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Safe Radon Levels: What Your Radon Test Number Means

No radon level is known to be completely safe, but the EPA sets 4 pCi/L (about 150 Bq/m³) as the level at which to fix a home, and suggests considering a fix between 2 and 4 pCi/L. The average US home is about 1.3 pCi/L; outdoor air is about 0.4. Decide on a long-term or averaged result, not one short test.

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The numbers

Radon levels at a glance

  • EPA: fix at 4 pCi/L (150 Bq/m³) or more.
  • EPA: consider fixing between 2 and 4 pCi/L.
  • WHO reference level: 100 Bq/m³, about 2.7 pCi/L.
  • Health Canada: 200 Bq/m³, about 5.4 pCi/L.
  • Average US home 1.3 pCi/L; outdoor air 0.4.

Last reviewed: October 9, 2026

Result interpreter

What does your radon number mean?

Enter the number on your report, the kind of test, where it sat and why you tested. You get EPA's next step for that situation, the value in both units, EPA's risk figures for the nearest levels and where the number sits among real home tests. The interpreter never adjusts your reading.

3.9 pCi/L = 144 Bq/m³ · 2 to just under 4 pCi/L

Below EPA's action level, in the 2–4 range where EPA suggests considering a fix.

EPA's next step

  • A short-term test is less likely than a long-term test to show your year-round average, because radon varies day to day and season to season.
  • A long-term test (more than 90 days, ideally including the heating season) settles whether you are above or below 4.
  • Close to 4: EPA's example is that when two short-term tests average 4.1 pCi/L, there is about a 50% chance the year-round average is somewhat below 4. EPA also says no level of radon is safe.
  • Basement readings are usually the highest in a home. EPA tests the lowest lived-in level: the basement if it is used regularly, otherwise the first floor.

Other guidelines

144 Bq/m³ is at or above the WHO reference level of 100 Bq/m³ (about 2.7 pCi/L) and below Health Canada's 200 Bq/m³ guideline (about 5.4 pCi/L), which applies to an annual average.

EPA lifetime estimate (per 1,000 people)

  • At 2 pCi/L: about 4 never smoked · about 32 smokers
  • At 4 pCi/L: about 7 never smoked · about 62 smokers

EPA's table rows nearest your number, not a personal risk.

Pennsylvania comparison

31.7% of 811,755 Pennsylvania home tests (2016–2025) read higher than 3.9 pCi/L. A comparison with other homes in one high-radon state, not a risk measure; EPA's guidance above applies either way.

Runs in your browser; nothing you enter is sent anywhere. This is general information from EPA guidance, not medical advice.

Chart

Radon levels chart

US reports give radon in picocuries per liter of air (pCi/L); most other countries use becquerels per cubic meter (Bq/m³). One pCi/L is exactly 37 Bq/m³, so EPA's 4 pCi/L is 148 Bq/m³, which EPA rounds to 150.

LevelWho sets it / what it isNever smoked, per 1,000Smokers, per 1,000PA tests at or above
20 pCi/L · 740 Bq/m³EPA risk tableAbout 36About 2604.6%
8 pCi/L · 300 Bq/m³Twice EPA's action levelAbout 15About 12014.8%
5.4 pCi/L · 200 Bq/m³Health Canada guideline (annual average)––23.2%
4 pCi/L · 150 Bq/m³EPA action level: fix the homeAbout 7About 6231.7%
2.7 pCi/L · 100 Bq/m³WHO reference level––46.7%
2 pCi/L · 75 Bq/m³EPA: consider fixing between 2 and 4About 4About 3259.3%
1.3 pCi/L · 50 Bq/m³Average US indoor level (EPA)About 2About 20–
0.4 pCi/L · 15 Bq/m³Average outdoor level (EPA)Not listedAbout 3–

Risk figures: EPA lifetime estimates of lung cancer per 1,000 people exposed over a lifetime (EPA 402-R-03-003). "PA tests at or above" uses 811,755 Pennsylvania residential tests, 2016–2025.

Radon levels chart: EPA, WHO and Health Canada levels on one scaleA vertical scale from 0 to 10 picocuries per liter, with becquerels per cubic meter alongside (1 pCi/L = 37 Bq/m³). Marked levels: 0.4 average outdoor air; 1.3 average US home; 2 EPA consider fixing; 2.7 WHO reference level of 100 Bq/m³; 4 EPA action level, about 150 Bq/m³; 5.4 Health Canada guideline of 200 Bq/m³; 8 twice the EPA action level. Readings above 10 continue off the top of the scale.Bq/m³pCi/L10+ continues00274414862228296103708 · twice the action level5.4 · Health Canada (200 Bq/m³)4 · EPA action level: fix2.7 · WHO reference (100 Bq/m³)2 · EPA: consider fixing1.3 · average US home0.4 · average outdoor air
The same levels on one scale. Bands shade from neutral below 2 to deeper amber above 8. Health Canada and WHO set their levels in Bq/m³; the pCi/L values are conversions.

Why the numbers differ

Why EPA says 4, WHO says 2.7 and Canada says 5.4

All three agree on the science: there is no known safe level, and risk rises with long-term exposure. WHO estimates lung cancer risk rises about 16% for every 100 Bq/m³ increase in long-term average radon. The different numbers are policy choices about where to recommend action.

EPA says fix at 4 pCi/L and consider fixing from 2 to 4, noting that most homes today can be reduced to 2 pCi/L or below. WHO recommends a national reference level of 100 Bq/m³, and no more than 300 Bq/m³ where that is not achievable. Health Canada sets 200 Bq/m³ as an annual average and asks for a test of at least three months, ideally in fall or winter. Each is applied to a long-term average, not a single short reading.

Health risk

Lung cancer risk by radon level

Radon levelNever smoked: per 1,000Smoked: per 1,000EPA says
20 pCi/L (740 Bq/m³)About 36About 260Fix your home
10 pCi/L (370 Bq/m³)About 18About 150Fix your home
8 pCi/L (300 Bq/m³)About 15About 120Fix your home
4 pCi/L (150 Bq/m³)About 7About 62Fix your home
2 pCi/L (75 Bq/m³)About 4About 32Consider fixing between 2 and 4 pCi/L
1.3 pCi/L (50 Bq/m³)About 2About 20Average indoor radon level
0.4 pCi/L (15 Bq/m³)Not listedAbout 3Average outdoor radon level

EPA, Health Risk of Radon. Lifetime risk of lung cancer death per 1,000 people exposed over a lifetime, from EPA's Assessment of Risks from Radon in Homes (EPA 402-R-03-003). EPA notes a former smoker's risk may be lower than a current smoker's, and higher than a never-smoker's.

EPA's best estimate is about 21,000 lung cancer deaths a year in the US from radon (uncertainty range 8,000 to 45,000), about 2,900 of them among people who never smoked. These are population estimates for a lifetime of exposure, not a forecast for any one person. Smoking multiplies radon's effect, which is why the two columns differ so much.

Real data

Where your number sits: 811,755 real home tests

Pennsylvania's Department of Environmental Protection publishes every residential radon result reported to it. Of 811,755 home tests started from 2016 to 2025, 31.7% read 4 pCi/L or more and 59.3% read 2 or more. A 3.9 reading was higher than about 68%of them, yet it still sits in EPA's "consider fixing" range: where a number ranks says nothing about whether to act on it.

Nationally, labs reported 2,013,544 home tests to the CDC for 2008–2017: 35.7% were 4 or more and 24.7% were 2 to 4. To see how common high results are where you live, look up radon levels by zip code.

Distribution of Pennsylvania home radon test results0–1 pCi/L: 15.4%; 1–2 pCi/L: 25.3%; 2–3 pCi/L: 16.9%; 3–4 pCi/L: 10.8%; 4–6 pCi/L: 11.0%; 6–8 pCi/L: 5.8%; 8–10 pCi/L: 3.5%; 10–20 pCi/L: 6.7%; 20+ pCi/L: 4.6%0–1 pCi/L15.4%1–2 pCi/L25.3%2–3 pCi/L16.9%3–4 pCi/L10.8%4–6 pCi/L11.0%6–8 pCi/L5.8%8–10 pCi/L3.5%10–20 pCi/L6.7%20+ pCi/L4.6%
Share of 811,755 Pennsylvania residential radon tests (2016–2025) in each range, pCi/L. Amber bars are at or above EPA's 4 pCi/L action level. Source: PA DEP, data.pa.gov.

Variation

Why two radon tests on the same house disagree

EPA says radon levels tend to vary from day to day and season to season, which is why a short test is less likely than a long one to show your year-round average. Pennsylvania's data shows how large the swings can be.

Pennsylvania basement radon tests at 4 pCi/L or more, by start monthJanuary: 37.8% of 59,912 tests; February: 35.3% of 63,894 tests; March: 33.7% of 70,407 tests; April: 33.9% of 62,210 tests; May: 34.1% of 63,033 tests; June: 29.3% of 63,246 tests; July: 24.9% of 58,978 tests; August: 29.5% of 58,015 tests; September: 36.3% of 52,709 tests; October: 39.6% of 56,972 tests; November: 37.4% of 50,677 tests; December: 35.1% of 41,541 testsJanuary37.8%59,912February35.3%63,894March33.7%70,407April33.9%62,210May34.1%63,033June29.3%63,246July24.9%58,978August29.5%58,015September36.3%52,709October39.6%56,972November37.4%50,677December35.1%41,541
Basement tests at or above 4 pCi/L by the month the test started, Pennsylvania 2016–2025. Different homes each month, not the same house retested.
Pennsylvania radon tests at 4 pCi/L or more, by floorBasement: 33.8% of 701,594 tests; First floor: 21.4% of 59,415 tests; Slab on grade: 17.5% of 23,207 tests; Crawl space: 13.2% of 7,692 tests; Second floor: 13.8% of 4,449 testsBasement33.8%n=701,594First floor21.4%n=59,415Slab on grade17.5%n=23,207Crawl space13.2%n=7,692Second floor13.8%n=4,449
Tests at or above 4 pCi/L by the floor tested, Pennsylvania 2016–2025, with the number of tests. Different homes on each floor.
  • Season. 39.6% of basement tests started in October were 4 or more, and 37.8% in January, against 24.9% in July. A common explanation is a closed-up, heated house drawing more soil gas in during the cooler months.
  • Floor. 33.8% of basement tests were 4 or more, against 13.8% on second floors. Radon enters from the soil, so the lowest level usually reads highest.
  • Test length.A 2-day test catches that week's weather; a 90-day-plus test averages it out.
  • Side by side or back to back.EPA's home-sale guide explains the trade-off: two tests at the same time check precision, while two in sequence capture more of the variation over time.
  • Near 4.EPA's example: two short tests averaging 4.1 pCi/L leave about a 50% chance the year-round average is somewhat below 4.
  • Placement. Kits in a kitchen, bathroom, draft or by an open window, or run in a storm or high winds, can mislead.

The practical rule: average your results rather than keeping the one you like, and decide on a long-term test or the average of paired tests.

Testing

Short-term vs long-term radon tests, and when to retest

Short-term tests stay in the home 2 to 90 days; long-term testsrun more than 90 days and are more likely to reflect the year-round average. EPA's three steps are in the diagram.

Place the kit in the lowest lived-in level (the basement if it is used regularly, otherwise the first floor), in a room used regularly but not a kitchen or bathroom, at least 20 inches above the floor. For a 2- or 3-day test, close windows and outside doors at least 12 hours before starting. Retest after any fix, then every two years, and test again if you start using a lower level.

EPA radon testing steps: short-term test, follow-up, decide, retestStep 1: take a short-term test of 2 to 90 days. If the result is below 4 pCi/L, consider fixing between 2 and 4, and test again in the future. If it is 4 or higher, Step 2: take a follow-up test, a long-term test for a better year-round picture, or a second short-term test if you need results quickly or the first result was above 8. Step 3: fix the home if the long-term result is 4 or more; consider fixing if the average of the two short-term tests is 4 or more. After a fix, retest, then every two years.Step 1 · Short-term test2 to 90 days, lowest lived-in levelbelow 44 or higherBelow 4 pCi/L2–4: consider fixingBelow 2: no actionTest again in the futureStep 2 · Follow upLong-term test (>90 days)or a second short-termtest now if first > 8Step 3 · DecideLong-term 4 or more: fixTwo short tests average4 or more: consider fixingAfter a fixRetest to confirm, then every 2 yearsHigher short-term results make the case forfixing more certain (EPA). Act on averages.
EPA's testing steps from A Citizen's Guide to Radon (EPA 402/K-12/002).

Home sales

Radon results in a home sale

EPA's real-estate guide allows three approaches, each for at least 48 hours in the lowest level a buyer could use: two short-term tests at the same time, two back to back, or a continuous monitor. If the average is 4 pCi/L or more, EPA recommends fixing the home. EPA also describes steps to prevent tampering with the test. Some states require radon disclosure; check your state's radon disclosure rule. For typical costs and who pays for radon mitigation in a sale, see the calculator.

Next steps

If your result is high

The usual fix is sub-slab depressurization: a pipe through the floor and a fan that draws soil gas from under the slab and vents it above the roof. Sealing alone is not a fix, because EPA says it has not been shown to lower radon significantly or consistently, though sealing floor cracks and a sealed sump lid help a system work. Exhaust-only basement ventilation can depressurize the room and pull more radon in.

Hire a mitigator licensed by your state or certified by a national proficiency program; your state radon office can point you to one. The radon mitigation cost calculator gives planning ranges, not quotes. If part of the house is over a crawl space, our sister site explains whether crawl space encapsulation reduces radon. Homes on a private well can also get radon from water, which is tested separately.

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FAQs

Safe radon levels: common questions

Is a radon level of 2.9, 3.5 or 3.9 high?

Each is below EPA's 4 pCi/L action level but in the 2–4 range where EPA suggests considering a fix, and above the WHO reference level of 100 Bq/m³ (about 2.7 pCi/L). If it came from a short-term test, a long-term test settles where your year-round average sits. The interpreter on this page shows EPA's next step for your exact number.

Is 5.2 or 6.3 a high radon level?

Both are above EPA's 4 pCi/L action level. From a short-term test, EPA says to follow up with a long-term test or a second short-term test, and to fix the home if the long-term result, or the average of the two short tests, is 4 or more. It is not an emergency: the risk comes from years of exposure, so there is time to test properly and compare quotes.

What radon level is alarming or dangerous?

EPA has no separate emergency level. It recommends fixing at 4 pCi/L or more, and says that if a first short-term test is more than twice that (above 8 pCi/L), take a second short-term test immediately. EPA's lifetime risk estimates rise steadily with the level, from about 7 per 1,000 never-smokers at 4 pCi/L to about 36 at 20 pCi/L.

Is it okay to live in a house with radon?

Every house has some radon, and outdoor air averages about 0.4 pCi/L. EPA's advice is to test, fix the home if the result is 4 pCi/L or more, and consider fixing between 2 and 4. EPA says most homes can be brought to 2 pCi/L or below. Radon has no immediate symptoms; ask a doctor about personal health concerns.

What month is radon highest?

Radon varies day to day and season to season, according to EPA, and tends to run higher when a house is closed up for heating. In Pennsylvania's 2016–2025 data, 39.6% of basement tests started in October were 4 or higher, against 24.9% in July. Those are different homes, not the same home retested, so treat it as a pattern, not a rule.

Is it safe to sleep in a basement with radon?

It depends on the measured average where you sleep, not on the room type. EPA tests the lowest lived-in level, so a basement bedroom is the right place to test. If the long-term or averaged result there is 4 pCi/L or more, EPA's advice is to fix the house, which lowers radon throughout it, rather than to stop using the room.

What is 4 pCi/L in Bq/m³?

Exactly 148 Bq/m³, because 1 pCi/L equals 37 Bq/m³; EPA rounds it to 150. Going the other way, the WHO reference level of 100 Bq/m³ is about 2.7 pCi/L and Health Canada's 200 Bq/m³ guideline is about 5.4 pCi/L.

Related pages

Where to go next

Radon Levels by Zip Code

EPA zone and the share of high tests in your county, plus your state radon office.

Read next

Radon Mitigation Cost Calculator

Planning ranges for a system and the fan's running cost.

Read next

Basement Floor Cracks

Which cracks to seal, and why sealing alone is not a radon fix.

Read next

Basement Ventilation

When fans help, and when they pull in soil gas.

Read next

Sources

Sources

Method: Pennsylvania figures are generated by scripts/radon/levels-summary.mjs from data.pa.gov dataset vkjb-sx3k (queried 2026-10-09): building purpose "Residential", test start 2016-01-01 to 2025-12-31, readings from 0 to under 999 pCi/L (811,790 tests before filtering; 35 dropped). The mitigation field is "not collected" for every row. This page is general information drawn from EPA, WHO and Health Canada guidance; it is not medical advice. Last reviewed October 9, 2026.