WHO vs EPA Radon Levels: 2.7 vs 4.0 pCi/L, and Which to Follow

1. Why you see two different numbers

Read three articles about radon and you can come away confused. One says you should fix your home at 4.0 pCi/L. Another says the World Health Organization recommends acting at a lower level. A third quotes a figure in becquerels that looks nothing like either. None of these sources is wrong. They are simply citing different organizations that set different thresholds for different reasons.

The two you will run into most are the US Environmental Protection Agency (EPA) and the World Health Organization (WHO). The EPA sets the operative standard used in the United States. The WHO publishes guidance meant for governments worldwide. Once you understand what each number is for, the apparent contradiction goes away, and the practical decision for your home gets simpler.

2. The EPA action level: 4.0 pCi/L

In the United States, the EPA action level is 4.0 pCi/L (picocuries per liter). At or above that level, EPA recommends installing a radon mitigation system to bring the home down. This 4.0 figure is the number embedded in most US real estate transactions, lender requirements, and mitigation contracts, so it is the one most homeowners encounter first.

EPA does not treat 4.0 as a hard wall, though. Its own guidance encourages homeowners to consider taking action when a home reads between 2.0 and 4.0 pCi/L, because risk is still present in that range. So even within the US standard, the message is that 4.0 is where action is recommended, not where risk begins.

3. The WHO reference level: 2.7 pCi/L

The World Health Organization recommends a reference level of 100 becquerels per cubic meter (Bq/m³), which converts to roughly 2.7 pCi/L. WHO describes it as a level above which action should be considered to reduce radon exposure. It is lower than the US action level on purpose: WHO set it from a health-based view of how radon exposure relates to lung cancer risk.

WHO also recognizes that 100 Bq/m³ is not achievable everywhere given local conditions and resources. Its guidance allows countries to adopt a national level no higher than about 300 Bq/m³ (roughly 8 pCi/L) where the lower target is not feasible. That built-in flexibility is a clue to the bigger point: a reference level is a target shaped by both health science and practical reality, which is exactly why countries land on different numbers.

4. pCi/L vs becquerels: the unit confusion

Part of what makes radon thresholds look inconsistent is that the world uses two different units. The United States measures radon in picocuries per liter (pCi/L). Most other countries, and the WHO, use becquerels per cubic meter (Bq/m³). They measure the same thing, just on different scales.

Keep the unit in mind whenever you compare guidance from different countries or monitor brands. A reading that looks alarmingly high or reassuringly low may simply be expressed in the unit you are not used to.

5. Why the two numbers differ

The core difference is what kind of number each one is. The WHO 2.7 pCi/L figure is a health-based reference level, anchored to the relationship between long-term radon exposure and lung cancer risk. If the only goal were to minimize risk on paper, you would push the target as low as measurement and mitigation allow.

The EPA 4.0 pCi/L figure is an action level, which is a policy decision. It weighs health risk against feasibility: how reliably homes can be measured and reduced, what mitigation can realistically achieve, and the cost of acting nationwide. A policy level has to be a number people can actually meet at scale, which is why it sits a bit higher than a pure health-based target.

6. Neither number means "safe"

This is the most important point, and it is the one both organizations agree on. Radon risk is continuous. There is no threshold below which exposure carries no risk at all. A home at 3.9 pCi/L is not safe and a home at 4.1 pCi/L is not suddenly deadly. The risk rises gradually with the level and the years of exposure.

That is why the framing matters. The EPA action level and the WHO reference level are both points at which action is recommended, not lines between safe and dangerous. Lowering radon below either number still reduces risk. The reason radon deserves attention at all is its place in the data: radon is the second leading cause of lung cancer in the United States after smoking, and the leading cause among people who have never smoked. For more on how that risk works, see our guide to radon and lung cancer risk.

7. What number should a homeowner act on

Here is the practical guidance. If you live in the United States, the operative standard is the EPA 4.0 pCi/L action level. At or above 4.0, the recommendation is clear: mitigate. Lenders, real estate transactions, and mitigation contractors all work from this number, so it is the one that drives most decisions in practice.

The WHO 2.7 pCi/L reference level is useful as context rather than as a competing rule. It tells you that risk is still meaningful below the US action level, which is why EPA itself suggests considering action between 2.0 and 4.0 pCi/L. If your home lands in that band, weigh a few things: how long you plan to live there, whether children or smokers are in the household, and how modest the cost of a mitigation system is relative to years of reduced exposure. Many homeowners reasonably choose to mitigate in that range.

None of this works without a measurement. Neither the EPA number nor the WHO number tells you anything about your specific home until you test it. If you have never tested, that is the first step, and our overview of why radon testing is essential explains why it matters everywhere, not just in high-zone areas.

8. Radon risk in metro Atlanta

For homeowners around Atlanta, this is not an abstract debate over thresholds. Much of metro Atlanta sits in higher-radon EPA zones, including several counties placed in Zone 1, the highest category on the EPA Map of Radon Zones, which predicts a county average at or above 4.0 pCi/L. The region's granite and uranium-bearing bedrock is a major reason elevated readings turn up so often here.

In a region where elevated radon is common, the gap between the EPA and WHO numbers is more than trivia. A home that reads 3.0 pCi/L clears the US action level but sits above the WHO reference level, putting you squarely in the range where considering action is sensible. Testing tells you where your home falls, and from there the decision is yours to make with clear numbers in hand.

9. Frequently asked questions