Understanding Radon Measurement Units
Radon concentration is measured in different units depending on location:
Measurement Units: Bq/m³ and pCi/L
Becquerels per Cubic Meter (Bq/m³) - International Reference Unit
Bq/m³ is the official unit used by most countries worldwide, including Canada, the United Kingdom, and is the reference unit of the WHO.
What it measures: The number of radioactive decay events per second in one cubic meter of air.
Named after: Henri Becquerel, discoverer of radioactivity
Typical values in Bq/m³:
- •Outdoor air: ~15 Bq/m³
- •Average indoor level: ~48 Bq/m³
- •WHO action level: 100 Bq/m³
- •EU action level: 300 Bq/m³
Picocuries per Liter (pCi/L) - United States Unit
pCi/L is primarily used in the United States. It may appear in scientific publications, certain equipment, reports, or documentation from the U.S., but the official international reference unit is Bq/m³.
What it measures: The number of radioactive decay events per minute in one liter of air.
Named after: Marie and Pierre Curie, radiation research pioneers
Typical values in pCi/L:
- •Outdoor air: ~0.4 pCi/L
- •Average indoor level: ~1.3 pCi/L
- •US EPA action level: 4 pCi/L
Conversion between units:
- •1 pCi/L = 37 Bq/m³
- •1 Bq/m³ = 0.027 pCi/L
In this guide: All values are presented in both units for international comprehension.
Quick Reference Conversion Table
Common levels in both units:
| Bq/m³ | pCi/L | Description |
|---|---|---|
| 15 | 0.4 | Typical outdoor air |
| 48 | 1.3 | Global average indoor |
| 74 | 2.0 | Low indoor level |
| 100 | 2.7 | WHO action level |
| 148 | 4.0 | US EPA action level |
| 200 | 5.4 | Canadian action level |
| 300 | 8.1 | EU action level |
| 370 | 10.0 | High level |
| 740 | 20.0 | Very high level |
Note: All subsequent examples in this guide will show both units.
What Different Radon Levels Mean
Understanding your results in context:
Below 48 Bq/m³ (1.3 pCi/L)
Status: Below global average indoor level
Risk level: Lower than typical indoor exposure, but not zero
Recommendations:
- •No immediate action required
- •Consider periodic retesting (every 2-5 years)
- •Maintain good ventilation
- •Retest if you make major structural changes
- •Some health authorities still recommend mitigation below this level
Important: No radon level is completely safe. Even low levels carry some risk, though substantially less than higher concentrations.
48-100 Bq/m³ (1.3-2.7 pCi/L)
Status: Above average, below WHO action level
Risk level: Measurable but relatively low
Recommendations:
- •Consider mitigation, especially if:
- •Many homes can be reduced to this range or lower
- •Cost of mitigation often modest compared to health benefit
- •Periodic retesting recommended
Note: WHO recommends action at 100 Bq/m³ (2.7 pCi/L), making this range a gray area where mitigation is advisable but not universally mandated.
100-200 Bq/m³ (2.7-5.4 pCi/L)
Status: Exceeds WHO action level, meets or exceeds many national standards
Risk level: Significant long-term health risk
Action: Mitigation strongly recommended
Timeline: Should be addressed within months, not years
What to expect:
- •Professional mitigation can reduce levels by 50-99%
- •Most homes in this range can be reduced to below 74 Bq/m³ (2 pCi/L)
- •Cost similar to common home repairs
- •Substantial reduction in cancer risk
If you're buying/selling:
- •Buyers commonly request mitigation before purchase
- •Sellers should consider addressing before listing
- •May affect property value and sale timeline
200-740 Bq/m³ (5.4-20 pCi/L)
Status: High radon level - exceeds most international action levels
Risk level: Serious health concern requiring prompt action
Action: Professional mitigation needed soon
Timeline: Address within weeks to a few months
Health context:
- •For smokers: Risk of lung cancer increases substantially
- •For non-smokers: Risk still significant and measurable
- •Long-term exposure at these levels responsible for many radon-related lung cancers
Mitigation expectations:
- •Systems can reliably reduce even these levels by 90-99%
- •Target reduction to below 100 Bq/m³ (2.7 pCi/L)
- •Follow-up testing essential to verify effectiveness
Living situation:
- •Minimize time in high-radon areas until mitigated
- •Especially important for bedrooms and frequently occupied spaces
- •Consider temporary sleeping arrangements in higher areas of home
Above 740 Bq/m³ (20 pCi/L)
Status: Very high radon level - immediate attention required
Risk level: Severe health hazard
Action: Seek professional assessment immediately
Timeline: Address as soon as possible (within days to weeks)
Immediate steps:
- •Contact qualified mitigation contractor immediately
- •Get multiple professional assessments
- •Consider temporary relocation during mitigation if levels exceed 1,480 Bq/m³ (40 pCi/L)
- •Minimize time in affected areas, especially sleeping
- •Do not wait for second test - begin mitigation process
What to expect:
- •Even very high levels can be reduced to below 100 Bq/m³ (2.7 pCi/L)
- •May require more extensive systems
- •Costs may be higher than average but still typically $2,000-5,000
- •Post-installation verification essential
Special considerations:
- •Check for unusual radon sources (building materials, indoor water features)
- •Investigate all potential entry points
- •May need combination of mitigation techniques
International Action Levels Compared
Different countries and organizations recommend action at different levels:
Health Organization Recommendations
World Health Organization (WHO):
- •Action level: 100 Bq/m³ (2.7 pCi/L)
- •Rationale: Most protective standard based on health risk analysis
- •If not achievable: Maximum 300 Bq/m³ (8.1 pCi/L)
International Commission on Radiological Protection (ICRP):
- •Reference level: 100-300 Bq/m³ (2.7-8.1 pCi/L)
- •Optimization: As low as reasonably achievable below reference level
National Standards
United States:
- •EPA action level: 148 Bq/m³ (4 pCi/L)
- •Consider action: 74-148 Bq/m³ (2-4 pCi/L)
- •Goal: Indoor levels no higher than outdoor (15 Bq/m³ / 0.4 pCi/L)
Canada:
- •Action level: 200 Bq/m³ (5.4 pCi/L)
- •Previous level (2007): 800 Bq/m³ (22 pCi/L) - significantly reduced
European Union:
- •Existing buildings: 300 Bq/m³ (8.1 pCi/L)
- •New construction: 200 Bq/m³ (5.4 pCi/L)
- •Workplaces: 300 Bq/m³ (8.1 pCi/L)
United Kingdom:
- •Action level: 200 Bq/m³ (5.4 pCi/L)
Australia:
- •Action level: 200 Bq/m³ (5.4 pCi/L)
Germany:
- •Reference value: 100 Bq/m³ (2.7 pCi/L)
Ireland:
- •Reference level: 200 Bq/m³ (5.4 pCi/L)
Trend: International standards are converging toward lower action levels (100-200 Bq/m³) as health evidence strengthens.
Which Standard Should You Use?
If your level is:
- •Below all standards: Minimal action needed, periodic retesting
- •Above WHO but below national level: Consider your risk factors (smoking, time in space, age)
- •Above your national standard: Mitigation recommended
- •Above all standards (>300 Bq/m³ / 8 pCi/L): Mitigation strongly advised regardless of location
Conservative approach: WHO's 100 Bq/m³ (2.7 pCi/L) represents the most health-protective threshold and is a reasonable target for mitigation regardless of your country's official action level.
Short-Term vs Long-Term Test Results
Different test durations provide different information:
Short-Term Test Results (2-90 days)
Advantages:
- •Quick results for fast decisions
- •Useful for real estate transactions
- •Identifies immediate concerns
Limitations:
- •May not reflect year-round average
- •Influenced by weather and season
- •Can vary significantly from true annual level
Interpreting short-term results:
- •If below 100 Bq/m³ (2.7 pCi/L): Risk is lower, but consider long-term test for better annual average
- •If 100-200 Bq/m³ (2.7-5.4 pCi/L): Follow up with second short-term test or long-term test
- •If above 200 Bq/m³ (5.4 pCi/L): Sufficient evidence to proceed with mitigation
- •If very high (>740 Bq/m³ / 20 pCi/L): No additional testing needed before mitigation
When results are borderline:
If your short-term result is near an action level (e.g., 90-110 Bq/m³), there's uncertainty about whether your year-round average is above or below the threshold. A follow-up test provides more clarity.
Long-Term Test Results (>90 days)
Advantages:
- •More accurate annual average
- •Accounts for seasonal variation
- •Better for long-term decision making
- •Less influenced by temporary conditions
Recommended duration:
- •Minimum: 91 days
- •Optimal: 6-12 months (full year preferred)
Interpreting long-term results:
Long-term results are more reliable for deciding whether to mitigate:
- Below 74 Bq/m³ (2 pCi/L): Low annual exposure
- 74-100 Bq/m³ (2-2.7 pCi/L): Consider mitigation based on risk factors
- Above 100 Bq/m³ (2.7 pCi/L): Mitigation recommended
- Above 200 Bq/m³ (5.4 pCi/L): Mitigation strongly recommended
Advantage over short-term:
Less uncertainty about whether to mitigate since results better reflect actual long-term exposure.
Using Two Short-Term Tests
An alternative to long-term testing:
Method:
- •Conduct two short-term tests sequentially or simultaneously
- •Average the results
- •Use average to make mitigation decision
Sequential approach:
1. First test: Initial screening
2. If elevated, second test: Confirmation
Simultaneous approach:
- •Place two test devices side-by-side (10 cm / 4 inches apart)
- •Average both results
- •Faster confirmation than sequential
When to use this approach:
- •Need faster results than long-term test provides
- •Real estate transaction deadlines
- •When first short-term test shows elevated levels
- •For quick confirmation before mitigation decision
Averaging results:
- •Add both results together
- •Divide by two
- •Use this average for decision making
Example:
- •Test 1: 130 Bq/m³ (3.5 pCi/L)
- •Test 2: 170 Bq/m³ (4.6 pCi/L)
- •Average: 150 Bq/m³ (4.05 pCi/L) - exceeds WHO level, consider mitigation
When Test Results Are Uncertain
Results near action levels create uncertainty:
The Borderline Zone
When results fall within ±20% of an action level:
Example: 80-120 Bq/m³ (2.2-3.2 pCi/L) around WHO's 100 Bq/m³ level
Why uncertainty exists:
- •Natural radon fluctuation
- •Measurement error
- •Short-term tests may not reflect annual average
- •Weather and seasonal effects
If short-term test is 110 Bq/m³ (3.0 pCi/L):
- •~50% chance true annual average is below 100 Bq/m³
- •~50% chance it's above 100 Bq/m³
Resolution strategies:
1. Long-term test: Most definitive answer
2. Second short-term test: Average with first result
3. Conservative approach: Treat as if above action level
4. Risk-factor analysis: Consider smoking, occupancy patterns, age
Factors That Create Variation
Results can vary due to:
Seasonal changes:
- •Higher in winter (closed windows, heating creates more suction)
- •Lower in summer (open windows, less indoor-outdoor pressure difference)
- •Fall/spring may be intermediate
Weather conditions:
- •Barometric pressure changes
- •Precipitation
- •Wind conditions
- •Soil moisture
Building operation:
- •HVAC system use
- •Window/door opening patterns
- •Exhaust fan operation
- •Fireplace/combustion appliance use
Testing conditions:
- •Whether closed-building conditions maintained
- •Duration of test
- •Placement location
- •Device quality
These variations are why:
- •Long-term tests are more reliable
- •Multiple short-term tests averaged are better than single test
- •Very high results are unlikely to be measurement errors
- •Borderline results need follow-up
Making Decisions with Uncertain Results
When faced with borderline results:
Conservative approach (recommended):
- •Treat borderline high results as actionable
- •Cost of mitigation vs. lifetime cancer risk favors action
- •Even "successful" mitigation at borderline levels provides benefit
Risk-factor weighting:
- •Anyone in home smokes
- •Young children present (more years of exposure ahead)
- •High occupancy of tested area (e.g., bedroom in basement)
- •Elderly occupants (more vulnerable to cancer)
- •Previous radon exposure history
Financial considerations:
- •Mitigation cost: $1,200-3,000 typical
- •Cancer treatment cost: $100,000+
- •Quality of life impact: Significant
- •Peace of mind: Valuable
Cost-benefit analysis favors mitigation even when results are uncertain.
When in doubt:
- •Get second professional opinion
- •Conduct additional testing
- •Consult with radon professional about your specific situation
- •Remember: it's easier and cheaper to mitigate now than deal with health consequences later
Next Steps Based on Your Results
Clear action plan for each scenario:
If Results Are Low (<74 Bq/m³ / 2 pCi/L)
Immediate action: None required
Ongoing monitoring:
- •Retest every 2-5 years
- •Test before/after major renovations
- •Test if moving to different area of home
- •Test if nearby homes show high levels
Keep records:
- •Save test results
- •Note test date and location
- •File for future reference and home sales
If Results Are Moderate (74-200 Bq/m³ / 2-5.4 pCi/L)
Immediate action:
1. Confirm with follow-up test if only short-term test conducted
2. Get quotes from 2-3 certified mitigation contractors
3. Decide on mitigation based on risk factors
Timeline: Address within 3-6 months
Cost planning:
- •Get multiple estimates
- •Compare contractor qualifications
- •Budget $1,200-3,000 typically
Temporary measures:
- •Improve ventilation (though not reliable long-term solution)
- •Minimize time in high-radon areas
- •Don't use result-prone areas as bedrooms
If Results Are High (>200 Bq/m³ / 5.4 pCi/L)
Immediate action:
1. Contact certified mitigation contractors this week
2. Get professional assessments and quotes
3. Schedule installation within 1-2 months
Timeline: Begin mitigation process immediately
Temporary measures until mitigation:
- •Reduce time in affected areas
- •Increase natural ventilation when possible
- •Move sleeping areas to upper floors if applicable
- •Don't delay mitigation unnecessarily
Follow-up:
- •Post-installation testing mandatory
- •Verify reduction achieved
- •Maintain system properly
If Results Are Very High (>740 Bq/m³ / 20 pCi/L)
Emergency response:
1. Contact multiple mitigation contractors TODAY
2. Request expedited service
3. Consider temporary relocation if above 1,480 Bq/m³ (40 pCi/L)
Timeline: Install system within days to 2 weeks
Immediate protective measures:
- •Minimize time in affected areas
- •Sleep in upper floors
- •Maximize ventilation
- •Don't use affected areas for extended periods
Investigation:
- •May indicate unusual radon source
- •Check building materials
- •Inspect for concentrated entry points
- •Consider soil gas testing
Follow-up:
- •Post-installation testing within 24-48 hours
- •Additional testing at 30 days
- •Verify system operating properly
- •Check warning device frequently
Related Guides
Source Attribution
This content incorporates material from U.S. Environmental Protection Agency publications, which are in the public domain. Content has been adapted for an international audience with unit conversions, translations to multiple languages, and global framing of concepts while preserving all scientific facts and health guidance.
This content is in the public domain under 17 U.S.C. § 105. Content has been adapted for an international audience and translated to multiple languages while preserving scientific accuracy.