Understanding pCi/L: How Radon Is Measured

By Steve Crews · Last updated June 25, 2026

Where pCi/L comes from

Radon is radioactive, meaning its atoms are unstable and spontaneously break apart, releasing energy in the process. Measuring how fast that happens is the core of radon measurement.

The curie is the original unit for radioactivity, named after Marie Curie. One curie equals exactly 37 billion disintegrations per second, which is far too large a number to be useful when you are talking about the faint radioactivity in household air. A picocurie is one trillionth of a curie. At that scale, 1 pCi represents about 2.2 decay events per minute. Still a real physical event, but small enough to work with.

“Per liter” completes the unit. You are measuring the rate of radioactive decay in each liter of air sampled. So when a lab reports 3.5 pCi/L, it is telling you that in every liter of your indoor air, radon atoms are breaking apart at roughly 7.7 times per minute.

The becquerel (Bq) is the modern international unit for radioactivity, named after Henri Becquerel, who discovered the phenomenon. One becquerel is one disintegration per second. Most countries outside the US report radon in becquerels per cubic meter, so you will sometimes need to convert. The math is straightforward: 1 pCi/L equals approximately 37 Bq/m3. That means the EPA’s 4 pCi/L action level sits at about 148 Bq/m3, and the US average of 1.3 pCi/L is roughly 48 Bq/m3.

What the numbers actually look like in a real home

The EPA has collected decades of data on US housing, and the picture is fairly consistent. Average indoor radon in American homes sits around 1.3 pCi/L. Outdoor air, where radon disperses into the open atmosphere, runs about 0.4 pCi/L. Neither number is cause for concern on its own.

But concentrations vary enormously between houses. A home sitting over uranium-rich soil in parts of Pennsylvania, Iowa, or Montana can read 10, 20, or even 40 pCi/L. A neighbor on the same street might read 2 pCi/L. Construction details, soil type, foundation condition, ventilation patterns, and even the season all influence what shows up on a test. Two houses that look identical from the outside can differ by a factor of ten.

That variability is exactly why the EPA and the U.S. Surgeon General recommend testing rather than guessing. See our guide to normal and average radon levels for a closer look at how readings distribute across states and geological zones.

The EPA’s 4 pCi/L action level

The EPA set 4 pCi/L as its radon action level after weighing health risk against what is realistically achievable in existing homes. The U.S. Surgeon General issued a national health advisory that echoes this threshold, urging Americans to test and to reduce radon when levels are elevated.

The number is not a line between safe and unsafe. The EPA is explicit about this: the agency also recommends considering action when readings fall between 2 and 4 pCi/L, and it acknowledges that no radon concentration is completely free of health risk. A result of 3.9 pCi/L is not a clean bill of health; it is a lower level of risk than 6 pCi/L, which is itself less than 15 pCi/L. Risk is a gradient, not a cliff.

Think of the action level as the point where the EPA says the cost and effort of mitigation clearly pays off in reduced health risk. Below 4 pCi/L, the math gets closer, but the risk does not disappear.

How international guidelines compare

Seeing guidelines from other countries can seem confusing until you apply the conversion factor.

Health Canada sets its guideline at 200 Bq/m3, which converts to roughly 5.4 pCi/L. That is higher than the US action level in raw numbers, but the two agencies are not disagreeing about the health science. Canada’s threshold reflects a different judgment about what is practically achievable in Canadian housing stock. Both agencies recommend remediation and both acknowledge that lower is better.

The World Health Organization takes the most protective stance of the three, recommending a reference level of 100 Bq/m3 (about 2.7 pCi/L), with a ceiling of 300 Bq/m3 where the lower target is genuinely not feasible. The WHO classifies radon as a confirmed human carcinogen and explicitly encourages countries to set guidelines as low as they can reasonably achieve. A reading that falls between the US action level and the WHO reference level is still worth taking seriously.

The upshot: different countries, same science, slightly different lines drawn for practical reasons. In all three systems, reducing radon from 10 pCi/L to 1 pCi/L is unambiguously a win.

Reading a radon test report

Short-term charcoal canister tests, the kind you leave in place for 2 to 7 days and mail to a lab, come back with a single pCi/L number. Some reports add a margin of uncertainty, expressed as a plus-or-minus figure. A result of 3.8 +/- 0.4 pCi/L means the lab’s best estimate is 3.8, but the true value could reasonably fall anywhere from 3.4 to 4.2 pCi/L. At the edge of the action level, that range matters when you are deciding whether to act immediately or do a follow-up test.

Long-term tests, placed for 90 days to a year, reduce that uncertainty considerably because they average out daily swings from weather, HVAC use, and how often you open windows. A long-term result is much closer to your actual year-round exposure than any single short-term snapshot.

Both test types report in pCi/L in the United States. AARST-NRPP, the leading national certification body for radon measurement and mitigation professionals, specifies closed-house conditions and minimum placement durations for valid short-term tests. Following those protocols is what gives you a number the EPA guidelines actually apply to.

Many state radon programs sell or mail kits at reduced cost, sometimes for $10 to $30 or even free. A standard commercial kit runs a bit more. Check price on Amazon for a current selection, but check your state radon office first before spending anything. Our DIY radon testing guide covers exactly where to place the kit and what to do with the result.

Why the action level is not a safe level

This is the part that trips up a lot of homeowners. A result of 3.8 pCi/L is below the EPA action level. It is not a passing grade.

Radon risk builds over time. Years of exposure at 3 pCi/L accumulate, particularly for anyone in the home who smokes. The EPA estimates that radon risk is roughly ten times higher for smokers than for non-smokers at the same radon concentration, because tobacco smoke and radon damage lung tissue through overlapping mechanisms.

The practical implication: if your home tests between 2 and 4 pCi/L, think about your specific situation. How much time does your household spend on the lowest level of the home? Does anyone smoke? How long do you plan to stay? Mitigation at lower concentrations is often cheaper and simpler than at higher ones, and the benefit compounds over the years you remain in the house. For a detailed breakdown of what different levels mean for risk, see our guide on what are safe radon levels in a home.

FAQ

What does pCi/L actually stand for? pCi/L stands for picocuries per liter. A picocurie is one trillionth of a curie, the classic unit of radioactivity named after Marie Curie. In practical terms, 1 pCi/L means roughly 2.2 radioactive decay events are occurring per minute in every liter of air. It is a measure of how much radon activity is present, not a chemical concentration like parts per million.

Is 4 pCi/L the safe limit for radon? No. The EPA’s 4 pCi/L action level is the threshold at which the agency recommends corrective action, not a dividing line between dangerous and safe. The EPA and the U.S. Surgeon General also recommend considering action between 2 and 4 pCi/L. No radon level carries zero risk; lower is always better.

How do I convert pCi/L to Bq/m3? Multiply pCi/L by 37 to get Bq/m3. So 4 pCi/L equals about 148 Bq/m3. Going the other direction, divide Bq/m3 by 37. Health Canada’s 200 Bq/m3 guideline converts to about 5.4 pCi/L, and the WHO’s 100 Bq/m3 reference level works out to roughly 2.7 pCi/L.

Why does the US use pCi/L instead of Bq/m3? The United States adopted the curie-based system before the international becquerel standard became dominant, and pCi/L stuck in US labs, EPA guidance, and state regulations. Most countries that set their radon guidelines later defaulted to SI units and Bq/m3. Both units measure the same physical phenomenon; the conversion factor of 37 gets you from one to the other in either direction.