Radon Levels Chart: What Counts as Safe?

Radon test results are reported in picocuries per liter of air, written as pCi/L. It's a measure of radioactivity concentration in the air you're breathing, and it's the standard unit used across the United States for both short-term and long-term radon testing. If you've just gotten a test result back and aren't sure what the number means, this guide walks through how to read it.

The EPA action level

The Environmental Protection Agency sets 4.0 pCi/L as the action level for indoor radon. That means if a reliable test (or the average of two tests) comes back at 4.0 pCi/L or higher, the EPA recommends fixing the home with a mitigation system. This number isn't a hard line between "dangerous" and "fine," it's the point at which the EPA judged that the benefit of fixing the problem clearly outweighs the cost and effort for most households.

Rough guide to radon levels

• Below 2 pCi/L: Close to typical outdoor background levels. Generally considered low risk, though the EPA notes there is no fully risk-free level of radon exposure.
• 2 to 4 pCi/L: Below the action level, but the EPA suggests homeowners consider mitigation if it's practical, since reducing radon further still reduces long-term risk.
• 4 pCi/L and above: The EPA's official action level. Mitigation is recommended.
• 10 pCi/L and above: Considered a higher-priority situation. Many contractors recommend confirming quickly with a second test and scheduling mitigation sooner rather than later.
• 20 pCi/L and above: Uncommon, but does happen in certain high-radon-potential areas. Homes at this level typically need a carefully designed system, sometimes with more than one suction point.

These ranges are general guidance, not a medical or legal standard. Your specific situation, including how much time household members spend in lower levels of the home, is worth discussing with a mitigation professional if your result is in a gray area.

Why there's no "zero" on the chart

Radon is present at some low level almost everywhere, indoors and outdoors, because it comes from the ground itself. Outdoor air typically averages somewhere around 0.4 pCi/L. Indoor air in a well-ventilated home with a favorable foundation can sometimes get close to that. But because radon comes from an ongoing natural process (uranium decay in soil), there's no practical way to get a home's indoor air to a true zero. The goal of mitigation isn't elimination, it's getting the concentration down to a level the EPA and the broader radon science community consider low risk.

Short-term vs. long-term testing, and why it matters for your number

The pCi/L number you get depends partly on how the test was done:

• Short-term tests (typically 2 to 4 days) are fast and useful for an initial screening, but they capture a snapshot. Weather, whether windows were open, and even the season can swing the result.
• Long-term tests (90 days or more) average out those swings and generally give a more accurate picture of the radon level you're actually exposed to over time, since levels are often higher in winter when a house is sealed up and running heat.

Because of that variability, the standard recommendation is: if a short-term test comes back at or above 4.0 pCi/L, do a second test to confirm before deciding on mitigation. If the second test also comes back elevated, or if you'd rather skip straight to a more accurate picture, a long-term test or immediate mitigation are both reasonable paths.

Where to test in your home

The EPA recommends testing on the lowest livable level of the home, meaning a floor you actually use regularly, such as a finished basement, rather than an unfinished crawl space or storage area. If you spend a lot of time in a basement office or a basement bedroom, that's the room to test. Testing the main floor only, when there's a finished basement in regular use, can understate your actual exposure.

What to do with your result

• Below 2 pCi/L: No action needed beyond periodic retesting, especially after any major foundation work or renovation.
• 2 to 4 pCi/L: Consider mitigation, particularly if the home has occupants who spend a lot of time in lower levels, or if a simple system would be inexpensive given your foundation.
• 4 pCi/L or above: Retest to confirm, then contact a licensed or certified mitigation contractor. See our cost guide for what to expect, and our directory to find a licensed contractor near you.

After mitigation

A properly installed system typically brings levels down to well under 2 pCi/L, often close to outdoor background levels. A post-installation test is the standard way to confirm the system is working, and periodic retesting every couple of years afterward is a good habit, since fans do eventually wear out and stop pulling air the way they should.