GuidesAirWaterScore
How to Read a National Best/Worst County Ranking Without Over-Trusting One Number
A single national list of best and worst counties compresses two different scores, two different scales of coverage gaps, and thousands of counties into one rank. What the rank can tell you, what it can't, and why a close rank is not a meaningful difference.
What the list actually is
The best & worst counties ranking is a national top and bottom 100, built from the same composite formula as the ZIP score tool, applied to every US county that has both an EPA air monitor and at least one active community water system on record. That “both” is doing real work — it’s not a ranking of all 3,100-plus US counties, and the counties that don’t appear on it aren’t being scored low. They’re simply not eligible, because one of the two data sources has nothing on file for them.
The eligibility filter is itself a filter
A county needs an EPA-monitored air record and at least one active water system on file to appear anywhere on this list — first, best, worst, or in between. Roughly half of US counties have no air monitor at all, so the eligible pool already skews toward more populated counties, where monitors tend to sit. Before treating a county’s absence from either end of the list as good news, check whether it’s in the eligible pool at all — a county can be entirely unmonitored and simply never show up.
No population floor — a real design choice, not an accident
Unlike some rankings that require a minimum population before a place qualifies, this one applies none. A county’s rank rests entirely on its own record, whether that record comes from one small water system or fifty large ones. That means a sparsely populated county can land at the very top or the very bottom of the national list on the strength of a single system’s compliance history or a single monitor’s readings — the same way a one-monitor state can top or bottom the state air ranking. That’s not a flaw the tool is hiding; it’s a direct consequence of ranking every eligible county on equal footing rather than only the big ones. Before reacting to an extreme rank, check how many people the underlying systems and monitor actually cover.
Why some counties are labeled “Mixed” instead of scored
When a county’s air and water scores diverge sharply — by 30 points or more — the tool refuses to average them into one number and labels the county “Mixed” instead. That keeps a strong score on one side from quietly covering for a weak score on the other, but it also means two counties near each other on the list can be there for very different reasons: one balanced across both halves, another divided sharply between them.
What a low rank doesn’t mean
A county near the bottom of this list does not necessarily have unsafe water or dangerous air right now. This is a compliance and monitoring record built from historical EPA data, not a live safety measurement or a health assessment. A county can rank low because a small system serving a fraction of its residents carries an unresolved violation, or because its one monitor recorded an unusually bad stretch of days — not because every resident is affected equally. For the fuller picture behind any one rank, look up the specific ZIP code or water utility involved rather than stopping at the county’s position on a 200-county national list.
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 the county ranking tool's own methodology and eligibility rules.