The EPA's Radon Action Levels Explained
Radon is measured in picocuries per liter of air (pCi/L). The EPA has established the following guidance levels for residential radon:
Reference Points: Average Radon Levels
- Average outdoor radon: 0.4 pCi/L — the background level in ambient air
- Average US indoor radon: 1.3 pCi/L — national average for all home types
- Average Maine indoor radon: Estimated 2–5 pCi/L — significantly above national average
- EPA action level: 4 pCi/L — fix your home at or above this level
- WHO reference level: 2.7 pCi/L — more conservative international guideline
Lifetime Cancer Risk by Radon Level
The following table shows the EPA's estimated lifetime lung cancer risk at different radon concentrations, assuming 70 years of exposure in the home (a conservative worst-case assumption):
| Radon Level (pCi/L) | Risk for Non-Smokers | Risk for Smokers | EPA Guidance |
|---|---|---|---|
| 0.4 (outdoor average) | 3 in 1,000 | 7 in 1,000 | Normal background |
| 1.3 (US home average) | 2 in 1,000 | 20 in 1,000 | Average risk |
| 2 | 4 in 1,000 | 32 in 1,000 | Consider fixing |
| 4 (action level) | 7 in 1,000 | 62 in 1,000 | Fix your home |
| 8 | 18 in 1,000 | 150 in 1,000 | Fix promptly |
| 20 | 36 in 1,000 | 260 in 1,000 | Fix urgently |
Source: EPA, "A Citizen's Guide to Radon." Risk estimates are for lifetime (70-year) exposure.
Why Maine's Numbers Are Higher
Maine's granite bedrock contains higher concentrations of uranium than many other states. This geology releases radon at higher rates, which means Maine homes are more likely to have elevated levels. The Maine CDC reports that roughly 1 in 3 Maine homes tests above the EPA action level — a rate far above the national average of about 1 in 15 homes.
This is especially true in central and western Maine, where granitic rock formations are most prevalent. Even newer, well-built homes in these regions regularly test at 4 pCi/L or above.
How to Test Your Home
There are two main types of radon tests:
- Short-term test (48–96 hours): Charcoal canister placed in the lowest livable area of the home. Fast results, best for initial screening or real estate transactions. Available at hardware stores for $15–$40.
- Long-term test (90 days to 1 year): Alpha track detector that measures average radon over time. More accurate because it accounts for seasonal variation. Best for understanding your true average exposure.
Both types should be placed in the lowest livable level of your home — a finished basement, or the first floor of a home without a basement. Windows should remain closed except for normal entry and exit during the test period.
Frequently Asked Questions
My home tested at 5 pCi/L. Is that really dangerous?
At 5 pCi/L, you are above the EPA action level. The risk isn't immediate or acute, but sustained exposure at this level over years significantly increases lung cancer risk — particularly for smokers. We recommend scheduling mitigation within the next few months. It is not a panic situation, but it is worth acting on.
I just bought a home. The previous owner did nothing about the radon. Should I be worried?
Test first to know the current level. If the previous owner had tested results, review them, but test again — radon levels can change after renovations, landscaping, or changes in HVAC. If levels are at or above 4 pCi/L, schedule mitigation. You haven't necessarily accumulated dangerous exposure from a short ownership period, but going forward, you should address it.
Will my radon level always be the same?
No. Radon levels fluctuate based on season (typically higher in winter when homes are sealed), weather (barometric pressure affects radon entry), and home conditions (HVAC operation, windows, etc.). A single short-term test gives a snapshot; a long-term test gives a more reliable average. For most real estate and health decisions, a properly conducted short-term test is sufficient.
My basement tested at 6 pCi/L but the first floor tested at 2 pCi/L. Which matters?
Both matter, but the basement result drives the mitigation decision. Radon tends to be highest in the lowest level. The EPA recommends basing action on the lowest livable level — even if you rarely use the basement, radon can migrate to upper floors. At 6 pCi/L in the basement, mitigation is recommended.
Know Your Level. Take Action.
Use our radon risk calculator to see what your test result means in practical terms — or contact us to schedule a mitigation assessment.
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