If you have ever researched radon internationally, you may have encountered test results or guidelines expressed in units that look unfamiliar — Bq/m³ from European sources, WL from occupational health documents, or pCi/L from U.S. EPA guidance. All three measure the same phenomenon (radon radioactivity in air) but express it differently. Understanding the conversions and the context in which each unit is used lets you compare international research, interpret older documents, and understand why your neighbor’s European renovation report quotes a different number than your EPA-sourced action level.
picocuries per liter (pCi/L) — The U.S. Standard
The picocurie per liter is the standard unit for radon concentration in air used by the U.S. EPA, U.S. state radon programs, and most North American regulatory frameworks.
What It Measures
A curie (Ci) is a unit of radioactivity equal to 37 billion disintegrations per second — defined as the activity of one gram of radium-226. A picocurie (pCi) is one trillionth of a curie, or 0.037 disintegrations per second. Picocuries per liter expresses how many radon disintegrations per second are occurring in one liter of air.
At EPA’s action level of 4.0 pCi/L, approximately 0.148 radon disintegrations occur per second per liter of air in your home — or about 9 per minute per liter.
Key Reference Values in pCi/L
- Outdoor average (U.S.): ~0.4 pCi/L
- Indoor average (U.S.): ~1.3 pCi/L
- EPA “consider mitigating” level: 2.0 pCi/L
- EPA action level: 4.0 pCi/L
- EPA “fix immediately” level: 8.0 pCi/L (or higher — no waiting for confirmatory test)
becquerels per cubic meter (Bq/m³) — The International Standard
The becquerel per cubic meter is the SI (International System of Units) standard for radon concentration. It is used by the World Health Organization, European Union radon regulations, and most countries outside North America.
What It Measures
A becquerel (Bq) is one radioactive disintegration per second. Becquerels per cubic meter (Bq/m³) expresses how many radon disintegrations per second occur in one cubic meter of air. Because 1 cubic meter = 1,000 liters, the conversion between Bq/m³ and pCi/L involves both the volume conversion and the unit conversion.
Conversion Formula
1 pCi/L = 37 Bq/m³ (exact conversion)
1 Bq/m³ = 0.027 pCi/L
Practical conversions:
- EPA action level (4.0 pCi/L) = 148 Bq/m³
- WHO reference level (100 Bq/m³) = 2.7 pCi/L
- EU reference level (300 Bq/m³ for existing buildings) = 8.1 pCi/L
- EU reference level (200 Bq/m³ for new construction) = 5.4 pCi/L
Key Reference Values in Bq/m³
- Outdoor average: ~15 Bq/m³
- Indoor average (U.S.): ~48 Bq/m³
- WHO reference level: 100 Bq/m³ (2.7 pCi/L)
- EU reference level (existing buildings): 300 Bq/m³ (8.1 pCi/L)
Working Levels (WL) — The Occupational Standard
The working level (WL) is an older unit developed for measuring radon exposure in uranium mines and other occupational settings. It measures the combined energy of all short-lived radon decay products (progeny) in one liter of air — not radon itself. It remains in use in some occupational health, regulatory, and older literature contexts, but is rarely used in modern residential radon programs.
What It Measures
One working level (1 WL) is defined as any combination of short-lived radon progeny in one liter of air that will result in the emission of 1.3 × 10⁵ MeV of alpha energy upon complete decay. At equilibrium between radon and its progeny, 1 WL corresponds to approximately 200 pCi/L of radon.
Conversion Formula
At typical indoor equilibrium ratios (approximately 0.4 equilibrium factor):
1 WL ≈ 200 pCi/L of radon (at complete equilibrium)
EPA’s 4.0 pCi/L action level ≈ 0.02 WL
Working level months (WLM) is the cumulative exposure metric — one WLM equals exposure to 1 WL for one working month (170 hours). Occupational exposure limits and mining health regulations are often expressed in WLM per year.
Quick Conversion Reference
| pCi/L | Bq/m³ | WL (approx.) | Context |
|---|---|---|---|
| 0.4 | 15 | 0.002 | Outdoor average |
| 1.3 | 48 | 0.007 | U.S. indoor average |
| 2.7 | 100 | 0.014 | WHO reference level |
| 4.0 | 148 | 0.02 | EPA action level |
| 8.0 | 296 | 0.04 | EPA immediate action |
| 8.1 | 300 | 0.04 | EU reference level (existing buildings) |
| 20.0 | 740 | 0.1 | High-risk residential |
Frequently Asked Questions
Why does the WHO action level seem lower than EPA’s?
The WHO reference level of 100 Bq/m³ (2.7 pCi/L) is lower than EPA’s 4.0 pCi/L (148 Bq/m³) because the WHO chose to set a more conservative reference level reflecting updated health evidence. EPA’s 4.0 pCi/L level was set in the 1980s and has not been formally revised, though EPA acknowledges that radon between 2.0 and 4.0 pCi/L still poses meaningful risk and recommends considering mitigation in that range.
If my test result is in Bq/m³, how do I know if I should mitigate?
Divide your Bq/m³ result by 37 to get the pCi/L equivalent. If the result is 148 Bq/m³ or higher (4.0 pCi/L), EPA recommends mitigation. If you are following WHO guidance, the reference level is 100 Bq/m³ (2.7 pCi/L). If following EU guidance for existing buildings, the reference level is 300 Bq/m³ (8.1 pCi/L).
My old test report shows results in WL — how do I convert to pCi/L?
Multiply the WL value by 200 to get the approximate equivalent pCi/L at complete equilibrium. For example, 0.02 WL × 200 = 4.0 pCi/L. Because actual indoor equilibrium factors vary (typically 0.3–0.5, not 1.0), WL-to-pCi/L conversions have some inherent uncertainty. For modern residential decisions, use pCi/L or Bq/m³ from a current lab test.
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