Best & Worst Counties for Air and Water Quality

How this is calculated

The same composite formula as the Air & Water Score by ZIP Code tool, run directly on every county that has both an EPA-monitored air record and at least one active community water system — 972 counties nationwide as of this build. Counties with only one half of the data (no monitor, or no active water system) are excluded from this particular ranking, but still get a single-axis score on the cleanest air and worst water state rankings.

air score    = clamp(100 - AQI90 / 2, 0, 100)
water score  = 100 - population-weighted violation penalty
composite    = 50% air + 50% water

If air and water scores differ by 30 points or more, the county is
labeled "Mixed" instead of averaged — the average would hide
whichever half is actually worse.

No adjustable weights panel on this page — with 972 counties to recompute, that needs a client-side fetch of the whole dataset just to move one slider. Use the ZIP score tool to change the air/water mix or the violation weights and see the effect on one place directly.

Sources

• EPA AirData — annual AQI by county (2025)
• EPA ECHO — SDWA (SDWIS) bulk data download (2026Q2)

Last reviewed: September 2026 · Data in use: AQI 2025, SDWIS 2026Q2 · Dataset built 2026-09-02