The short answer
Radon is measured in picocuries per liter (pCi/L). Outdoor air averages about 0.4 pCi/L and U.S. homes average about 1.3 pCi/L. The EPA recommends fixing any home at 4.0 pCi/L or higher and considering a fix between 2.0 and 4.0 pCi/L, because there is no known safe level of radon. The World Health Organization uses a lower reference level of 2.7 pCi/L. The higher the number, the higher the lifetime lung cancer risk, and every level on this chart is fixable.
1. The radon levels chart
The bands below follow the EPA thresholds. Two numbers matter most: 2.0 pCi/L, where the EPA says to start considering a fix, and 4.0 pCi/L, the action level where the EPA says to fix the home.
| Radon level | What it means | What to do |
|---|---|---|
| 0.4 pCi/L | Average outdoor air (EPA). The practical floor for any home. | No action. This is as low as indoor air realistically gets. |
| 1.3 pCi/L | Average indoor level for U.S. homes (EPA). | No action needed. Retest every two years or after major changes to the home. |
| Below 2.0 pCi/L | Low. Below the range where the EPA suggests considering a fix. | Keep the result. Retest every two years, and after renovations, HVAC changes, or foundation work. |
| 2.0 to 3.9 pCi/L | Moderate. The EPA says to consider fixing in this range. The WHO reference level of 2.7 pCi/L sits inside it. | Confirm with a second or long-term test. Mitigation is a reasonable choice, especially with children, smokers, or a finished basement. |
| 4.0 to 7.9 pCi/L | High. At or above the EPA action level of 4.0 pCi/L. | Fix your home. Confirm with a follow-up test if the first was a short-term test, then mitigate. |
| 8.0 to 19.9 pCi/L | Very high. Two to five times the action level. | Fix your home. The EPA says a short-term result more than twice the action level should be followed by a second short-term test, then mitigation. |
| 20 pCi/L and above | Extreme. Five or more times the action level. | Fix your home as soon as practical. Limit time spent on the lowest level until the system is running. |
Thresholds and averages from the EPA publication A Citizen's Guide to Radon and the EPA page on its radon action level. WHO reference level from the WHO Handbook on Indoor Radon.
2. Look up your exact reading
Most people arrive here with a specific number. Find yours below. Risk figures are the EPA's lifetime estimates per 1,000 people exposed at that level, from the table in section 3.
Outdoor air. A home reading this low is essentially at the outdoor background. Nothing to do.
Below the U.S. indoor average of 1.3. Low. Retest every two years to catch changes.
The U.S. indoor average. Low, but not zero risk. The EPA estimates about 2 in 1,000 never-smokers exposed at this level over a lifetime could get lung cancer.
Still below 2.0, the bottom of the "consider fixing" range. Low. Worth a long-term test if the number came from a two-day kit in mild weather, since winter readings often run higher.
The start of the range where the EPA says to consider fixing. Lifetime risk at 2 pCi/L is about 4 in 1,000 for never-smokers and about 32 in 1,000 for smokers.
Moderate. Just under the WHO reference level of 2.7. Confirm with a second test. Mitigation is optional and reasonable.
The World Health Organization reference level (100 Bq/m³). Many homeowners treat this as their personal action line. The EPA still classifies it as "consider fixing."
Upper moderate. Close enough to 4.0 that a short-term result here can easily be above 4.0 in another season. A long-term test or a second short-term test settles it. Mitigation is a sound decision.
The EPA action level. Fix your home. Lifetime risk at 4 pCi/L is about 7 in 1,000 for never-smokers and about 62 in 1,000 for smokers, roughly the risk of dying in a car crash for a never-smoker.
High, but a short-term average this close to 4.0 has real uncertainty. The EPA notes that if the average of two short-term tests is 4.1, there is about a 50 percent chance the year-round average is somewhat below 4.0. That is a reason to confirm, not a reason to skip the fix. Most homeowners at this level mitigate.
High. Well past measurement noise. Mitigate. A second short-term test is still good practice if the first was a do-it-yourself kit.
Very high, twice the action level. The EPA says a first short-term result more than twice 4.0 should be followed immediately by a second short-term test, and the home should be fixed if the second test is 4.0 or higher. Lifetime risk at 8 pCi/L is about 15 in 1,000 for never-smokers.
Very high. Lifetime risk at 10 pCi/L is about 18 in 1,000 for never-smokers and about 150 in 1,000 for smokers. Schedule mitigation without waiting for a second season of data.
Extreme, five times the action level. Lifetime risk at 20 pCi/L is about 36 in 1,000 for never-smokers and about 260 in 1,000 for smokers. Mitigate as soon as practical and limit time on the lowest level until the system is running.
Readings this high are uncommon but they do occur. They are fully fixable. Systems can reduce radon by up to 99 percent, so even a very high reading normally comes down below 4.0 and often below 2.0.
3. The EPA lifetime risk table
The EPA publishes lifetime lung cancer risk estimates by radon level, separately for people who smoke and people who never smoked. The figures below are the EPA's, reproduced from A Citizen's Guide to Radon. They describe how many people out of 1,000 could get lung cancer if exposed to that level over a lifetime.
| Radon level | Smokers (per 1,000) | Never-smokers (per 1,000) | EPA guidance |
|---|---|---|---|
| 20 pCi/L | About 260 | About 36 | Fix your home |
| 10 pCi/L | About 150 | About 18 | Fix your home |
| 8 pCi/L | About 120 | About 15 | Fix your home |
| 4 pCi/L | About 62 | About 7 | Fix your home |
| 2 pCi/L | About 32 | About 4 | Consider fixing between 2 and 4 pCi/L |
| 1.3 pCi/L | About 20 | About 2 | Average indoor level. Reducing below 2 pCi/L is difficult |
| 0.4 pCi/L | Not listed | Not listed | Average outdoor level |
Source: EPA, A Citizen's Guide to Radon (EPA 402/K-12/002). Lifetime risk of lung cancer death from EPA Assessment of Risks from Radon in Homes (EPA 402-R-03-003). The EPA notes former smokers fall between the two columns.
Two things stand out. First, risk climbs steadily rather than jumping at 4.0. The never-smoker figure roughly doubles from 2 to 4 pCi/L and doubles again from 4 to 8. Second, smoking multiplies the radon risk by roughly seven to ten times at every level. We cover that interaction in our guide to radon and smoking.
4. Why 4.0 is an action level, not a safety line
The most common misreading of a radon chart is treating 4.0 pCi/L as the boundary between safe and dangerous. It is not. The EPA states plainly that no level of radon is safe and that levels below 4 pCi/L still pose some risk. The action level is where the agency decided the risk clearly justifies the cost of a fix for every home. Below that, it is a judgment call, and the EPA still recommends considering a fix from 2.0 up.
The World Health Organization drew its line lower, at 100 Bq/m³ (about 2.7 pCi/L), with an upper bound of 300 Bq/m³ (about 8.1 pCi/L) where a country cannot achieve the lower figure. Neither agency is wrong. They weighed cost and achievability differently. If your reading falls between 2.7 and 4.0, you are above the WHO reference and inside the EPA's consider-fixing range, which is why so many homeowners in that band choose to mitigate. We compare the two standards in WHO vs EPA radon levels.
Congress set a long-term national goal that indoor radon should be no higher than outdoor air, about 0.4 pCi/L. The EPA acknowledges that goal is not yet achievable in every home, but says most homes today can be reduced to 2 pCi/L or below. That is the realistic target after mitigation, not simply "under 4."
5. Why one reading is not a fixed number
A radon level is not a property of the house the way square footage is. It moves hour to hour and season to season with soil moisture, barometric pressure, wind, and how the home is heated and ventilated. Winter readings are typically higher than summer readings because a closed, heated house pulls more soil gas indoors. A two-day test is a snapshot. A 90-day or longer test is closer to the true annual average, which is what the risk tables are built on.
This is why the EPA's own testing protocol is two steps. Take a short-term test. If it reads 4 pCi/L or higher, take a follow-up test, either a second short-term test or a long-term test, before deciding. If the first result is more than twice the action level, take a second short-term test right away rather than waiting months for a long-term result. And the EPA notes that when two short-term tests average 4.1, there is about a 50 percent chance the year-round average is somewhat below 4.0. A reading close to the line is a reason to confirm, not a reason to relax.
A single low reading in July does not clear a north Georgia home. If your only test was a short-term kit in mild weather, treat a result of 2.0 or above as provisional and confirm it with a winter test or a long-term test. Our guide to why radon levels fluctuate explains the swings in detail.
6. pCi/L vs Bq/m³
The United States reports radon in picocuries per liter. Most of the world, including the WHO, uses becquerels per cubic meter. One pCi/L equals 37 Bq/m³. If your monitor or report shows Bq/m³, use this conversion to find your place on the chart.
| pCi/L | Bq/m³ | Reference point |
|---|---|---|
| 0.4 | 15 | EPA average outdoor air |
| 1.3 | 48 | EPA average U.S. indoor level |
| 2.0 | 74 | Bottom of EPA consider-fixing range |
| 2.7 | 100 | WHO reference level |
| 4.0 | 148 | EPA action level |
| 8.1 | 300 | WHO upper limit for national reference levels |
| 20 | 740 | Five times the EPA action level |
For the full conversion ladder and the history behind each unit, see radon measurement units explained.
7. What to do at each level
Below 2.0 pCi/L
Keep the report with your house records. Retest at least every two years, and sooner if you finish a basement, replace the HVAC system, add insulation or new windows, or have foundation work done. Any of those can change the pressure balance that draws soil gas indoors.
2.0 to 3.9 pCi/L
Confirm the number with a second short-term test in a different season or a long-term test. Then decide. Mitigation at this level is optional under EPA guidance and recommended by the WHO standard once you pass 2.7. Households with children, current or former smokers, or a basement used daily usually choose to fix it. We walk through that decision in should you mitigate between 2 and 4 pCi/L.
4.0 pCi/L and above
Fix the home. If your only result is a single short-term test, confirm it with a follow-up test first, unless it is more than twice the action level, in which case run the second short-term test immediately and move to mitigation if it holds. A sub-slab depressurization system is the standard fix. The EPA says systems can reduce radon by up to 99 percent and that most homes can be brought to 2 pCi/L or below. Confirm the result with a post-mitigation test no sooner than 24 hours after the fan starts and within 30 days of installation, then retest at least every two years. Pricing is covered on our radon mitigation cost page.
Reading a report rather than a monitor? Lab reports for charcoal or electret kits include the measurement period and sometimes a margin of error. Our guide to how to read radon test results explains each line of a typical report.
8. Frequently asked questions
This article is general information for homeowners, not medical, legal, or engineering advice. Risk figures are EPA population estimates and do not predict any individual outcome. Radon levels vary from home to home and over time, and agency guidance can change. The only way to know your home's radon level is to test it.
