Radon in Your Home: Testing, Safe Levels & Mitigation

What is Radon?

Radon is a radioactive gas produced by the natural breakdown of uranium in soil, rock and groundwater. Uranium is present in trace amounts almost everywhere, so radon is constantly being released from the ground and drifting into the air. Outdoors it disperses to harmless concentrations. Indoors, where it can accumulate, it becomes a health problem.

The danger comes from what radon does after you breathe it in. As radon decays, it produces short-lived radioactive particles that lodge in lung tissue and emit alpha radiation as they continue to break down. Over years of exposure, that radiation damages lung cells and raises the risk of lung cancer. The EPA estimates radon is responsible for roughly 21,000 lung cancer deaths in the U.S. each year, making it the second-leading cause of lung cancer after smoking and the leading cause among people who have never smoked. Smokers face a much higher combined risk, because tobacco smoke and radon damage compound each other.

Radon has no smell, color or taste, and it produces no immediate symptoms. Unlike carbon monoxide, which can make you sick within hours and is caught by a cheap alarm, radon does its damage silently over decades. That is why testing, rather than waiting for a sign, is the whole strategy.

How Radon Enters a Home

Radon moves through the soil as a gas and follows the path of least resistance. A house acts like a weak vacuum on the ground beneath it: warm indoor air rises and escapes through the upper levels, and the resulting slight negative pressure at the foundation pulls soil gas up through any opening it can find. Common entry points include:

• Cracks in slab floors and foundation walls, including hairline shrinkage cracks
• Construction joints where the floor slab meets the wall
• Gaps around service penetrations for plumbing, gas lines, and electrical conduit
• Sump pits and floor drains that open directly to the soil or drain tile
• Hollow-block walls, which act as a channel for soil gas
• Exposed soil in crawl spaces
• Well water, which can release dissolved radon into the air when you shower or run taps (a secondary route in most homes, but worth knowing if you are on a private well)

Because the driving force is pressure difference, radon levels rise when the house is closed up and the heating system is running. They also change with weather, soil moisture, and how you use exhaust fans. That variability is why one short reading is never the final word.

Radon Levels: What the Numbers Mean

Radon in air is measured in picocuries per liter (pCi/L) in the U.S. Two EPA thresholds matter:

• 4 pCi/L — the EPA action level. If a confirmed test reads 4 pCi/L or higher, the EPA recommends you fix your home. This is not a “safe” line so much as the level at which mitigation is clearly worth it; the EPA is explicit that there is no known safe level of radon.
• 2 to 4 pCi/L — consider fixing. Risk still exists in this range, and the EPA says to consider mitigation. Modern systems can often bring levels well under 2 pCi/L, so many homeowners who test in this band choose to fix anyway, especially if someone in the home smokes or if the space is a bedroom or home office.

For scale, the EPA cites an average indoor level of about 1.3 pCi/L and an average outdoor level of about 0.4 pCi/L. The World Health Organization uses a lower reference level of 100 becquerels per cubic meter, which works out to roughly 2.7 pCi/L, so international guidance leans more conservative than the U.S. action level.

Who Is at Risk?

Every home can have a radon problem. Elevated levels have been found in new and old houses, drafty and well-sealed ones, homes with basements and homes without. That said, some situations raise the odds:

• Basements and below-grade living space. Any room in direct contact with soil, especially a finished basement bedroom or office where people spend hours, is the highest-exposure part of the house.
• Slab-on-grade homes. No basement does not mean no radon. The slab sits on soil, and cracks or plumbing penetrations open straight into living space.
• Regions with uranium-rich geology. Granite, shale, and certain glacial deposits tend to produce more radon. The EPA maps every U.S. county into three radon zones; Zone 1 counties have a predicted average indoor level above 4 pCi/L. Large parts of Iowa, Pennsylvania, Ohio, Colorado and Minnesota fall into Zone 1, along with many counties across the northern Plains, the Appalachians and the Rocky Mountain states.
• Tight, energy-efficient homes. Less air exchange means whatever radon enters stays longer.
• Private wells in high-radon areas. Waterborne radon adds to airborne levels when water is agitated indoors.

Two cautions about zone maps. First, they predict averages, not individual homes; a Zone 3 (lowest-risk) county still contains houses over 4 pCi/L. Second, the EPA notes that granite countertops, a frequent worry, contribute a negligible amount of radon in nearly all cases compared to what comes up from the soil. Use the map to gauge urgency, then test regardless.

To find your county's zone and your state radon office, use the EPA's radon zone and state contact page. Many state programs sell test kits at cost or give them away during January, National Radon Action Month.

How to Test for Radon

There are three practical approaches, and they answer slightly different questions.

Short-Term Tests (2 to 90 days)

The most common are activated-charcoal canisters and charcoal liquid scintillation kits that sit in the home for 2 to 7 days, then get mailed to a lab. They are inexpensive and give you a result within a couple of weeks. The trade-off is that radon fluctuates day to day, so a short test is a snapshot, not an annual average. Short-term tests are the right tool for a first screening and for real estate transactions, where speed matters.

Long-Term Tests (more than 90 days)

Alpha-track and electret ion chamber detectors are left in place for three months to a year. Because they integrate across seasons and weather, they give a much better estimate of your true year-round exposure. If your short-term result landed anywhere near the action level, a long-term follow-up is the most reliable way to decide whether to mitigate.

Continuous Digital Radon Monitors

Consumer electronic monitors read radon continuously and display both a short-term (typically 1 to 7 day) and a long-term average. They cost more up front than a kit but are reusable, show you how levels swing with weather and season, and are the easiest way to confirm a mitigation system is still working years later. For anyone who spends real time in a basement, a monitor is the better long-term investment; treat it as a screening and tracking tool, and confirm high readings with a lab-analyzed test or a certified measurement professional before spending on mitigation.

Testing Protocol That Gives a Valid Result

• Test the lowest lived-in level. A finished basement counts. An unused crawl space does not — test the lowest floor people actually occupy.
• Use closed-house conditions for short-term tests. Keep windows and exterior doors closed (except for normal entry and exit) for 12 hours before the test starts and throughout the test. Do not run whole-house fans; normal HVAC operation is fine.
• Place the detector correctly. At least 20 inches above the floor, away from drafts, exterior walls, direct sun, high humidity (not a bathroom or kitchen), and out of reach of kids and pets.
• Prefer the heating season. Winter testing under closed conditions produces the most conservative reading. A summer test with windows open can badly under-report.
• Follow up before you act. The EPA protocol: if a short-term result is 4 pCi/L or higher, run a second test. If it was more than double the action level (above 8 pCi/L), do a second short-term test right away; otherwise a long-term test gives the better picture. Base the mitigation decision on the average of two short-term tests or on the long-term result.

Radon Mitigation: How It Works

The good news about radon is that it is one of the most fixable indoor air problems. The dominant method is active sub-slab depressurization (ASD), sometimes called sub-slab suction:

1. A hole is cored through the basement slab (or a crawl space is covered with a sealed plastic membrane).
2. A PVC pipe is set into the gravel or soil beneath the slab and routed up through the house or along an exterior wall.
3. An inline fan, mounted in the attic, garage or outside, runs continuously and pulls soil gas from under the slab.
4. The pipe discharges above the roofline, where radon disperses harmlessly outdoors.
5. Major entry points — sump pits, slab cracks, wall-floor joints — are sealed so the fan works on the soil rather than sucking conditioned air out of the house.

By reversing the pressure difference that was drawing radon in, ASD keeps soil gas from entering in the first place. The EPA notes that well-designed systems can reduce radon levels by up to 99 percent, and finished systems routinely test well below 2 pCi/L. A U-tube manometer on the pipe shows at a glance that the fan is drawing suction. Fans typically last many years but do eventually fail, so retesting every two years, or keeping a continuous monitor running, is the way to catch a dead fan before radon creeps back.

What Mitigation Typically Costs

A professionally installed ASD system typically runs from about $800 to $2,500. Where you land in that range depends on foundation type (a single poured slab is simplest; a mix of slab, block wall and crawl space needs more suction points and membrane work), home size, how far the pipe has to travel, and regional labor rates. Ongoing cost is the fan's electricity, usually on the order of a few dollars a month, plus a replacement fan every decade or so.

Sealing cracks alone is not a reliable fix; the EPA does not recommend it as a stand-alone method because soil gas finds new paths. Passive systems (the same pipe without a fan) are sometimes built into new construction and can be activated later by adding a fan if a test shows the need.

When to Hire a Certified Radon Professional

You can do the testing yourself. Mitigation is a different matter. Sizing the fan, choosing suction points, sealing correctly and making sure the system does not backdraft a furnace or water heater all take training, and a poorly built system can leave you with a false sense of security. Hire a professional when:

• Your confirmed level is at or above 4 pCi/L and you are ready to mitigate
• You are buying or selling a home and need a test that will hold up in the transaction
• A DIY or previously installed system is not bringing levels down
• You have a mixed or unusual foundation (partial basement plus crawl space, for example)

Look for certification from the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB), and check whether your state licenses radon contractors; a number of states require it. The EPA maintains a page on how to find a radon test kit or a qualified measurement and mitigation professional, which routes you to your state program and the national certification directories. Get more than one quote, ask for a post-mitigation test as part of the job, and confirm the system will be labeled and warrantied.

Radon and the Rest of Your Indoor Air

A HEPA air purifier will not remove radon; it is a gas, not a particle, and the fix is at the foundation rather than in the room. Radon does sit alongside the other indoor hazards worth addressing at the same time. If you are working through your home, pair a radon test with a carbon monoxide alarm check, a look at moisture and mold in the same below-grade spaces where radon collects, and the ventilation and filtration steps in our guide to improving indoor air quality.