What does my radon test result mean?

Below 2.0 pCi/L: below EPA's consider-action range; retest every 2 years. 2.0-3.9: EPA suggests considering mitigation; WHO (2.7 pCi/L reference) recommends it. 4.0 or above: EPA action level, mitigate. 10+ pCi/L: high, urgent — limit time in lowest level until mitigated. 20+ pCi/L: very high, immediate action.

Is the EPA's 4.0 pCi/L action level safe?

Not exactly — risk is continuous. There's no threshold below which radon is risk-free. 4.0 pCi/L is the U.S. regulatory threshold balancing health risk against widespread mitigation feasibility. The WHO recommends action at 2.7 pCi/L. Both agencies acknowledge that lower is better; they differ on where to draw the action line.

Why are my Colorado radon readings so different between tests?

Seasonal variation. Colorado winter readings can be 30-50% higher than summer readings on the same home because the stack effect is stronger and homes are tighter. A 2-day test captures one snapshot; a 90-day long-term test averages across seasons. Dramatically different readings (5x or more) may also indicate placement errors.

Radon Levels Explained: What 2, 4, 10, and 20 pCi/L Actually Mean

Your radon test result came back as a number. What does that number actually mean? This page walks through every level in the practical range — outdoor air, the U.S. indoor average, the borderline zone, the EPA action level, and the high readings — and explains what each one tells you about your home.

The reference points

Level (pCi/L)	Reference point
~0.4	Outdoor air average (U.S.)
~1.3	U.S. indoor air average (single-family)
2.0	EPA "consider action" lower bound
2.7	WHO recommended action level (100 Bq/m³)
4.0	EPA action level (mitigate at or above)
~6.4	Colorado indoor average (CDPHE, recent figure)
10.0	EPA flag — limit exposure until mitigation
20.0+	Very high, urgent mitigation

Below 2.0 pCi/L — below the consider-action range

Your home is in the lower half of the U.S. distribution. The EPA does not recommend action at this level. WHO would still flag it (the WHO reference is 2.7 pCi/L), but neither agency recommends mitigation below 2.0.

What to do:

Retest every 2 years. Conditions change over time.
Retest after major remodels that affect the foundation or HVAC.
Test before listing or buying.

2.0–3.9 pCi/L — EPA says "consider mitigation"

The EPA recommends considering mitigation in this range. The WHO (2.7 pCi/L action level) would actively recommend mitigation in the upper half of this range.

For Colorado homeowners specifically, the borderline zone is worth thinking carefully about:

Winter readings in this range are common. The seasonal average may be lower.
Summer readings in this range usually mean the seasonal average is higher.
Run a long-term test to get the annual picture before deciding.
Risk equivalents: Per EPA tables, 2 pCi/L gives roughly 4-in-1,000 lifetime lung cancer risk for never-smokers and 32-in-1,000 for smokers.^[1]
Lifestyle matters. If the lowest level is used daily (home office, basement bedroom, kids' playroom), the case for mitigation is stronger.

Full health risk context →

4.0 pCi/L — the EPA action level

This is the threshold where the EPA recommends mitigation and where CDPHE concurs. At 4.0 pCi/L:

Risk: ~7-in-1,000 lifetime lung cancer risk for never-smokers; ~62-in-1,000 for smokers.^[1]
Comparison: At 4 pCi/L, a smoker is about 5x more likely to die from radon than from a home fire over their lifetime, per EPA's risk comparison tables.
Action: Confirm with a second test or a professional continuous monitor. Then get at least two written quotes from DORA-licensed contractors.

4.0 pCi/L isn't a cliff. Risk at 4.1 isn't meaningfully different from risk at 3.9. The line is regulatory, not medical. But it is the line the U.S. uses, and it's what most real-estate transactions key off of.

4–10 pCi/L — clear action range

This is the most common range for Colorado homes that test high. Mitigation in this range is straightforward — a standard sub-slab depressurization system typically brings these homes below 2.0 pCi/L.

Risk at this range:

5 pCi/L: ~9-in-1,000 risk (never-smoker), ~80-in-1,000 (smoker)
7 pCi/L: ~12-in-1,000 risk (never-smoker), ~110-in-1,000 (smoker)
10 pCi/L: ~18-in-1,000 risk (never-smoker), ~150-in-1,000 (smoker)

The EPA also notes that at 10 pCi/L (smoker), risk is roughly 200x higher than dying from a home fire.^[1] The framing is meant to underscore that radon risk is real and underappreciated relative to risks people take seriously.

10+ pCi/L — high, urgent

EPA recommends not waiting in this range — minimize time in the lowest level until a mitigation system is installed.^[1] At this level:

Risk is well above the action-level threshold.
Mitigation is the same technique as for 4–10 range, but may need a larger fan or more suction points to bring levels down.
Use the lowest level less while the system is being installed.
Most Colorado contractors can do an emergency install within 7–10 days.

20+ pCi/L — very high

Less common but does occur, particularly in Colorado mountain communities or homes on exposed Pierre Shale outcrops. At 20+ pCi/L:

Risk per EPA tables is ~36-in-1,000 for never-smokers and ~260-in-1,000 for smokers.^[1]
Mitigation should happen quickly. Don't sleep in the basement until it's done.
The contractor may need diagnostic testing to design an appropriate system — multi-zone homes with very high readings sometimes need 3+ suction points.

What if the levels vary widely between tests?

Variation between tests is normal. A 2-day short-term test captures one snapshot of conditions; a 90-day long-term test averages across seasons. Don't be alarmed if a winter reading is 6 pCi/L and a summer reading on the same home is 3 pCi/L — that's typical Colorado seasonal variation.