Radon levels refer to the concentration of radon gas inside a building, measured in picocuries per liter (pCi/L) in the United States or becquerels per cubic meter (Bq/m3) internationally. The EPA recommends taking action to reduce radon when levels reach 4 pCi/L or higher, and there is no known safe level of exposure. Radon is a naturally occurring radioactive gas that seeps into homes from the ground, and it’s the second leading cause of lung cancer after smoking.
Where Radon Comes From
Radon is a byproduct of uranium breaking down in soil and rock. Uranium decays into radium, and radium decays into radon-222, the form of the gas that enters buildings. Because uranium exists naturally in the earth’s crust, radon is present virtually everywhere, but concentrations vary widely depending on local geology.
The gas moves through porous soil and enters homes through cracks in foundations, gaps around pipes, and any opening where the building contacts the ground. How much radon seeps in depends on soil porosity, moisture content, weather conditions, and the pressure difference between indoors and the soil beneath the building. Homes with lower air pressure than the surrounding soil essentially pull radon inside, which is why basements and ground floors typically have the highest concentrations.
What the Numbers Mean
In the U.S., radon is measured in picocuries per liter (pCi/L). The EPA sets 4 pCi/L as its action level, the threshold at which homeowners should install a mitigation system. But the agency also recommends considering mitigation for levels between 2 and 4 pCi/L, since no exposure level is considered truly safe. The average outdoor radon concentration is about 0.4 pCi/L, so even a reading of 2 pCi/L represents five times the background level.
Internationally, radon is measured in becquerels per cubic meter (Bq/m3). The International Atomic Energy Agency sets maximum reference levels at 300 Bq/m3 for homes and 1,000 Bq/m3 for workplaces. For context, the EPA’s 4 pCi/L action level converts to about 150 Bq/m3, which is more conservative than the international standard.
Why Radon Levels Matter for Health
Radon gas itself isn’t the direct problem. When you inhale radon, it decays inside your lungs and releases tiny bursts of radiation that damage the cells lining your airways. Over years, this damage can lead to lung cancer. The CDC estimates radon causes about 2,900 lung cancers each year among Americans who never smoked, and the risk is substantially higher for people who do smoke because the two exposures compound each other.
The relationship between radon exposure and lung cancer risk is linear, meaning there’s no clear cutoff below which the gas is harmless. Higher levels and longer exposure both increase risk. This is why even moderate readings in the 2 to 4 pCi/L range deserve attention, especially if you spend many hours at home.
How Radon Levels Vary by Location
The EPA divides the U.S. into three radon zones based on predicted indoor levels. Zone 1 areas (the highest risk) have average indoor levels that may exceed 4 pCi/L. These are concentrated across the northern Midwest, parts of the Appalachians, and sections of the Mountain West, where granite and shale formations contain higher concentrations of uranium. Zone 2 areas have moderate potential, with averages between 2 and 4 pCi/L. Zone 3 areas have the lowest potential, with averages below 2 pCi/L.
These zones are useful as a general guide, but they don’t predict what’s happening in any individual home. Two houses on the same street can have very different radon levels because of differences in foundation construction, soil conditions, and ventilation. The only way to know your home’s radon level is to test it.
How to Test Your Home
Radon test kits come in two types. Short-term kits measure radon over 2 to 90 days and give you a quick snapshot. Long-term kits measure for more than 90 days and provide a more accurate picture of your home’s year-round average. The longer the test runs, the better it reflects actual exposure, since radon levels fluctuate with weather, ventilation, and seasonal changes.
For most of the 20th century, radon levels in homes were consistently highest during winter heating months, roughly October through April. Keeping windows and doors closed reduced air exchange, and forced-air heating systems could distribute radon-laden air from the basement throughout the house. More recently, monitoring data in North America shows levels becoming more even across seasons, with some homes actually reading higher in summer. This makes long-term testing especially valuable, since a short winter test alone may not capture the full picture.
You can buy a short-term test kit from a hardware store or order one from your state radon office, often for free or at low cost. Place the kit in the lowest lived-in level of your home, away from drafts and exterior walls. If the short-term result comes back at or above 4 pCi/L, a follow-up long-term test or a second short-term test can confirm whether mitigation is needed.
Reducing High Radon Levels
The most effective mitigation method is called soil depressurization. A contractor drills a small hole through the basement slab or foundation, inserts a pipe that runs up and out through the roof, and attaches a small fan that pulls radon-laden air from beneath the building before it can enter your living space. The system runs continuously and is quiet, typically using about 20 watts of power, roughly the same as a small light bulb.
This approach consistently reduces indoor radon by more than 85%, often bringing homes well below the 4 pCi/L action level. Some homes with extremely high levels may need additional suction points or sealing of foundation cracks for the best results. Passive versions of the same system, which rely on natural airflow instead of a fan, also exist but are less effective and are most commonly installed during new construction as a precaution.
Other strategies include sealing cracks and gaps in the foundation and improving ventilation, but these work best as supplements rather than standalone fixes. A properly installed active depressurization system is the standard approach recommended by the EPA and is typically installed in a single day.