Radon Level Measurement: pCi/L vs Bq/m3 Explained
Radon level measurement is reported in different units depending on the country and the device, and the difference is meaningful: a number that looks alarming in one unit may be normal in another, and a number that looks acceptable in one threshold framework may exceed action levels in another. This guide explains pCi/L versus Bq/m³, the EPA versus WHO threshold frameworks, the math to convert between them, and what specific concentration ranges actually mean for homeowner decisions. The content summarizes EPA, CDC, and WHO guidance current as of 2026; it is informational, not a substitute for a certified measurement professional.
The two units that appear in radon reports
Picocuries per liter (pCi/L) is the US measurement standard, used by EPA and on most US-purchased consumer test kits. Becquerels per cubic meter (Bq/m³) is the SI metric standard, used by WHO, most European agencies, and Canada’s national radon program. Conversion: 1 pCi/L equals 37 Bq/m³, and 1 Bq/m³ equals 0.027 pCi/L. The EPA action threshold of 4 pCi/L equals 148 Bq/m³.
Why the units differ
Picocuries refer to activity — the rate of radioactive decay — based on the historical curie unit (3.7 × 10¹⁰ decays per second). Becquerels are the SI equivalent, defined as one decay per second. Both units measure the same physical quantity: radioactive decay rate per volume of air. The pCi/L unit predates SI adoption and remained dominant in US regulatory practice; Bq/m³ became standard internationally after SI harmonization.
EPA versus WHO action thresholds
EPA recommends mitigation at 4.0 pCi/L (148 Bq/m³) and consideration of mitigation between 2.0 and 4.0 pCi/L (74-148 Bq/m³). WHO’s reference level is 2.7 pCi/L (100 Bq/m³), and Canada’s national guideline is 5.4 pCi/L (200 Bq/m³). The differences reflect different policy choices about acceptable risk levels and feasibility of widespread mitigation, not different scientific understanding of dose-response.
Quick conversion math
To convert pCi/L to Bq/m³, multiply by 37. To convert Bq/m³ to pCi/L, divide by 37 (or multiply by 0.027). Common conversions: 2 pCi/L equals 74 Bq/m³ (WHO close to action). 4 pCi/L equals 148 Bq/m³ (EPA action). 5 pCi/L equals 185 Bq/m³. 10 pCi/L equals 370 Bq/m³. 20 pCi/L equals 740 Bq/m³. The math is straightforward but easy to forget mid-conversation with a contractor or surveyor.
What different concentration ranges mean for homeowners
Below 2 pCi/L (74 Bq/m³): low concentration, no immediate action. Retest in 2 to 5 years or after major renovation. 2 to 4 pCi/L (74-148 Bq/m³): borderline. EPA recommends considering mitigation, especially in homes with smokers, young children, or pregnant occupants. WHO would call for action in this range. 4 to 10 pCi/L (148-370 Bq/m³): elevated. Mitigation recommended under EPA framework. Above 10 pCi/L (370 Bq/m³): high. Mitigation strongly recommended without delay.
Why a single number is not the whole story
Radon concentrations fluctuate daily with weather, HVAC operation, and seasonal changes. A 48-hour short-term test captures a snapshot that may run higher or lower than the home’s true annual average. EPA’s recommended pathway for borderline results (2 to 6 pCi/L) is a follow-up long-term test (alpha-track over 90+ days) or a second short-term test, with mitigation triggered if both confirm elevated levels. For more on how the test types compare, see our charcoal vs alpha-track vs CRM comparison.
Measurement uncertainty matters
Every radon measurement carries an uncertainty band, typically ±10 to 20 percent for short-term tests and ±5 to 10 percent for long-term integrated alpha-track readings. A result of 4.2 pCi/L with ±20 percent uncertainty could represent a true concentration anywhere from 3.4 to 5.0 pCi/L. Homeowners should read the uncertainty figure on the test report, not just the central value, when interpreting a borderline result.
How long-term integrated readings compare to short-term snapshots
An alpha-track detector deployed for 12 months gives an annual-average concentration that smooths seasonal variation. A 48-hour charcoal test deployed in January typically reads higher than the annual average because winter heating amplifies the stack effect that pulls soil gas into the home. The same home tested in July often reads lower. Both readings are valid measurements of different time windows; the long-term integrated reading is the better representation of cumulative exposure.
Continuous radon monitor reports
CRMs produce hourly concentration data across the deployment period and report an average plus a graph of the time series. The graph is informative — a flat curve at 6 pCi/L suggests steady-state soil-gas entry, while a sawtooth curve oscillating between 2 and 12 pCi/L suggests intermittent driving forces like HVAC cycling or barometric pressure swings. The average is the headline number but the graph tells the operational story.
Reading a European or Canadian report in the US context
If a homeowner receives a report from a European or Canadian lab, the number will be in Bq/m³. To translate to EPA’s framework: 150 Bq/m³ equals approximately 4 pCi/L (EPA action). 200 Bq/m³ equals approximately 5.4 pCi/L (Canada action). 300 Bq/m³ equals approximately 8.1 pCi/L (well into EPA mitigation territory). 500 Bq/m³ equals approximately 13.5 pCi/L (high concentration regardless of framework).
What the lung-cancer risk modeling actually shows
EPA’s BEIR VI risk model estimates lifetime lung-cancer risk increases by approximately 6 to 16 percent per 1 pCi/L (37 Bq/m³) of long-term exposure, depending on smoking status. The risk is essentially linear with no safe lower bound — there is no concentration at which radon poses zero risk, only concentrations at which the marginal risk is small enough to accept. The 4 pCi/L action threshold is a policy choice that balances feasibility against incremental risk, not a “safe” line. For broader context on radon health risk, see our Front Range radon testing hub.
How units appear on consumer devices
US-made consumer devices (Airthings Wave, Corentium Home, AccuStar charcoal kits) report in pCi/L by default, often with a Bq/m³ toggle in the device settings. European-made devices may default to Bq/m³ with a pCi/L toggle. Lab reports from US labs are in pCi/L; lab reports from non-US labs are usually in Bq/m³. Homeowners should confirm the unit before interpreting any specific number.
Combining short-term and long-term measurements
A common Colorado pattern: 48-hour CRM during a real-estate transaction shows 6.2 pCi/L. The buyer purchases. Post-purchase, the homeowner installs a long-term alpha-track for 12 months. The annual-average alpha-track reads 4.8 pCi/L. Both are correct measurements of different time periods. The annual-average reading is the better basis for long-term mitigation decisions; the 48-hour snapshot was the appropriate basis for the time-constrained transaction.
Why winter and summer tests can disagree
Winter heating creates a stack effect — warm air rises through the home, pulling cold air (and soil gas) in through foundation penetrations. Concentrations rise. Summer cooling reverses some of this, and open windows during shoulder seasons dilute indoor concentrations. A home that reads 8 pCi/L in January may read 3 pCi/L in July. The annual-average alpha-track integrates over both and gives the more decision-useful number.
Mitigation system performance reading
After a sub-slab depressurization system is installed, post-mitigation testing measures the system’s effectiveness. EPA recommends a post-mitigation short-term test within 30 days to confirm levels drop below 4 pCi/L. Many homeowners leave a consumer continuous monitor in place for the first year to track post-installation performance. A correctly designed and installed system typically reduces concentrations by 80 to 99 percent from pre-mitigation levels.
When to call a professional vs. DIY measurement
DIY for general homeowner screening, baseline assessment, or post-mitigation surveillance. Hire a certified AARST-NRPP or NRSB professional for any real-estate transaction, any lender-required test, any pre-mitigation assessment that will inform system design, and any test where the result will be cited in a disclosure or insurance context. The unit choice (pCi/L versus Bq/m³) does not change the recommendation; the certified professional’s report works equally well in either unit framework.
References
- EPA Health Risk of Radon — U.S. Environmental Protection Agency
- CDC Radon Information — Centers for Disease Control and Prevention
- CDPHE Radon Program — Colorado Department of Public Health and Environment
- American Lung Association Radon Page — American Lung Association
Front Range homeowners weighing measurement and mitigation can reach out through our contact page for a vetted certified provider.