The real numbers, where they come from, and what they mean for your ZIP code.
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"There's no radon in the desert" is the most persistent myth we deal with. The data below is why we keep answering it with a tripod and a monitor instead of an argument.
Arizona has tracked indoor radon results since 1993 through the state health department's indoor radon surveillance work. Across roughly 7,000 tested homes: an average of 2.9 pCi/L, with 74% of homes below the EPA action level of 4.0 pCi/L and 26%, about one home in four, above it. The highest reading recorded in the state is 55.8 pCi/L, roughly fourteen times the action level. For context, outdoor air runs about 0.4 pCi/L and the EPA suggests considering mitigation anywhere above 2.0. ADHS's radon program and data live at azdhs.gov.
The EPA divides every US county into three radon zones by predicted average. Maricopa County, along with Pima and most southern counties, sits in Zone 2, moderate potential with predicted averages between 2 and 4 pCi/L. Several northern Arizona counties, including Coconino and Yavapai, are Zone 1, the highest designation. The county-by-county map is published by the EPA here as a PDF. Two caveats the map itself insists on: zones predict county averages, not your house, and elevated homes have been found in all three zones. We've measured double-digit readings inside Zone 2.
Radon comes from uranium decaying in rock and soil, so geology draws the map. The recurring hot spots: North Scottsdale, Cave Creek, Carefree, Fountain Hills and Rio Verde, where homes sit on and among granite outcrops; the Verde Valley and Prescott; the Tucson foothills; and much of northern Arizona. In the Valley, the pattern we see over and over is simple: the closer to decomposed granite terrain, the more often tests come back above 4.0. Flat central and west Valley alluvium tests lower on average, but "lower on average" still includes the occasional 8.
No basements here, so people assume no problem. But slab-on-grade homes sit directly on the soil, and every slab crack, plumbing penetration and expansion joint is a pathway. Then our sealed-up living does the rest: Phoenix homes spend winter closed against the cold nights and four summer months closed around the AC, and closed houses concentrate whatever seeps in. It's why we treat testing as a year-round service rather than a winter one.
Test once, professionally or with a kit, and let the number decide. Under 2.0, retest in a couple of years or after any remodel that touches the slab. Between 2.0 and 4.0, think about mitigation the way you'd think about a slow roof leak. Over 4.0, fix it; the cost guide shows exactly what that runs in the Valley, and the mitigation page shows what the fix involves. Buying or selling in a hot-spot area, read the real estate radon page before your inspection window opens.
Questions about your area? Call (520) 423-5900. We'll tell you what we've been measuring near you.