Why Two ZIP Codes Can Show the Exact Same Air & Water Score

Two ZIP codes fed into the compare tool sometimes come back with the identical air score, the identical water score, and the identical composite — not close, identical. That isn't a coincidence and it isn't a bug. It's a direct result of how both halves of this site's score are actually measured.

Both halves of the score are computed once per county, not once per ZIP code

The air subscore comes from EPA AirData's annual AQI file, which reports "one record per year per CBSA or county" (US EPA AirData — pre-generated data files). There is no per-ZIP air monitoring record anywhere in that file — every ZIP code that resolves to the same county reads the same county-year record, because that's the only record that exists.

The water subscore is built the same way. EPA's SDWA geographic-areas file tracks which "county that the facility serves" a water system belongs to, using a COUNTY_SERVED field and an AREA_TYPE_CODE of CN for county-level service areas (US EPA ECHO — SDWA data downloads). This site's own build pipeline resolves every ZIP code to one dominant county, then pools every active water system serving that county into a single score weighted by how many people each system serves. Two ZIP codes that resolve to the same county are drawing on the exact same pool of water systems, weighted the exact same way, so the water half ties too.

Put together: the Air & Water Score by ZIP Code tool computes both halves once per county and then looks the county up by ZIP — it doesn't compute a separate number for every ZIP inside that county. Feed the compare tool two ZIP codes that map to the same county, and it returns the same air score, the same water score, and the same composite for both, because underneath, it's running the same county lookup twice.

Why ZIP codes and counties don't line up in the first place

This only comes up because a ZIP code and a county aren't the same kind of thing. A ZIP code is a mail-delivery route, not a boundary drawn to match anything else. The Census Bureau's own ZIP Code Tabulation Areas (ZCTAs) — the generalized-area version of a ZIP code used for public data like this — are their own separate geography, built from Census block boundaries, and can even "form noncontiguous areas (e.g., islands or enclaves)" (US Census Bureau — ZIP Code Tabulation Areas) rather than nesting neatly inside a single county.

Because of that mismatch, most ZIP codes still fall entirely inside one county, but a real share don't. This site's own ZIP-to-county mapping, built from the Census 2020 ZCTA-to-county relationship file, found that 10,186 of the nation's 33,791 ZCTAs — about 30% — span more than one county. For those, the mapping picks whichever county holds the larger share of the ZIP's land area and names that county on the result page rather than hiding the choice. But that 30% is the exception. For the roughly 70% of ZIP codes sitting cleanly inside one county, any two of them will tie on this tool every time.

A tie here means the county matches, not that the places are the same

A tied score means one specific thing: the two ZIP codes you entered resolve to the same county, so the county-level air record and the pooled county-level water record are identical for both. It does not mean the two ZIP codes are interchangeable places to live. A county can be large, and the water score itself already has to average across every system inside it — precisely because a single county can contain more than one water utility with more than one compliance record. Two ZIP codes on opposite sides of the same county can sit on completely different water systems and still return a tied score here, because this tool resolves both to the county level and can't see finer than that.

If a tie surprises you, the fastest check is the county name printed under each result. Same county both times means the tie is mechanical and expected — it's telling you about the resolution of the underlying EPA data, not making a claim that the two ZIP codes are the same. It's a limit worth knowing before using a tie, or the lack of one, to make a moving decision.

What actually produces a real gap

A gap between two ZIP codes only shows up when they resolve to different counties, because that's the only case where the underlying EPA record actually changes. And even then, a small, single-digit gap between two similarly-monitored counties is closer to noise than signal — a slightly different mix of measured days or utilities can move a score a few points on its own. The question worth asking first when two ZIP codes compare differently, or tie, is almost always "are these the same county," not "how precise is this number."

Frequently asked questions

Why did comparing two ZIP codes return the exact same score twice?

Because both ZIP codes resolve to the same county, and both the air and water subscores are computed once per county rather than once per ZIP code — EPA's AirData file publishes one record per county per year, and its SDWA data identifies water-system service areas by county, not by ZIP.

Does a tied score mean the two ZIP codes are the same place to live?

No. It means the EPA county-level record this tool uses is identical for both ZIP codes, not that every address, street, or water utility inside that county is the same.

Why do some ZIP codes span more than one county?

Because a ZIP code is a postal delivery route, not a boundary drawn to match a county line. The Census Bureau's own ZCTAs, the closest public geography to a ZIP code, are built from Census block boundaries and don't have to nest inside one county — this site's own mapping found about 30% of ZCTAs actually do span more than one.

How does this site decide which county a ZIP code belongs to when it spans more than one?

It picks the county holding the largest share of the ZIP code's land area, and it names that chosen county on the result page rather than leaving the pick unstated.

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 primary sources cited in the body.