Radon Levels in Maine by County

A county-by-county breakdown of radon risk across Maine — EPA Zone classifications, the percentage of homes that test above the action level, and what to expect in your area.

The bottom line for Maine

Maine has the highest concentration of EPA Zone 1 counties of any state in the country. Eight of the ten counties we serve are Zone 1 (predicted indoor levels above 4 pCi/L). The state average is roughly 30% of homes above the action level — five times the national average of 6%.

County-by-County Snapshot

Click any county to see the towns we serve there.

County	EPA Zone	~% Homes > 4 pCi/L	Notes
Kennebec County ›	Zone 1	38%	Inland granite-rich bedrock; capital region (Augusta, Waterville, Hallowell) consistently tests among the highest in the state.
Cumberland County ›	Zone 1	28%	Southern coastal county. Portland metro shows somewhat lower averages near the coast; inland Cumberland (Windham, Gorham) trends higher.
Androscoggin County ›	Zone 1	35%	Lewiston-Auburn metro. Older mill-era housing stock plus granite geology produces consistently elevated readings.
Penobscot County ›	Zone 1	33%	Bangor area. High well-water usage compounds airborne radon issues; aeration treatment common here.
York County ›	Zone 1	26%	Southernmost Maine. Coastal sandy soils slightly reduce risk vs inland, but York and Sanford regularly test above 4 pCi/L.
Sagadahoc County ›	Zone 1	30%	Smallest county by area but rich in granite. Bath and Brunswick regularly produce 4–10 pCi/L readings.
Lincoln County ›	Zone 1	32%	Mid-coast. Mix of granite ledge and clay; results vary widely by neighborhood.
Knox County ›	Zone 2	22%	Mid-coast region. Closer to coast = somewhat lower averages, but Camden and Rockland still see frequent elevated readings.
Hancock County ›	Zone 2	24%	Includes Mount Desert Island (Bar Harbor). Mixed bedrock; many seasonal homes that benefit from continuous monitors.
Oxford County ›	Zone 1	36%	Western mountains and lakes region. Highest-radon-potential geology in the state; many homes test above 10 pCi/L.

Sources: EPA Map of Radon Zones (Publication 402-R-93-071), Maine CDC Radon Surveillance Reports, and ~17 years of in-field testing observations from working Maine mitigators. County percentages are estimates rounded to the nearest whole number; individual home readings vary widely based on foundation type, ventilation, and bedrock composition.

What Do EPA Zones Mean?

Zone 1

Highest Risk

Predicted average > 4 pCi/L

The EPA's highest-risk classification. Roughly 30–40% of homes test above the action level. Test every home; expect mitigation may be needed.

Zone 2

Moderate Risk

Predicted average 2 – 4 pCi/L

Moderate risk classification. ~15–25% of homes test above the action level. Test every home; mitigation needed in many.

Zone 3

Lowest Risk

Predicted average < 2 pCi/L

Lowest-risk classification — but radon can still be present. Maine has zero Zone 3 counties. Even our lowest-risk areas (coastal Knox, Hancock) are Zone 2.

Why Are Maine's Levels So High?

Granite-rich bedrock

Maine sits on uranium-bearing granite formations stretching from Oxford County through the central highlands. As uranium decays, it produces a continuous supply of radon that migrates upward through soil and rock fractures.

Tight modern construction

Energy-efficient Maine homes seal in heat — and radon. Newer construction (post-1990) often tests higher than older drafty farmhouses, not lower.

Stack effect in cold winters

Heated air rising through Maine homes in winter creates negative pressure at the basement level, actively pulling soil gas (and radon) into the home. Winter readings typically run 30–50% higher than summer.

Private well water

About half of Maine households use private wells, and Maine wells routinely contain very high concentrations of dissolved radon. When water is used in the home, that radon volatilizes into indoor air.

Want to Know Your Home's Number?

Test, then call us if you need to mitigate. We answer the phone with honest advice — even if you decide to wait.

Call (207) 483-5637