Archived release. This page is frozen at the data release below and will not change. Current data is at the latest version of this list.
Insights › Combined exposures
Explainer · How combined exposures work
Some ZIP codes carry two separate geologic exposures at once — modeled indoor radon and modeled groundwater arsenic. This page explains what each measurement is and what federal reference each is checked against, then lists the ZIP codes where both run high. Pick a state below to see its own list.
National explainerData release 2026National ranking · state pages linked below
484
ZIP codes listed
1.5% of the 32,940 US ZIP codes
14
States with any
of 50 states + DC
87%
In the top 3 states
Iowa, Minnesota, Ohio
100
Cross both federal lines
modeled radon above 4 pCi/L and modeled arsenic above 10 µg/L
Where they are
Three states hold the large majority of this list. That concentration is the finding, not a caveat: radon and arsenic both come from the rock a community sits on, so where both run high together follows specific bedrock and does not exist in most of the country. It is also the honest reading of our coverage — groundwater arsenic can only be modeled for about three quarters of US ZIP codes, so a national list of “the worst” would quietly claim we had checked places we had not.
Ordered by whichever of the two modeled measurements ranks lower nationally, so the top of this list is high on both rather than carried by one. Select any row to open that ZIP code’s own health page.
Royal, IA · 51357
Ranked on its weaker measurement: 99.4th percentile nationally
Radon
99.4th
Arsenic
99.8th
George, IA · 51237
Ranked on its weaker measurement: 99.0th percentile nationally
Radon
99.9th
Arsenic
99.0th
Greenville, IA · 51343
Ranked on its weaker measurement: 98.9th percentile nationally
Radon
98.9th
Arsenic
99.7th
Sioux Rapids, IA · 50585
Ranked on its weaker measurement: 98.9th percentile nationally
Radon
99.9th
Arsenic
98.9th
Rembrandt, IA · 50576
Ranked on its weaker measurement: 98.9th percentile nationally
Radon
99.1th
Arsenic
98.9th
Lakefield, MN · 56150
Ranked on its weaker measurement: 98.7th percentile nationally
Radon
98.7th
Arsenic
98.8th
Sherburn, MN · 56171
Ranked on its weaker measurement: 98.6th percentile nationally
Radon
99.3th
Arsenic
98.6th
Rock Rapids, IA · 51246
Ranked on its weaker measurement: 98.5th percentile nationally
Radon
99.8th
Arsenic
98.5th
Dickens, IA · 51333
Ranked on its weaker measurement: 98.5th percentile nationally
Radon
98.5th
Arsenic
99.9th
Linn Grove, IA · 51033
Ranked on its weaker measurement: 98.4th percentile nationally
Radon
99.4th
Arsenic
98.4th
Ruthven, IA · 51358
Ranked on its weaker measurement: 98.4th percentile nationally
Radon
98.4th
Arsenic
99.6th
Ayrshire, IA · 50515
Ranked on its weaker measurement: 98.4th percentile nationally
Radon
98.4th
Arsenic
98.5th
Alvord, IA · 51230
Ranked on its weaker measurement: 98.2th percentile nationally
Radon
98.5th
Arsenic
98.2th
Peterson, IA · 51047
Ranked on its weaker measurement: 98.2th percentile nationally
Radon
99.3th
Arsenic
98.2th
Newell, IA · 50568
Ranked on its weaker measurement: 98.1th percentile nationally
Radon
98.4th
Arsenic
98.1th
Swea City, IA · 50590
Ranked on its weaker measurement: 98.1th percentile nationally
Radon
98.1th
Arsenic
98.5th
Terril, IA · 51364
Ranked on its weaker measurement: 98.1th percentile nationally
Radon
98.6th
Arsenic
98.1th
Melvin, IA · 51350
Ranked on its weaker measurement: 98.0th percentile nationally
Radon
99.6th
Arsenic
98.0th
Little Rock, IA · 51243
Ranked on its weaker measurement: 98.0th percentile nationally
Radon
98.0th
Arsenic
98.0th
Marathon, IA · 50565
Ranked on its weaker measurement: 98.0th percentile nationally
Radon
98.0th
Arsenic
98.6th
Showing 20 of 484. Every ZIP code on this list appears on its own state page below. See the full list →
Not on this list does not mean safe. Groundwater arsenic cannot be modeled for every ZIP code in the country, and a ZIP with no modeled value for either exposure is absent from this list because it was not measured — never because it was checked and found clear. A ZIP that was measured and simply does not carry both exposures is not listed either. Absence from this page is never a statement that a place is safe.
What "combined exposure" means
Radon and arsenic both come from the rock and soil a community sits on. Radon is a gas that can seep from the ground into homes; arsenic can dissolve from bedrock into the groundwater that wells draw from. Because both are tied to the same underlying geology, there are areas where a modeled reading of each runs high at the same time. A combined- exposure page simply reports where both modeled measurements are elevated together in the same ZIP codes.
It reports two facts side by side. It does not merge them into a single score, rank areas by how "bad" the pair is, or attach a number to how often they co-occur.
Why the figures come first. Radon and arsenic follow specific bedrock, so this overlap is concentrated — the figures above the list show where. Groundwater arsenic cannot be modeled everywhere, so the list covers less than the whole country. Each state page below carries that state's full set.
The two federal references
Where the modeled value crosses the line, this area's modeled radon runs above EPA's 4 pCi/L action level — the level at which the EPA recommends taking action to reduce indoor radon. It is an action level, a recommendation to act, never a regulatory limit.
EPA indoor-radon action level — 4 pCi/L. A recommendation to act, not a regulatory limit.
USGS models a high likelihood that groundwater arsenic here exceeds 10 µg/L — the level EPA sets as the limit for public water systems. Most private wells are federally unregulated, so this is a modeled likelihood, never a measured exceedance.
EPA drinking-water maximum contaminant limit — 10 µg/L. Applies to public systems; private wells are not covered.
The value gate · why the same section can read two ways
The radon sentence that names the 4 pCi/L action level renders only where the modeled radon value actually crosses 4 pCi/L. An area can rank among the highest 10% of US ZIP codes for modeled radon and still sit below 4 — so it gets a plain ranking sentence instead of the federal-number one. Here is the same section rendered for two illustrative areas.
Illustrative: modeled radon at 5.8 pCi/L — above the 4 pCi/L line
This area's section renders
USGS models a high likelihood that groundwater arsenic here exceeds 10 µg/L — the level EPA sets as the limit for public water systems — and this area's modeled radon runs above EPA's 4 pCi/L action level.
Illustrative: modeled radon at 3.4 pCi/L — in the top 10% nationally, below 4 pCi/L
This area's section renders
This area ranks among the highest 10% of US ZIP codes for both modeled radon and modeled groundwater arsenic.
Why the difference? Both areas are among the highest 10% of US ZIP codes for modeled radon — but only the first area's modeled value crosses 4 pCi/L, so only it earns the action-level sentence. The second sits in the tail, still below the line, so it reads as a ranking, not an exceedance. By EPA's estimate only roughly 6–7% of US homes exceed the 4 pCi/L action level, so the national top-10% boundary itself falls below 4 pCi/L — being in the tail does not mean crossing the line. The arsenic half is unaffected: "USGS models a high likelihood" is the honest framing at any value, because it describes a probability, not a measured reading. How we measure this →
See it for a state
Each state page lists the ZIP codes within that state where both modeled exposures are elevated, with the value gate applied row by row. We publish a state when it has at least one such ZIP code in the current data release.
The ranking above is national and covers only the states listed here. A state page carries every one of its ZIP codes, not just those high enough to reach the national list.
Modeled, not measured. Indoor radon uses the Li national ZCTA-level model; groundwater arsenic uses the USGS private-well probability model. Neither is a reading from your home or tap. A ZIP with an unobservable leg is shown as not measured, never as "not impacted." The two references are federal and different: radon's 4 pCi/L is an action level (a recommendation to act), arsenic's 10 µg/L is the maximum contaminant limit EPA sets for public water systems, and private wells are outside it. A ZIP reaches the list by ranking in the highest 10% of US ZIP codes for each exposure. The list is national; each state also has its own page with that state's full set. Read the full combined-exposure methodology →
The permalink /insights/radon-arsenic/2026-cc9c40e3 resolves to this exact data release and never changes; an always-latest version lives at /insights/radon-arsenic. Both are free to reuse under CC BY 4.0 with attribution.
Suggested citation — ZipVitals, “Combined exposure: modeled radon and modeled arsenic, by ZIP code,” 2026 release (env-co-occurrence v1). Permalink /insights/radon-arsenic/2026-cc9c40e3.
Every ZIP code on this list, as a CSV keyed to this exact release. Coverage denominators, source names and license terms travel inside the file header, so the numbers cannot be separated from the terms they came with.
Download the data (CSV, 484 rows)Indoor radon comes from the Li national ZCTA-level model; groundwater arsenic from the USGS private-well probability model. Both are modeled, never a reading from a home or a tap — the CSV header names each source and its terms.