What Radon Level Is Dangerous? Understanding pCi/L and EPA Thresholds

Radon is measured in picocuries per liter (pCi/L)—a unit that means very little to most people until they’re staring at a test result trying to figure out if they need to call a contractor. Here’s what those numbers actually mean.

The EPA’s Action Level: 4 pCi/L

The U.S. Environmental Protection Agency recommends mitigation when indoor radon levels reach or exceed 4 pCi/L. This is the point where the long-term lung cancer risk from radon exposure is considered high enough to warrant action.

At 4 pCi/L, the EPA estimates your lifetime risk of lung cancer from radon exposure is roughly 7 per 1,000 (for non-smokers) to 62 per 1,000 (for smokers). By comparison, the risk from drunk driving is about 2 per 1,000 over a lifetime.

The Consideration Level: 2 pCi/L

The EPA also sets a consideration level of 2 pCi/L—the point at which you should seriously consider whether to mitigate, even though the level hasn’t reached the action threshold.

The average indoor radon level in the US is 1.3 pCi/L. Anything above 2 pCi/L is above average. The EPA notes that there is no confirmed safe level of radon exposure—2 pCi/L still carries risk, just less than 4 pCi/L.

What Levels Mean in Practice

Radon Level	Risk Interpretation	Recommended Action
Below 2 pCi/L	Below average, lower risk	No action required; retest periodically
2–4 pCi/L	Above average risk	Consider mitigation; definitely retest
4–8 pCi/L	Elevated risk	Mitigate within months
8–20 pCi/L	High risk	Mitigate urgently
Above 20 pCi/L	Very high risk	Mitigate as soon as possible

The highest levels typically occur in lower floors of homes—basements and ground floors—particularly in regions with uranium-rich geology.

How Radon Risk Compares to Other Risks

Context helps when evaluating a number:

• Smoking and radon combined is the highest-risk combination. A smoker in a home at 8 pCi/L has a dramatically elevated risk of lung cancer compared to either risk factor alone.
• At 4 pCi/L, the EPA estimates the radon-related lung cancer risk is higher than the risks from arsenic in drinking water or benzene in outdoor air—both of which are regulated environmental risks.
• Outdoor air has an average radon level of about 0.4 pCi/L. Radon disperses rapidly outdoors.

Is There a Safe Level?

No. There is no established threshold below which radon definitively causes no harm. The EPA’s levels are action guidelines based on risk-benefit analysis of mitigation costs—not a declaration that levels below 4 pCi/L are safe.

If your home tests at 3.9 pCi/L and you can afford a $1,200 mitigation system, mitigating still makes sense.

What Affects Indoor Radon Levels

Geographic location. Radon originates from uranium decay in soil and rock. High-radon geologic zones include much of the Rocky Mountain region, the northern Midwest (Iowa, Minnesota), the Appalachian region, and parts of the Mid-Atlantic. But high levels can occur anywhere.

Floor level. Radon enters through foundation cracks, sumps, and gaps around pipes. Levels are highest in basements and ground-contact floors. Upper floors typically have lower levels.

Building characteristics. Tight, well-sealed construction can actually trap radon at higher concentrations. Homes with crawl spaces, sump pits, and certain foundation types vary in how much radon enters.

Season and weather. Radon levels fluctuate. They’re often higher in winter (sealed homes, negative pressure from heating systems) and lower in summer (open windows, different pressure dynamics).

Testing: Short-Term vs. Long-Term

Short-term tests (2–7 days) give a snapshot and are appropriate for initial screening or real estate transactions. Keep windows closed and normal HVAC operation during the test.

Long-term tests (90 days to 1 year) give a more representative average. They account for seasonal variation and are the better measure of actual chronic exposure.

If a short-term test shows elevated levels, confirm with a second test or a long-term test before committing to mitigation. Radon levels vary day to day.

How Mitigation Gets Levels Down

A properly installed mitigation system typically reduces radon levels by 80–99%. A home that tests at 15 pCi/L before mitigation might test at 0.5–1.0 pCi/L afterward. Mitigation systems are highly effective.

Post-mitigation testing is included in most professional installations to confirm the system is working. A result below 2 pCi/L post-mitigation is a good outcome.

Frequently Asked Questions

My home tested at 3.8 pCi/L—should I mitigate? The EPA doesn’t mandate it, but the risk difference between 3.8 and 4.0 pCi/L is negligible. If your budget allows, mitigation is a reasonable choice. At minimum, retest in a different season to confirm the level.

What’s the highest radon level ever recorded in a home? Levels above 100 pCi/L have been recorded in US homes—particularly in areas with highly radon-prone geology. Iowa and Minnesota have documented the highest average levels in the country.

Does radon level change between floors? Yes, significantly. A basement might test at 8 pCi/L while the first floor tests at 3 pCi/L and the second floor tests at 1.5 pCi/L. The EPA recommends testing the lowest livable level of the home.

If I live on the third floor of an apartment building, do I need to worry about radon? Upper-floor apartments are at much lower risk. Radon concentrations decrease significantly with height above the foundation. Ground-floor and basement units are the concern.

For cost information, see our radon mitigation cost guide.

Test Your Home, Then Decide

Radon is odorless, colorless, and invisible—the only way to know your exposure is to test. Test kits are available for $10–$30 from hardware stores, online retailers, or your state radon office. A professional mitigator can also conduct the test as part of a free or low-cost consultation.

Use RadonLocal to find certified radon testing and mitigation contractors near you.