A radon level at or above 4 pCi/L (picocuries per liter) is considered high by the U.S. Environmental Protection Agency and calls for action to reduce it. That’s the official threshold, but the EPA also recommends considering a fix when levels fall between 2 and 4 pCi/L. To put those numbers in context, the average outdoor air contains about 0.4 pCi/L of radon naturally.
The Key Numbers to Know
Radon is measured in picocuries per liter (pCi/L) in the United States and in becquerels per cubic meter (Bq/m³) in most other countries. The conversion is roughly 1 pCi/L = 37 Bq/m³. Here’s how the major thresholds break down:
- 4 pCi/L (150 Bq/m³): The EPA’s action level. At or above this, you should install a radon reduction system.
- 2 to 4 pCi/L (75 to 150 Bq/m³): The EPA recommends considering mitigation in this range as well, since there is no truly safe level of radon exposure.
- 0.4 pCi/L (15 Bq/m³): The average concentration in outdoor air, your natural baseline.
Other countries set their thresholds differently. Canada uses 200 Bq/m³ (about 5.4 pCi/L) as its guideline, recommending corrective action within one year if your home exceeds it. The International Atomic Energy Agency sets a maximum reference level of 300 Bq/m³ (about 8.1 pCi/L) for homes and 1,000 Bq/m³ for workplaces. So the U.S. standard is actually one of the more conservative in the world.
How Radon Levels Translate to Health Risk
Radon is the second leading cause of lung cancer after smoking, and the risk scales with both the concentration and how long you’re exposed. The EPA estimates lifetime lung cancer risk per 1,000 people at different levels, and the numbers look very different depending on whether someone smokes.
For smokers, out of every 1,000 people exposed over a lifetime, about 62 could develop lung cancer at 4 pCi/L, roughly 120 at 8 pCi/L, and about 260 at 20 pCi/L. Smoking and radon together multiply each other’s damage to lung tissue.
For people who have never smoked, the numbers are lower but still meaningful: about 7 per 1,000 at 4 pCi/L, 15 per 1,000 at 8 pCi/L, and 36 per 1,000 at 20 pCi/L. At 20 pCi/L, a non-smoker’s lung cancer risk is roughly five times what it would be at the action level. Even at the “take action” threshold of 4 pCi/L, the risk for non-smokers is comparable to dying in a car crash over a lifetime, which is why the EPA treats it seriously.
How to Test Your Home Accurately
Radon levels fluctuate throughout the day and across seasons, so how you test matters. There are two main approaches.
Short-term test kits measure radon over 2 to 90 days and give you a quick snapshot. If your short-term result comes back at or above 4 pCi/L, the CDC recommends taking a second short-term test to confirm. You then average the two results. If that average is 4 pCi/L or higher, it’s time to call a licensed mitigation professional.
Long-term test kits measure for more than 90 days and give a more reliable picture of your actual year-round exposure. A single long-term test result at or above 4 pCi/L is enough to act on without needing a second test. As the CDC puts it, the longer the test runs, the better it reflects your true radon exposure and daily living patterns, things like how often you open windows or run ventilation.
Test kits are inexpensive and available at most hardware stores. You place them in the lowest livable level of your home, typically a basement or ground floor, with windows and doors closed as much as possible during the test period.
Why Levels Vary So Much Between Homes
Two houses on the same street can have wildly different radon readings. Radon is a gas produced by the natural decay of uranium in soil and rock, and it seeps into buildings through cracks in foundations, gaps around pipes, and other openings where the structure meets the ground. The concentration you get depends on the uranium content of the soil beneath your specific foundation, how tightly your home is sealed, your ventilation patterns, and even the weather.
Basements and ground floors typically have the highest levels because they’re closest to the source. Upper floors tend to be lower, though not always low enough to ignore. Newer, more airtight homes can actually trap more radon indoors than older, draftier ones.
What Mitigation Looks Like
If your test results come back high, a radon mitigation system is straightforward and effective. The most common approach uses a vent pipe and fan to pull radon from beneath the foundation and release it above the roofline, where it disperses harmlessly into outdoor air. This method, called sub-slab depressurization, typically reduces indoor radon levels by up to 99%.
Installation usually takes less than a day and costs between $800 and $2,500 depending on your home’s construction and local pricing. After installation, you should retest to confirm the system brought levels below 4 pCi/L, ideally below 2 pCi/L. The system runs continuously and uses about as much electricity as a light bulb, so ongoing costs are minimal. Most professionals recommend retesting every two years to make sure the system is still performing.