Radon levels in your home: how high is too high

A radon level of 4 picocuries per liter (pCi/L) or higher is the point where the EPA says to fix a home. Between 2 and 4 pCi/L, the agency says a fix is worth considering. Below 2 pCi/L, most homes don't need one.

Maybe a home inspector handed you a report with a pCi/L number and no context, or you bought a test kit after hearing that radon levels in a home can run high without any smell or symptom to warn you. Either way, the number by itself doesn't tell you much. This post explains what counts as a high reading, how radon is tested, and where a continuous monitor fits next to a proper test.

Key Takeaways

• The EPA's action level is 4 pCi/L; it says to consider fixing a home between 2 and 4 pCi/L.
• The national average indoor reading is about 1.3 pCi/L; outdoor air averages about 0.4 pCi/L.
• A short-term radon test runs 2 to 90 days; a long-term test runs more than 90 days and averages out seasonal swings.
• Radon tends to read highest in winter, when a closed house pulls more soil gas in through the lowest level.
• Radon needs months of data to read reliably, unlike CO2 or PM2.5, which respond within hours.

What radon level is too high in a home?

The EPA draws the line at 4 pCi/L: at or above that reading, it recommends fixing the home. From 2 to 4 pCi/L, the EPA says a fix is worth considering, especially if the home has small children or if the reading has been climbing. Below 2 pCi/L, the agency doesn't recommend a fix.

For comparison, the national average indoor radon level is about 1.3 pCi/L, and the average outdoor level is about 0.4 pCi/L. The World Health Organization sets a lower reference level, about 2.7 pCi/L (100 becquerels per cubic meter), and recommends acting closer to that number where it's practical. There's no reading of zero: radon is in the ground almost everywhere, and every home lets some in.

Why radon levels change by season

Radon is a gas that comes up from the soil under a house, through cracks in the foundation, sump pits, and gaps around pipes. A basement or crawlspace usually reads highest, because it sits closest to the source.

The reading isn't constant. In winter, a closed house with the heat running pulls air up through the lowest level and out through the roof and upper floors, known as the stack effect. That draws more soil gas in behind it, which is why radon levels in a home often run highest in the coldest months and lowest when the windows are open. A single test taken in July can understate what a house sees in January.

How to test radon levels in your home

A short-term test kit, run for 2 to 90 days with the windows closed, is the fastest way to get a number. It's inexpensive and works for a quick check, such as before a home sale, but it only shows a snapshot.

A long-term test, run for more than 90 days, or a continuous monitor left in place year-round, gives a more reliable picture because it captures the swing between seasons instead of one closed-house reading. A Measured Air kit reports radon alongside CO2, PM2.5, and humidity, but its radon reading needs months of data to mean anything, which is a real tradeoff against a short-term kit: if you need a fast answer for a closing date, a short-term test still does that job better.

What to do when a radon reading comes back high

At 4 pCi/L or above, the EPA recommends hiring a qualified radon mitigation contractor, who typically installs a vent pipe and fan system that draws soil gas out from under the foundation and releases it above the roofline. At 2 to 4 pCi/L, the usual next step is a retest, since a single reading can vary.

The EPA also names radon as a leading cause of lung cancer in the United States, second only to smoking, and the leading cause among people who have never smoked. That's the EPA's statement, not something a kit can confirm on its own. A Measured Air kit reports the radon number; the EPA's health guidance explains what the number means for health.

Where to put a radon monitor at home

Put the monitor on the lowest level you actually spend time in. If the basement is finished and used daily, test there. If it's unfinished and rarely visited, test on the first floor instead, since that's the air people are actually breathing.

Keep the monitor away from drafts, exterior doors, windows, and vents, all of which pull in outside air and can push the reading down artificially. A closet, a corner of a bedroom, or a home office works better than a spot next to a window unit.

Radon next to the rest of your home's air

Measured Air makes kits that report on the air people actually breathe: PM2.5, CO2, VOCs, humidity, and radon, each next to the outdoor air at your address. On the homeowners page, every reading is kept in history, so a January radon average can sit next to last summer's, which is useful once you have enough months of data to compare.

Radon is the slowest-moving reading a kit reports. A CO2 spike tells you something changed by morning. A radon average takes a season to mean anything. Each one still points to an action: crack a window when CO2 climbs, call a mitigation contractor when radon sits at 4 pCi/L or above.

FAQ

What is a safe radon level in a home? Below 2 pCi/L, the EPA doesn't recommend any action. Between 2 and 4 pCi/L, a fix is worth considering. At 4 pCi/L or above, the EPA recommends fixing the home.

How long does a radon test take? A short-term test runs 2 to 90 days with the house closed up. A long-term test runs more than 90 days and gives a better year-round average, since radon levels shift with the seasons.

Does radon level change with the seasons? Yes. Homes typically read highest in winter, when doors and windows stay shut and heating draws more soil gas in through the lowest level, and lowest when the house is opened up in warmer months.

Where should I test for radon in my home? Test on the lowest level you regularly use, away from drafts, doors, and windows. If the basement isn't lived in, test the first floor instead.

Can an air quality monitor test for radon? Some continuous monitors, including a Measured Air kit, report a radon reading, but it needs months of data to be reliable. A short-term test kit still answers faster if you need a number quickly, such as before selling a home.

What causes high radon levels in a home? Radon rises from uranium breaking down in soil and rock, and it enters a house through foundation cracks, sump pits, crawlspaces, and gaps around pipes. A tightly sealed, well-heated house in winter can pull in more of it.

The bottom line

A radon level under 2 pCi/L needs no action. Between 2 and 4 pCi/L, the EPA says to consider a fix, and at 4 pCi/L or above, it recommends one. Because radon shifts with the seasons, a single short-term test or a few months of continuous readings tell very different stories, and the lowest level you actually use is the room to test.

A kit that keeps a running history makes it easier to see where a home's radon sits over time, next to the rest of what's in the air. Order a kit sized to your home and start with a baseline for every room that matters.