What Are Safe Radon Levels in a Home?

By Steve Crews · Last updated June 24, 2026

The EPA’s 4 pCi/L action level

The number you will see most often is 4 pCi/L, or picocuries per liter. The EPA has used this as its action level for decades, meaning the concentration at which the agency recommends homeowners fix their radon problem. At or above 4 pCi/L, act. Below 2 pCi/L, the risk profile is lower and retesting in a few years is typically sufficient.

The EPA also identifies a middle zone. Between 2 and 4 pCi/L, the agency says homeowners should “consider” mitigation. That is careful language, not a loophole. Risk in that range is real but lower, and several factors can tip the decision: whether anyone in the home smokes, whether children sleep in the basement, and how much the work would cost.

The U.S. Surgeon General has separately issued a national health advisory calling on Americans to test their homes and reduce elevated levels, citing radon as the second leading cause of lung cancer in the United States. That advisory is not a restatement of the EPA’s numbers. It is an independent call to action from a separate public health authority.

Both agencies point homeowners toward certified professionals for mitigation work. The leading national credential is AARST-NRPP (now part of the Indoor Environments Association / National Radon Proficiency Program). Mitigators certified through this program have passed technical exams, follow published ANSI/AARST standards, and renew their credentials with continuing education.

Why no radon level is risk-free

The honest account: radon causes lung cancer through a dose-response relationship. More radon, over more years, means more risk. That relationship does not have a zero-risk floor, and the EPA states this directly.

The risk estimates make this concrete. The EPA estimates that out of 1,000 people who have never smoked and live in a home at 4 pCi/L for a lifetime, roughly 7 will develop lung cancer from radon. At 2 pCi/L, that figure drops to about 4 per 1,000. These numbers are not large in absolute terms, but they describe a hazard that is entirely preventable.

For people who smoke, the picture changes substantially. At 4 pCi/L, the EPA estimates roughly 62 out of every 1,000 smokers face radon-related lung cancer over a lifetime. Radon and tobacco each damage lung tissue, and together the effects compound far beyond what either does alone.

The 4 pCi/L action level is a practical policy line, not a threshold where the biology changes. The goal of a well-installed mitigation system is not to hit exactly 4 pCi/L. Most good systems push levels to 1 or 2 pCi/L. Lower is always better.

How US standards compare to Canada and the WHO

The EPA’s 4 pCi/L is not the only number in use internationally, and understanding the differences is useful context.

The World Health Organization set a reference level of 100 Bq/m3, which converts to about 2.7 pCi/L. The WHO’s framing is aspirational: countries should aim to keep levels below that figure where practical, while setting national targets no higher than 300 Bq/m3. It is guidance for national policy, and it deliberately sits below the US action level to encourage minimization.

Health Canada uses 200 Bq/m3 (roughly 5.4 pCi/L) as its remediation guideline. That is slightly above the EPA threshold. C-NRPP (Canadian National Radon Proficiency Program) plays the same credentialing role in Canada that AARST-NRPP does in the United States.

Why do these numbers differ? Each jurisdiction weighed the same underlying scientific evidence against practical factors: typical housing construction, the cost of widespread mitigation, what levels homes can realistically achieve after remediation, and national policy priorities. A radon atom is equally carcinogenic in Colorado and Ontario. The policy lines reflect where each government drew a practical threshold, not a dispute about the underlying biology.

For US homeowners, the EPA’s 4 pCi/L is the standard your state radon program, home inspector, and any certified mitigator will reference.

What typical US readings look like

The EPA estimates the national average indoor radon level in US homes at about 1.3 pCi/L. Outdoor air averages around 0.4 pCi/L. Most homes fall somewhere in that range. Elevated readings cluster in the Midwest, the Northern Plains, Appalachia, and parts of the Mountain West, though high results appear in every state.

The EPA publishes a national radon zone map that divides counties into three categories. Zone 1 counties have predicted average indoor levels above 4 pCi/L. Zone 2 covers 2 to 4 pCi/L. Zone 3 falls below 2 pCi/L. This map is a planning reference, not a verdict on any individual home. Two houses on the same block can differ by a factor of three or more, depending on soil permeability, foundation type, ventilation, and how well the structure is sealed. Your neighbor’s test result tells you nothing reliable about yours.

Many state radon programs publish local testing data showing what percentage of homes in a county tested above the action level. That context is worth checking before or after you run your own test.

How to read your result and decide what to do

Below 2 pCi/L: Most authorities consider this range lower-priority. It is well below every major action threshold and close to what typical well-ventilated indoor air would show. Retesting in a few years, especially after any renovation that affects the basement or foundation, is sensible.

Between 2 and 4 pCi/L: The EPA says consider acting. Smokers or former smokers in the home push this more urgently in that direction, as does regular use of basement spaces by children. A mitigation system at this range is not a dramatic expense relative to the risk reduction it delivers.

At 4 pCi/L or above: This is where the EPA and Surgeon General both say act. A well-designed sub-slab depressurization system typically reduces levels by 80 to 99 percent. A home at 8 pCi/L can realistically reach 1 to 2 pCi/L after installation. See our radon mitigation cost guide for what the work typically runs.

Before any of this applies to you, you need a test result. A long-term test, run for 90 days or more, gives the most reliable annual average, but a short-term charcoal test is a practical first step. Our guide on short-term vs long-term radon tests covers when each makes sense. Many state programs offer free or subsidized kits; check your state’s radon office before buying one. If you prefer to order directly, a quality charcoal kit is available at most hardware stores or Check price on Amazon.

If you have not yet tested, or your last test predates a major renovation, the complete guide to testing your home for radon is a good place to start.

FAQ

Is any radon level considered safe? No. The EPA, the U.S. Surgeon General, and the WHO all state that some risk exists at any radon level, and risk increases with both concentration and duration of exposure. The EPA action level of 4 pCi/L is a practical policy threshold, not a line between dangerous and risk-free. Reducing exposure is worthwhile even below the action level.

My test came back at 3.2 pCi/L. Do I need a mitigation system? You are below the EPA action level, but you are in the range where the EPA recommends considering mitigation. If anyone in the home smokes, if children spend significant time in the basement, or if you want to reduce risk proactively, a mitigation system is a reasonable choice. A properly installed system can push your level to below 2 pCi/L. You are not required to act, but the option makes practical sense.

Why does the US action level differ from the WHO’s reference level? The WHO’s reference level of 100 Bq/m3 (about 2.7 pCi/L) is lower than the EPA’s 4 pCi/L (about 148 Bq/m3). Both draw from the same scientific evidence on lung cancer risk. The difference reflects how each body balanced health aspiration against the practical question of what levels homes can realistically achieve and at what cost. Neither standard says radon below the threshold is free of risk.

Does radon level vary over time in the same home? Yes, often significantly. Levels tend to run higher in winter when homes are sealed against the cold and lower in summer when ventilation increases. They can also shift after renovations or changes to HVAC systems. A long-term test captures these seasonal swings and gives a more representative annual average than a short-term test taken at a single point in time.