The average indoor radon level in U.S. homes is about 1.3 pCi/L (picocuries per liter), based on the EPA’s national residential radon survey. That’s roughly three times higher than the average outdoor level of 0.4 pCi/L. But averages can be misleading here, because radon varies enormously from house to house, even between neighbors on the same street. Your home could test well below 1 pCi/L or well above 10 pCi/L depending on local geology, your foundation type, and how your home is ventilated.
What the Numbers Mean
Radon is measured in picocuries per liter of air. The EPA sets its “action level” at 4 pCi/L, meaning any home testing at or above that level should have a mitigation system installed. The agency also recommends considering mitigation for homes between 2 and 4 pCi/L. There is no level of radon exposure considered completely safe, but 4 pCi/L is the threshold where the EPA says the health risk clearly justifies the cost of fixing the problem.
The World Health Organization uses a stricter standard. It recommends countries set their reference level at 100 Bq/m³ (about 2.7 pCi/L), and says the ceiling should not exceed 300 Bq/m³ (about 8.1 pCi/L) even in countries where lowering levels is difficult. So by WHO standards, the EPA’s action level is already on the higher side.
How Radon Gets Inside
Radon is a naturally occurring radioactive gas produced by the breakdown of uranium in soil and rock. It seeps upward through the ground and enters your home through cracks and gaps in the foundation, including openings around pipes, sump pits, and construction joints where walls meet the floor. Because it’s colorless and odorless, it accumulates indoors without any obvious signs. Homes with basements or slab-on-grade foundations tend to have higher levels than those built on crawl spaces, though no home type is immune.
Health Risks at Different Levels
Radon is responsible for an estimated 21,000 lung cancer deaths per year in the United States, making it the second leading cause of lung cancer after smoking. The risk scales with both your radon exposure and whether you smoke, because radon and tobacco have a synergistic effect on lung tissue.
At the national average of 1.3 pCi/L, a nonsmoker has roughly a 2 in 1,000 chance of developing lung cancer from radon. A smoker at the same level faces a 20 in 1,000 chance. That tenfold difference holds across exposure levels. At 4 pCi/L (the EPA action level), about 7 out of 1,000 nonsmokers would develop lung cancer, compared to 62 out of 1,000 smokers. At 20 pCi/L, the numbers jump to 36 per 1,000 for nonsmokers and 260 per 1,000 for smokers.
If you smoke and live in a home with elevated radon, addressing either risk factor significantly reduces your overall cancer risk. But the combination of the two is far more dangerous than either one alone.
Why Radon Levels Change With the Seasons
Indoor radon levels aren’t static. For most homes in the Northern Hemisphere, concentrations tend to be highest during the heating season from October through April. Two factors drive this pattern: homes are sealed up tighter in winter, reducing the amount of fresh air that dilutes radon indoors, and forced-air heating systems can pull radon-laden air from lower levels and distribute it throughout the house.
That said, the seasonal pattern isn’t universal. A Canadian study of paired winter and summer radon tests found that about 48% of homes showed minimal seasonal difference. Roughly 25% had meaningfully higher radon in winter, while about 28% actually had higher readings in summer. So you can’t assume your winter test will always be the worst-case number.
How to Test Your Home
Testing is the only way to know your home’s radon level. You can pick up a test kit at most hardware stores or order one online, often for under $20.
Short-term test kits measure radon over a period of 2 to 90 days and give you a quick snapshot. They’re useful as a first screening, especially if you need results fast (during a real estate transaction, for example). Long-term kits measure for more than 90 days and provide a much more accurate picture of your home’s year-round average. Because radon fluctuates with weather, season, and how you use your home, longer tests produce more reliable results.
Place the test in the lowest livable level of your home, ideally a room you spend time in regularly. Keep windows and exterior doors closed as much as practical during the test. If a short-term test comes back at or above 4 pCi/L, follow up with either a second short-term test or a long-term test to confirm before deciding on mitigation.
Fixing a Radon Problem
If your home tests above 4 pCi/L, a radon mitigation system can typically reduce levels by 80% or more. The most common approach uses a vent pipe and fan to pull radon from beneath the foundation and exhaust it above the roofline, where it disperses harmlessly. The national average cost for installation is about $1,200, with most homeowners paying between $800 and $1,500 depending on the home’s construction and local market rates.
After installation, you should retest to confirm the system is working. Most mitigated homes drop well below 2 pCi/L. The fan runs continuously and uses about as much electricity as a light bulb, so ongoing costs are minimal. Systems typically last for years with little maintenance, though periodic retesting every two to five years is a good idea to make sure everything is still functioning properly.