Radon Unit: pCi/L, Bq/m3, and What Numbers Mean
Radon unit references can be confusing to a homeowner reading a test report for the first time. US radon results are reported in picocuries per liter (pCi/L), while international standards use becquerels per cubic meter (Bq/m³). Older industrial-hygiene literature uses Working Levels (WL) and Working Level Months (WLM) as cumulative exposure metrics. The EPA action level of 4 pCi/L corresponds to 148 Bq/m³, and understanding the conversion between units makes radon literature from any source readable. This guide summarizes EPA, CDC, and CDPHE guidance current as of 2026 and walks through the units, the conversions, and what the numbers actually mean for residential exposure. Consult a certified radon professional for testing and your physician for any health concerns.
What is the standard radon unit in the United States?
The US standard radon unit is picocuries per liter (pCi/L). A picocurie is one-trillionth of a curie, the original unit of radioactive activity named after Marie and Pierre Curie. One curie equals 37 billion radioactive decays per second. A picocurie is therefore 0.037 decays per second, and a picocurie per liter means 0.037 radioactive decays per second occurring in one liter of air.
EPA, CDC, and US state radon programs report indoor radon concentrations in pCi/L. The EPA action level (the indoor concentration at which mitigation is recommended) is 4 pCi/L. The EPA mitigation goal (the level a certified mitigation system should achieve) is typically 2 pCi/L or lower. The Front Range radon testing guide covers what the pCi/L readings actually mean for testing decisions.
The international SI unit: becquerels per cubic meter
The International System of Units (SI) standard for radon concentration is becquerels per cubic meter (Bq/m³). A becquerel is one radioactive decay per second, and Bq/m³ means one decay per second occurring in one cubic meter of air. The Bq/m³ unit is used by the World Health Organization, the European Commission, Health Canada, and most international radon programs outside the United States.
Conversion between pCi/L and Bq/m³ uses a fixed factor: 1 pCi/L equals 37 Bq/m³. The conversion follows from the relationship between the units (a picocurie is 0.037 becquerels) and the volume conversion (1 liter equals 0.001 cubic meters):
- 1 pCi/L = 37 Bq/m³
- 4 pCi/L (EPA action level) = 148 Bq/m³
- 2 pCi/L (EPA mitigation goal) = 74 Bq/m³
- 10 pCi/L = 370 Bq/m³
- 0.4 pCi/L (typical outdoor ambient) = 14.8 Bq/m³
The WHO reference level for indoor radon is 100 Bq/m³, which converts to approximately 2.7 pCi/L. That level is lower than the EPA action level of 4 pCi/L. The difference reflects different agency frameworks for balancing health risk against achievable mitigation outcomes.
Why two different units exist
The US adopted pCi/L for radon reporting before the international shift toward SI units became universal. EPA documents and US state programs have continued using pCi/L for consistency with older measurement data and regulatory thresholds. Most other countries adopted Bq/m³ as the SI-aligned unit. Both units measure the same underlying physical quantity (radioactive activity per volume of air), so conversion is straightforward.
Working Levels (WL) and Working Level Months (WLM)
Working Level (WL) is a cumulative exposure unit developed in the 1950s for uranium-mining safety standards. A Working Level represents the alpha-particle energy emitted by radon decay products in a given volume of air over a given time. One Working Level equals approximately 200 pCi/L of radon in equilibrium with its short-lived decay products.
Working Level Months (WLM) is the cumulative exposure metric: one WLM equals exposure to 1 Working Level for 170 hours (the assumed monthly work-shift duration for an underground miner). Historical uranium-miner exposure standards used WLM as the regulatory metric. Federal occupational exposure limits for uranium miners historically capped cumulative exposure at 4 WLM per year.
Working Levels are still encountered in occupational and historical radon literature but are rarely used in residential testing reports. The conversion is:
- 1 WL ≈ 200 pCi/L (assuming standard equilibrium between radon and its decay products)
- 1 WLM = 1 WL × 170 hours (cumulative exposure dose)
- 4 WLM = annual occupational exposure limit for uranium miners (historical)
The residential context uses pCi/L because the exposure duration is the entire year (8,760 hours) rather than a 170-hour work shift. A typical residential annual exposure at 4 pCi/L equates to approximately 0.5 WLM per year, well below historical occupational limits but accumulating over decades.
Cumulative exposure units: pCi/L-years
Residential lifetime exposure is sometimes expressed in pCi/L-years, calculated as the average annual indoor radon concentration multiplied by the number of years of exposure. A home averaging 4 pCi/L for 30 years produces a cumulative exposure of 120 pCi/L-years. EPA risk estimates for residential lung-cancer mortality use pCi/L-years as the underlying metric.
The pCi/L-years framing matters because radon-induced lung cancer risk accumulates with total exposure over time, not just peak exposure during any single year. A home that averages 8 pCi/L for 10 years produces the same cumulative exposure (80 pCi/L-years) as a home averaging 2 pCi/L for 40 years, even though the year-by-year concentration differs substantially.
How EPA risk estimates use pCi/L-years
EPA’s residential lung-cancer risk model is calibrated to lifetime exposure rather than peak concentration. At 4 pCi/L over 70 years of exposure (approximately 280 pCi/L-years), EPA estimates a lung-cancer mortality risk of approximately 7 deaths per 1,000 people for non-smokers and approximately 62 deaths per 1,000 people for current smokers. The dramatic smoker-versus-nonsmoker difference reflects the multiplicative interaction between radon exposure and tobacco smoke.
How to read a typical radon test report
A short-term radon test report (typically 48 to 96 hours of measurement) reports an average concentration in pCi/L over the test period, plus any minimum and maximum values from the data log if a continuous radon monitor was used. The report compares the average to the EPA action level of 4 pCi/L and provides a recommendation: no action needed (below 2 pCi/L), consider follow-up testing (2 to 4 pCi/L), or mitigation recommended (above 4 pCi/L).
Long-term radon tests (90 days to 1 year of measurement) report an average concentration over the test period and are generally considered more representative of typical exposure than short-term tests because they smooth out the day-to-day variation driven by weather, HVAC operation, and seasonal changes. Looking at the long-term radon test kit guide covers the trade-off between short-term and long-term testing approaches.
How indoor concentrations compare to outdoor levels
Outdoor ambient radon levels in the United States typically average 0.4 pCi/L (about 15 Bq/m³). Outdoor radon disperses quickly into the atmosphere and rarely accumulates above ambient levels. Indoor radon accumulates because the home traps soil gas and limits dilution with outdoor air. The indoor-to-outdoor ratio in homes with elevated radon can reach 10-to-1 or higher.
The EPA action level of 4 pCi/L represents approximately 10 times the typical outdoor ambient. Homes with indoor radon at 10 pCi/L or higher represent 25 times the ambient level. Mitigation systems aim to bring indoor concentrations close to (though usually slightly above) the outdoor ambient by intercepting soil gas before entry and exhausting it outdoors.
How accuracy and uncertainty appear in test results
Every radon measurement carries some statistical uncertainty. Short-term tests of 48 hours typically have a measurement uncertainty of plus or minus 25 to 35 percent at concentrations near the action level. A short-term test reading 4.5 pCi/L could plausibly represent a true value between 3.0 pCi/L and 6.0 pCi/L. That uncertainty range is why most radon professionals recommend confirming a borderline initial result with a second short-term test or a long-term test before deciding on mitigation. A clearly elevated result (8 pCi/L or higher) is much less likely to be a false-positive measurement and warrants prompt mitigation planning.
Continuous radon monitors used by certified measurement professionals report measurement uncertainty alongside the average concentration. A reading of 5.2 pCi/L plus or minus 0.4 pCi/L is reasonably confident; a reading of 5.2 pCi/L plus or minus 1.8 pCi/L is far less so. Homeowners reading their first test report should look for the uncertainty range as part of evaluating whether the result is decisive or warrants confirmatory testing.
How temperature and humidity affect radon measurement
Radon measurements are reported in concentration units that assume standard conditions. Temperature and humidity inside the home can affect both the underlying concentration (through changes in air exchange rate and soil-gas migration) and the response of certain measurement devices. Continuous radon monitors typically include temperature and humidity sensors to support quality-control checks during the measurement period. Alpha-track and charcoal-canister tests are less sensitive to ambient conditions but require closed-house conditions during the measurement to produce accurate results. CDPHE and EPA both publish closed-house protocols that homeowners and inspectors follow during testing.
What other countries use as action levels
Different countries set different action levels based on local regulatory frameworks and risk-benefit assessments:
- United States (EPA) — 4 pCi/L (148 Bq/m³) action level
- World Health Organization — 100 Bq/m³ (2.7 pCi/L) reference level
- European Union — 300 Bq/m³ (8.1 pCi/L) reference level for new and existing dwellings
- Health Canada — 200 Bq/m³ (5.4 pCi/L) guideline
- United Kingdom (UKHSA) — 200 Bq/m³ (5.4 pCi/L) action level
The differences reflect each country’s regulatory framework and the technical and economic feasibility of widespread mitigation. The WHO reference level is the lowest because it focuses purely on health-risk minimization without explicit cost-benefit constraints.
Putting the units to work in practical decisions
Homeowners reading a radon test report from any source can use the unit framework to make practical decisions quickly. A US test result in pCi/L compared to the EPA action level of 4 pCi/L tells whether mitigation is recommended. A European or international result in Bq/m³ compared to the local action level (300 Bq/m³ in the EU, 200 Bq/m³ in Canada, 100 Bq/m³ for the WHO reference) tells the same story in different units. Converting between units takes one multiplication or division by 37, which most homeowners can do with a calculator in seconds.
Specialty units like Working Levels and Working Level Months are mostly historical artifacts that homeowners will not encounter on a typical residential test report. Cumulative pCi/L-years exposure metrics matter for understanding long-term risk but rarely appear on individual test reports. The two units that matter for most residential decisions are pCi/L (US) and Bq/m³ (international), and both convert cleanly with the 37-to-1 factor. The Colorado-specific radon testing guidance covers how pCi/L readings translate into local CDPHE-aligned recommendations for Front Range homeowners.
Common confusion points in unit reading
A few specific reading mistakes recur often enough that they deserve attention. Some test reports list results in pCi/L but omit the slash, writing “pCiL” or “pCi per L,” which can confuse first-time readers. A few older reports list results in becquerels per liter (Bq/L) rather than per cubic meter; the conversion is 1 Bq/L equals 1,000 Bq/m³ equals roughly 27 pCi/L, so a Bq/L number is a thousand times larger than the equivalent Bq/m³ number. Reading the unit carefully before drawing conclusions prevents misinterpretation, especially when comparing results across testing methods or sources.
References
- EPA radon health risk and measurement units — US Environmental Protection Agency
- CDPHE radon measurement guidance for Colorado — Colorado Department of Public Health and Environment
- CDC radon exposure and measurement overview — Centers for Disease Control and Prevention
Front Range homeowners ready to test or retest can connect through our contact page for referrals to CDPHE-certified radon measurement professionals.