Radon Levels Explained: What Your pCi/L Result Means

Understanding radon levels turns a laboratory number into a decision. Most people get a result, see a figure such as 3.6, and have no idea whether that is fine or alarming. This guide places your number against the thresholds EPA and CDC actually publish, explains why there are no safe radon levels as such, and sets out what to do for each band.

Last reviewed and updated: October 7, 2026. Written by Jeremy Jarvis.

What pCi/L means

Radon is reported in picocuries per liter of air, written pCi/L. It is a measure of radioactive decay rather than of volume or weight. One picocurie per liter means roughly two radioactive decays each minute in every liter of air.

The gas itself is not the main hazard. Radon decays into fine radioactive particles, and those particles are what lodge in lung tissue. Consequently, exposure is about how much you breathe and for how long, not about a single moment.

You may also see becquerels per cubic meter, written Bq/m3, used internationally. EPA gives the equivalent directly: 4 pCi/L is 150 Bq/m3.

EPA’s action level, and the band below it

EPA sets two reference points rather than one.

First, the action level. EPA recommends homes be fixed if the radon level is 4 pCi/L (150 Bq/m3) or more. Second, the consider-fixing band. Because there is no known safe level of exposure to radon, EPA also recommends that Americans consider fixing their home for radon levels between 2 pCi/L and 4 pCi/L (75 to 150 Bq/m3).

That second recommendation is the one most articles leave out, and it changes how you read a middling result.

Reading your radon level band by band

Notice that the advice never becomes “do nothing forever”. Even a low reading is a reading taken under particular conditions, in a particular season.

Why there is no safe radon level

CDC is explicit: there is no known safe level of radon. EPA frames its guidance the same way, which is precisely why the 2 to 4 pCi/L band exists at all.

This surprises people, because most household thresholds work as pass or fail. Radon does not. Risk falls as the concentration falls, and it keeps falling below the action level too.

CDC therefore advises aiming for the lowest radon level you can achieve in your home, because doing so reduces your risk of harmful health effects, including lung cancer. In practice that means treating 4 pCi/L as the point where action becomes strongly advised, not as the point where risk begins.

The averages worth knowing

Two figures give your result meaning.

• About 1.3 pCi/L is the average indoor radon concentration for America’s homes, according to EPA.
• About 0.4 pCi/L is the average concentration of radon in outdoor air, which EPA notes is roughly one tenth of the 4 pCi/L action level.

The outdoor figure matters because it is the practical floor. No mitigation system makes indoor air cleaner than the air outside, so a target below roughly 0.4 pCi/L is not achievable.

The indoor average matters for scale. EPA states that it based its estimate of 21,000 radon-related lung cancers a year on that national average indoor level. In other words, the 21,000 figure is not driven by rare extreme houses; it comes from ordinary ones.

Radon levels and smoking

CDC describes radon as the second leading cause of lung cancer after smoking. The two risks also combine, which is why guidance treats a smoking household as a stronger reason to act at a given level.

We are not going to put numbers on your personal risk here, because that depends on your level, your years of exposure and your smoking history. If you want that conversation, have it with a clinician rather than a website.

Why two tests of the same house disagree

Radon levels are not a fixed property of a building. They move.

1. Season. Closed, heated houses in winter usually read higher than open houses in summer.
2. Weather. Barometric pressure, wind and rain all change how much soil gas is drawn indoors.
3. Soil moisture. Saturated ground can cap the soil and push gas toward the house.
4. Test conditions. A window left open during a short-term test lowers the reading substantially.
5. Placement. A detector in the lowest lived-in level reads differently from one upstairs.
6. Test length. Two-day tests sample a single weather window, while 90-day tests average many.

None of this makes testing unreliable. It simply means one short-term number near the action level deserves a second opinion before you spend four figures.

Short-term and long-term radon levels

A short-term test, typically two to seven days, answers the question quickly. Long-term tests run for more than 90 days and describe what you actually breathe across seasons.

Use the short test to screen and the long test to decide, unless the short result is clearly high. Where a reading lands above 8 pCi/L, waiting 90 days to confirm makes little sense. In that case, confirm quickly and act.

Our radon test kit guide covers which kit suits which situation, and how to test for radon walks through running one properly.

Why “safe radon levels” is a misleading phrase

People search for safe radon levels constantly, and the phrase sets up the wrong mental model. It implies a boundary with safety on one side.

Radiation risk does not work that way. CDC states there is no known safe level of radon, so what people call safe radon levels are really just lower-risk levels. EPA’s own structure makes the point: if 4 pCi/L were a safety line, there would be no reason to recommend considering a fix at 2 pCi/L.

A more useful question than “are my radon levels safe” is “how much lower could they reasonably be, and what would that cost”. That question has an answer. The first one does not.

So when you see a chart of acceptable radon levels online, read it as a guide to urgency rather than permission to stop thinking.

Radon levels by state and why maps do not decide it

State and county radon maps are widely shared, and they are genuinely useful for awareness. They are not useful for deciding about your house.

The reason is scale. A zone map describes a regional tendency across thousands of square miles. Radon entry, by contrast, depends on what is directly beneath your slab: soil composition, how water moves underground, foundation type, and the number and size of cracks and service penetrations.

Ventilation and heating patterns matter too. A tightly built house holds soil gas longer than a draughty one. Consequently, two neighbouring houses of similar age can differ several-fold.

MDH makes the practical point for its own state: it recommends that every home be tested, and notes that a radon test is the only way to know how much radon is in your home. The same logic applies wherever you live, including in a low zone.

What actually drives radon levels in your house

Five things explain most of the variation between homes.

1. Source strength. Radon comes from the natural decay of uranium and radium in rocks and soil, so the ground itself sets the ceiling.
2. Entry routes. CDC notes radon gets in through small cracks or holes. Slab cracks, the perimeter joint, service penetrations, open sump pits and dirt crawl spaces are the usual paths.
3. Pressure difference. Warm air rising indoors slightly lowers pressure at floor level, which draws soil gas upward.
4. Air exchange. How often indoor air is replaced dilutes whatever enters.
5. How you use the space. A finished basement bedroom exposes you far more than an unused cellar at the same concentration.

Notice that only the first is fixed. The other four are things a mitigation system, or even better ventilation, can change.

Radon levels in apartments and upper floors

Radon is usually highest in the lowest level, because that is closest to the source. As a result, upper-floor apartments often read lower than ground-floor ones.

Lower does not mean zero, though. CDC states that radon can build up in any home or building, whether it has a basement, is sealed or drafty, or is new or old. Some building materials also release low levels.

If you rent, test the space you actually live in rather than relying on a building-wide figure. If you own a ground-floor or garden-level unit, treat it like a house and test properly.

Comparing your level with a neighbour

Comparing readings over the fence is natural, and it misleads more often than it helps. Soil composition changes across a single street, and so does the path water takes underground.

Foundation detail matters just as much. One house may sit on four inches of clean gravel that vents freely. Next door may have a slab poured straight onto clay, with a different crack pattern and a different sump arrangement.

Ventilation completes the picture. Two houses with identical soil gas entry can hold very different indoor concentrations if one is airtight and the other leaks.

So treat a neighbour’s number as a reason to test, never as a substitute for testing. It is common for adjacent homes to differ by a factor of three or more, and nobody can tell which way round from the street.

What a good radon report should contain

Whether the test came from a kit or a professional, the paperwork should let someone else check your reasoning later.

• The measured result in pCi/L, stated plainly.
• The test type, whether short-term passive, long-term passive or continuous.
• Start and end dates, and the total hours the detector was exposed.
• The exact placement: which level of the home, which room, and the height.
• Whether closed-house conditions were kept, and any windows opened.
• The laboratory name, and its certification or participation in a proficiency programme.

Keep that record with your house documents. When you retest in two years, the comparison is only meaningful if you know the first test was run the same way.

Finally, resist rounding in your own favour. A result of 3.9 pCi/L is not “about 3”, and 4.1 pCi/L is not “basically 4”. People searching for safe radon levels often do this unconsciously, and it is the one bias worth guarding against when the number lands near the action level.

How often to retest

Treat a radon result as having a shelf life.

• Every two years as routine, even after a reassuring number.
• After foundation repair, a new slab, underpinning or major drainage work.
• Once you finish a basement, because an unused space becomes a lived-in one.
• When a mitigation system is installed, replaced or serviced.
• Before buying or selling, since the result forms part of the transaction.
• If your heating, ventilation or windows change substantially.

Each of those changes either the entry routes, the pressure balance or your exposure time. Any of the three moves the number.

What to do with your number

• Write down the result, the test type, the dates and the weather during the test.
• Compare it with 4 pCi/L first, then with the 2 to 4 pCi/L band, then with the 1.3 pCi/L average.
• Confirm anything at or near the action level with a second test.
• If the confirmed level is 4 pCi/L or above, contact a state-certified or nationally certified mitigation contractor.
• If it sits between 2 and 4 pCi/L, run a long-term test and weigh a fix, especially if you sleep at that level.
• Keep the paperwork. Buyers, sellers and future contractors will all ask.

Radon is unusual among household hazards: it is invisible, it is measurable, and the fix is well understood. Once you know your level, our guide to radon mitigation systems explains what reducing it involves.

Frequently asked questions about radon levels

What is a safe radon level?

There is not one. CDC states there is no known safe level of radon, and EPA takes the same position. What exists is an action level: EPA recommends homes be fixed at 4 pCi/L or higher, and recommends considering a fix between 2 pCi/L and 4 pCi/L. Lower is always better.

What radon level is dangerous?

EPA’s action level of 4 pCi/L is the point at which it recommends fixing the home. That is a risk-management threshold rather than a line between safe and unsafe. Because risk accumulates with time and concentration, a moderate level in a room you sleep in every night matters more than the same level in a store room.

What is the average radon level in a home?

EPA gives the average indoor radon concentration for America’s homes as about 1.3 pCi/L. The average concentration in outdoor air is about 0.4 pCi/L, roughly one tenth of the 4 pCi/L action level. Those two numbers are useful for placing your own result.

Is 3 pCi/L bad?

It sits in EPA’s consider-fixing band of 2 to 4 pCi/L, and it is more than twice the national indoor average. It does not trigger EPA’s fix recommendation, but it is not a clean bill of health either. A long-term test is a sensible next step before deciding.

Do radon levels change through the year?

Yes. Levels move with weather, soil moisture and how the house is heated and ventilated. Winter readings are often higher because the house is closed and warm air rising indoors draws soil gas upward. That is why a long-term test describes your exposure better than a single short one.

Does a low reading mean I never need to test again?

No. Retest every couple of years, and after foundation work, a new slab, or finishing a basement. Any change to the floor, the drainage or how you use the lowest level can change the result.

Sources and how we researched this guide

This guide is research-based. Every level, average and health statement is quoted from EPA, CDC or the Minnesota Department of Health. We do not interpret your individual result or give medical advice; mitigation is work for a certified contractor. Found something out of date? Tell us and we will check it against the source.

• EPA: What is EPA’s action level for radon and what does it mean?
• EPA: Health risk of radon
• CDC: About radon

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