Radon Levels by State: Complete 2026 Map and Testing Guide

Radon Risk by State: Where You're Most Likely to Find It

Radon comes from the natural decay of uranium in soil and rock — it's not distributed evenly across the U.S. The EPA divides the country into three radon potential zones based on historical testing data and geological factors. Here's what the data shows.

Highest-Risk States (EPA Zone 1)

Zone 1 counties account for 40% of U.S. homes. Predicted average indoor radon in these counties: 2.0 pCi/L or higher.

Highest-risk states by concentration:

• Iowa: 28% of homes tested exceed 4 pCi/L
• Minnesota: 27% exceed action level
• Nebraska: 25% exceed 4 pCi/L
• Pennsylvania: 24% exceed action level
• Colorado: 21% exceed 4 pCi/L
• Ohio: 19% exceed action level
• Missouri: 18% exceed 4 pCi/L
• Maine: 17% exceed action level
• Kansas: 17% exceed 4 pCi/L
• Montana: 16% exceed action level

Other states with significant Zone 1 coverage: South Dakota, North Dakota, Wyoming, Wisconsin, Indiana, and Illinois.

Moderate-Risk States (EPA Zone 2)

Zone 2 covers much of the East Coast, Mid-Atlantic, and scattered areas nationally. Predicted average: 0.4–2.0 pCi/L.

These states still have frequent elevated radon but not at the same rates as Zone 1. Testing is equally important here because individual homes vary widely.

Lower-Risk States (EPA Zone 3)

Zone 3 includes most of Texas, Florida, the Pacific Northwest, and scattered areas. Predicted average: below 0.4 pCi/L.

This doesn't mean radon doesn't exist — it means the statistical likelihood is lower. Testing is still recommended, especially if you're buying a home or if a neighbor's home tests high.

The Reality Behind the Averages

Zone designations are useful but incomplete. Two homes in the same EPA Zone 1 county can have radon levels 10 times apart. Soil type, foundation design, air pressure differences, and construction year all create variation.

Examples:

• A 1940s stone foundation home on a crawl space in Pennsylvania might test 18 pCi/L
• A 2020 slab-on-grade home in the same county might test 0.8 pCi/L
• Basement vs. crawl space, finished vs. unfinished, sealed vs. unsealed — all change the radon concentration

Takeaway: Your state's average tells you whether to test. Your home's test result tells you whether to mitigate.

How to Test for Radon

Short-Term Tests (2–7 Days)

Cost: $25–$50 (DIY kit) or $150–$300 (professional)

The EPA recommends a minimum 48-hour test — 2 days with doors and windows closed, normal living conditions. For higher accuracy, a 7-day test (1 week) is standard.

DIY kits from hardware stores work fine for initial screening — activate, leave for 48 hours, mail to the lab. Professional tests typically last 3–5 days and are more accurate.

When to use: Initial screening, quick assessment, reasonable for Zone 3 states.

Long-Term Tests (90+ Days)

Cost: $200–$500 (professional)

Captures seasonal variation. Radon levels fluctuate by season (typically higher in winter when homes are sealed). A 90-day test gives a more accurate picture of your actual exposure.

When to use: Zone 1 and Zone 2 states, before major decisions (buying, mitigating), if a short-term test was inconclusive.

Continuous Radon Monitors (Real-Time)

Cost: $300–$800

A device you place in your home that continuously measures radon over 48 hours or longer, giving real-time readouts. Some professionals use these to confirm elevated radon before quoting mitigation.

When to use: Professional assessments, to watch radon patterns, post-mitigation verification.

Testing Tips for Accuracy

• Close doors and windows: EPA protocol requires closed-home conditions for 12 hours before test start, then during the entire test
• Place in the lowest occupied level: Radon enters at foundation level; test where you actually spend time (basement, crawl space, or first floor)
• Avoid placement near HVAC returns or doors: These can artificially lower readings
• Don't test during renovation: Radon levels spike when you're opening walls or digging; test in normal conditions
• If elevated, retest: If your first test shows 4+ pCi/L, the EPA recommends a follow-up test to confirm before mitigation

What to Do With Your Test Results

Below 2 pCi/L: Low risk. Retest every 2 years or after major home renovations.

2–4 pCi/L: Gray zone. The EPA suggests considering mitigation. Check with a certified mitigator — they'll assess your home and give a cost estimate.

4+ pCi/L: The EPA action level. Schedule a mitigation assessment. A certified mitigator will design a system tailored to your foundation type and the radon level.

Above 10 pCi/L: High-risk range. Same process — get a mitigation quote immediately — but this level warrants faster action.

Next Steps

If you're in a Zone 1 or Zone 2 state, test first. If elevated, find a certified mitigator → for a free or low-cost assessment. If you're in Zone 3, testing is less urgent unless buying a home or you have reasons to suspect radon (geology, neighbor testing high, etc.).

Find testing pros or mitigators in your state → | How much does mitigation cost? →