sharply from the rest of its state. If you’re making a decision about a specific place rather than comparing states in the abstract, the ZIP score tool is the right level of detail — it resolves to the actual county behind your ZIP code rather than a statewide blend.

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 monitor network coverage and the state ranking tool's own methodology.