What 4.0 pCi/L Actually Means: Understanding the EPA Radon Action Level

By Chris, Founder of RadonResources.com · Reviewed against current EPA guidance

A radon result of 4.0 pCi/L means your home has reached the level at which the U.S. Environmental Protection Agency recommends fixing it. It does not mean the air flipped from safe to dangerous at that exact point. The EPA chose 4 pCi/L as a practical action level after weighing lung cancer risk against the cost of mitigation, what mitigation systems could reliably achieve and how accurate radon tests were. Risk rises gradually with every increase in concentration, so a home at 3.8 and a home at 4.2 are in very similar territory. This article explains what the number measures, where it came from, what the risk at that level looks like and what to do if your result lands at or near it.

What 4.0 pCi/L measures

Radon in the United States is reported in picocuries per liter of air, written pCi/L. A picocurie is a tiny unit of radioactivity, so the figure describes how much radioactive decay from radon is happening in each liter of air you breathe. Most other countries report radon in becquerels per cubic meter (Bq/m³). One pCi/L equals 37 Bq/m³, which puts 4.0 pCi/L at about 148 Bq/m³. The EPA often rounds this to 150.

For context, outdoor air averages about 0.4 pCi/L, and the average indoor level in U.S. homes is about 1.3 pCi/L. A reading of 4.0 is roughly ten times the outdoor average and about three times the typical indoor level. The EPA estimates that nearly 1 in 15 U.S. homes has radon at or above the action level.

Where the 4 pCi/L action level came from

The 4 pCi/L figure traces back to a 1970s Colorado guideline for homes built on uranium mill tailings. When high indoor radon turned up in ordinary homes in the mid-1980s, the EPA took a nonregulatory approach and issued a recommended action level rather than a legal limit. According to a National Academies review of radon guidelines, the agency compared several possible levels, including 2, 3, 4, 8 and 20 pCi/L, before settling on 4.

That choice balanced several factors. Lower action levels prevented more lung cancer across the population, but the EPA also weighed the cost per life saved, whether mitigation systems of the time could consistently bring homes below the target and the practical limits of radon test accuracy. The agency judged that 4 pCi/L was achievable in the vast majority of homes. In other words, the number reflects policy and engineering realities as well as health data.

Congress set a different long-term goal in the Indoor Radon Abatement Act of 1988: indoor radon no higher than outdoor air, or about 0.4 pCi/L. That goal has never been practical for most homes, and the EPA notes that reducing radon below 2 pCi/L is difficult. The gap between the two figures helps explain why the EPA also recommends considering action between 2 and 4 pCi/L.

How 4.0 pCi/L compares with other guidelines

The World Health Organization recommends a national reference level of 100 Bq/m³, about 2.7 pCi/L, and says countries that can’t reach that should stay at or below 300 Bq/m³ (about 8.1 pCi/L). Many countries have adopted reference levels between those figures. The U.S. action level of 4 pCi/L sits in the middle of the international range. It isn’t unusually strict or unusually lax, but it is higher than the level the WHO prefers.

What the lung cancer risk looks like at 4 pCi/L

Radon causes lung cancer, and the EPA estimates it is responsible for about 21,000 lung cancer deaths in the United States each year. The concern is long-term exposure, measured over years of living in a home, not a single day or week. Radon generally doesn’t cause immediate symptoms, so how you feel tells you nothing about your level.

The EPA’s risk charts show how the risk changes with concentration. If 1,000 people who never smoked were exposed to 4 pCi/L over a lifetime, the EPA estimates that about 7 could get lung cancer. At 2 pCi/L that estimate falls to about 4, and at 8 pCi/L it rises to about 15. For people who smoke, the numbers are far higher: about 62 per 1,000 at 4 pCi/L, because smoking and radon together raise the risk much more than either does alone.

These are population estimates built on assumptions about lifetime exposure. They describe risk across large groups and can’t predict what will happen to any one person. They do show why the EPA treats 4 pCi/L as a point to act rather than a boundary of safety. The agency states there is no known safe level of radon, and that levels below 4 pCi/L still carry some risk.

Why 3.9 and 4.1 are closer than they look

A result just under 4 can feel like a pass and a result just over it like a fail. In practice, the difference between them is small. Radon levels move up and down with weather, season, soil moisture and how a house is heated and ventilated. A short-term test captures only a few days of that variation, and every measurement method has some built-in uncertainty.

That’s why a single borderline reading is better treated as a signal to gather more information than as a final answer. A home that tests at 3.7 in late spring might read higher during a closed-up winter, and a home at 4.3 might average slightly lower over a full year. If you’d like to understand how the time of year affects results, see our guide to radon testing in winter vs. summer.

What to do if your result is at or near 4 pCi/L

The EPA’s testing guidance gives a clear sequence for an initial short-term result of 4 pCi/L or higher:

• If you want a better picture of your year-round average, run a long-term test lasting more than 90 days. Fix the home if that result is 4 pCi/L or higher.
• If you need an answer sooner, run a second short-term test. If the average of the two tests is 4 pCi/L or higher, the EPA recommends fixing the home.
• The higher the first result, the more sense it makes to choose the faster short-term follow-up rather than waiting months for a long-term test.

If your result is between 2 and 4 pCi/L, the EPA recommends that you consider fixing the home. Factors that might push you toward mitigation include a smoker in the household, a bedroom or living space on the lowest level, or plans to stay in the home for many years. Re-testing every few years is also reasonable, since radon can change after renovations or foundation settling.

Home sales follow a faster timeline. Buyers usually rely on a short-term test during the inspection period, often run as two side-by-side detectors or with a continuous monitor. Our radon test kit comparison covers which options suit a first screening, a follow-up test or a real estate transaction.

How mitigation changes a 4.0 result

A home at or modestly above 4 pCi/L is usually a straightforward mitigation project. The most common method is active soil depressurization, which uses a vent pipe and an inline fan to draw radon from beneath the foundation and release it above the roofline. Sealing cracks and openings helps the system work more efficiently, but sealing alone is usually not a dependable way to lower radon.

How far a system can bring levels down depends on the foundation, the soil and the quality of the installation. Many properly installed systems bring homes well below 4 pCi/L, and some get below 2, though the EPA notes that going below 2 is difficult in many homes. A post-mitigation test, run after the system is operating, is the only way to confirm the result. Our radon contractor checklist covers certifications to look for and questions to ask before hiring.

Mitigation lowers your future exposure. It can’t undo exposure you’ve already had, but reducing an elevated level reduces the long-term risk tied to continued exposure.

How often readings near 4 show up in submitted data

Readings around the action level are common among homeowners who share their results with us. Of the 5,438 radon readings submitted to RadonResources.com, 947 were reported as exactly 4 pCi/L, making it one of the most frequently reported values in the data. About 91 percent of submitted readings were at or above 4.

That percentage is much higher than the EPA’s national estimate of roughly 1 in 15 homes, and the reason matters. People who get an elevated result are more likely to look for information and submit their number, so this data reflects who chooses to share, not the typical U.S. home. It’s useful for seeing what other homeowners in your area have reported, but it can’t tell you your own level. You can browse user-submitted radon levels or check your state radon page for local context.

Frequently asked questions

Is 4.0 pCi/L dangerous?

A level of 4.0 pCi/L is high enough that the EPA recommends fixing the home, because long-term exposure at that level carries a measurable lung cancer risk. It isn’t an emergency in the sense of requiring you to leave the house. The concern builds over years, which gives you time to confirm the result with a follow-up test and arrange mitigation with a qualified professional.

Should I mitigate if my result is exactly 4.0?

A result of exactly 4.0 meets the EPA’s threshold, since the recommendation applies to homes at or above 4 pCi/L. If that number came from a single short-term test, confirm it first with a long-term test or a second short-term test. If the long-term result or the average of two short-term tests is 4.0 or higher, the EPA recommends fixing the home.

Does the 4.0 action level apply to a basement no one uses?

The EPA recommends testing the lowest level of the home that is lived in or could be used regularly, such as a finished basement. A rarely used, unfinished basement will usually read higher than the floors above it. If you test there, the result may overstate your everyday exposure. Testing the main living level as well can help you decide what the number means for your household.

What is 4.0 pCi/L in Bq/m³?

Using the standard conversion of 37 Bq/m³ per pCi/L, 4.0 pCi/L equals about 148 Bq/m³. The EPA rounds this to 150 Bq/m³ in some materials. Readings from European and Canadian monitors are often shown in Bq/m³, so converting helps when you compare your result with U.S. guidance.

Reading your number in context

The 4.0 pCi/L action level is a practical threshold chosen to balance health risk against what testing and mitigation can reliably deliver. It works best as a decision point. A confirmed result at or above it calls for mitigation, a result between 2 and 4 is worth weighing based on your household, and any result is worth re-checking over time as your home and its conditions change.

Sources and further reading

• U.S. Environmental Protection Agency, Health Risk of Radon (risk charts, action level, estimated annual deaths, average indoor and outdoor levels).
• U.S. Environmental Protection Agency, A Citizen’s Guide to Radon (follow-up testing rules, 1 in 15 homes estimate, no known safe level, congressional long-term goal).
• National Research Council, Evaluation of Guidelines for Exposures to Technologically Enhanced Naturally Occurring Radioactive Materials, chapter on indoor-radon guidelines and recommendations (origin and basis of the 4 pCi/L action level).
• World Health Organization, WHO Handbook on Indoor Radon: A Public Health Perspective (100 Bq/m³ reference level).
• U.S. Environmental Protection Agency, Consumer’s Guide to Radon Reduction (mitigation methods and post-mitigation testing).
• RadonResources.com user-submitted radon readings (5,438 submissions; self-reported and not a representative sample).

This article is for general educational purposes and is not medical advice. Questions about your personal health risk should be discussed with a qualified healthcare provider.