Radon Testing: How to Test Your Home and What the Results Mean

Your basement looks fine. The air smells clean. Nothing about the room suggests a problem. But radon is invisible and odorless, it gives you no cue to act. The only way to know whether it is accumulating in your home is to test for it. Radon is one of the more straightforward indoor air quality problems: it's dangerous, it's common, and it's fixable. A $15 test kit and a few days of patience will tell you where you stand.

This guide covers what radon is, why it matters, how to test for it accurately, and what to do if your levels are elevated.

What Radon Is and Where It Comes From

Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock. Uranium is present in virtually all soils, so radon is being generated everywhere, the concentrations just vary depending on local geology, soil permeability, and building characteristics.

The gas migrates upward through soil and enters buildings through any opening in contact with the ground: cracks in concrete slabs, gaps around pipes and utility penetrations, joints between walls and floors, sump pump openings, and porous concrete block walls. It can also dissolve in groundwater and enter through well water, though this is a secondary pathway in most homes.

Once inside, radon accumulates. A well-sealed, energy-efficient home can trap radon more effectively than a drafty older house, one of the ironies of building tight without mechanical ventilation. The lowest levels of a building, where the structure contacts the ground, typically have the highest concentrations. Upper floors are usually lower, though not always negligibly so.

Radon is measured in picocuries per liter (pCi/L) in the United States and in becquerels per cubic meter (Bq/m³) internationally. The conversion is 1 pCi/L = 37 Bq/m³.

Why Radon Matters: The Health Risk

Radon is the second leading cause of lung cancer after smoking. The EPA estimates that radon causes approximately 21,000 lung cancer deaths per year in the United States. For non-smokers, it is the leading cause of lung cancer.

The mechanism is direct. Radon itself is a noble gas, chemically inert, inhaled and exhaled without reacting. But as it decays, it produces radioactive daughter products (polonium-218 and polonium-214) that are solid particles. These particles lodge in lung tissue and emit alpha radiation, damaging DNA in the cells they contact. Over years of exposure, this damage accumulates and can initiate cancer.

The risk is proportional to both concentration and duration of exposure. Living in a home with 4 pCi/L for decades carries roughly the same lung cancer risk as smoking half a pack of cigarettes per day. At 8 pCi/L, the risk doubles. For smokers, radon exposure multiplies the already-elevated risk substantially, the two carcinogens are synergistic, not merely additive.

There is no known safe level of radon exposure. The risk does not drop to zero at any threshold, it diminishes as concentrations decrease, but some risk persists even at low levels. This is why building biology and international health guidelines set action thresholds well below the EPA's 4 pCi/L.

Testing Thresholds: How Much Is Too Much

Different organizations set different action levels, and understanding the range helps you make informed decisions about your own home.

SBM-2008 Standard

The SBM-2008 standard from the Institute of Building Biology uses a four-tier system that reflects a precautionary approach to radon exposure:

Notice that the SBM-2008 "severe concern" range begins at 50 Bq/m³, well below the EPA action level. This reflects the building biology principle of reducing exposures as far as practically achievable, rather than accepting a regulatory threshold as "safe enough."

EPA Action Level

The EPA recommends taking action to reduce radon if levels are at or above 4 pCi/L (148 Bq/m³). The agency also suggests considering mitigation for levels between 2 and 4 pCi/L, though this is framed as a recommendation rather than an action level. The 4 pCi/L threshold is not a safety line, it's a risk-management threshold based on practical and economic considerations at the time it was set.

WHO Guideline

The World Health Organization recommends a reference level of 100 Bq/m³ (2.7 pCi/L). Where this is not achievable, the WHO suggests a maximum of 300 Bq/m³. The WHO guideline sits between the SBM-2008 standard and the EPA action level, and it reflects the international consensus that the EPA's 4 pCi/L is higher than ideal.

Testing Methods

Radon testing is straightforward and inexpensive compared to most indoor air quality assessments. The methods fall into three categories, each suited to a different purpose.

Short-Term Tests (2–7 Days)

Short-term radon tests use charcoal canisters or electret ion chambers placed in the home for two to seven days. After the exposure period, the device is sealed and mailed to a lab for analysis. Results come back within a week or two.

Cost: $15–25 per test. Many state radon programs offer discounted or free test kits.

Strengths: Fast, cheap, and widely available. A good starting point if you have never tested or need a quick screening, such as during a real estate transaction.

Limitations: Radon levels in a home fluctuate significantly with weather, season, soil moisture, barometric pressure, and ventilation patterns. A two-day test captures a snapshot, not an average. A test during a rainy week in winter with the house sealed up may read substantially higher than the same home tested during a dry week in summer with windows open. A short-term test can tell you whether radon is likely a concern, but it cannot tell you your actual long-term exposure. If a short-term test returns elevated results, it justifies action, but a low short-term result doesn't guarantee safety. Follow up with long-term testing.

Long-Term Tests (90+ Days)

Long-term tests use alpha track detectors placed in the home for 90 days to a full year. The detector contains a small piece of plastic film that records alpha particle tracks from radon decay. After the exposure period, the detector is mailed to a lab, where the tracks are counted to calculate the average radon concentration over the entire testing period.

Cost: $25–40 per detector.

Strengths: This is the definitive test. A long-term measurement captures seasonal variation, weather fluctuations, and changes in building use patterns. The result represents what you actually breathe over months of normal living, not an artificial snapshot under closed-house conditions. If you're making decisions about mitigation based on a test result, a long-term test is the one you want.

Limitations: You have to wait. A 90-day minimum means results aren't available quickly. This makes long-term tests unsuitable for real estate transactions with tight timelines. They also require that the detector remain undisturbed in the same location for the entire testing period.

Continuous Radon Monitors

Digital radon monitors are electronic devices that measure radon continuously and display real-time or near-real-time readings. Most consumer-grade monitors update hourly or daily and calculate running averages over days, weeks, and months.

Cost: $100–250 for a consumer-grade monitor.

Strengths: Ongoing monitoring without the wait-and-mail cycle of passive detectors. You can watch levels change in response to weather, ventilation, and seasonal shifts. If you install a mitigation system, a continuous monitor lets you verify it's working in real time. Some models log data and connect to smartphone apps for tracking trends.

Limitations: Consumer monitors are less precise than lab-analyzed detectors, particularly over short measurement intervals. The first few days of readings from a newly placed monitor are unreliable, most need a week or more of data before the readings stabilize. They're more expensive than passive tests, though the cost is one-time rather than per-test. For a definitive baseline measurement, a lab-analyzed alpha track detector is still the standard.

Where to Place the Test

Placement matters. A test in the wrong location can understate the problem or produce misleading results.

• Test the lowest occupied or occupiable level. If you have a finished basement that anyone uses, for sleeping, working, or recreation, test there. If the basement is unfinished but could be finished in the future, test there as well. This gives you the worst-case reading for the building.
• Place the detector about three feet off the ground. Not on the floor, not on a high shelf. Radon concentrations vary with height, and chest-height placement approximates the breathing zone.
• Keep it away from exterior walls, windows, and doors. At least 20 inches from any exterior wall and away from drafts. Airflow from outside dilutes the local reading and doesn't represent what's accumulating in the room.
• Avoid kitchens, bathrooms, and laundry rooms. Humidity, ventilation fans, and intermittent airflow in these rooms affect readings.
• Leave the detector undisturbed. Don't move it during the test period. Don't place it in direct sunlight or near a heat source.

Closed-House Conditions

For short-term tests, maintain closed-house conditions beginning 12 hours before the test starts and continuing throughout the test period. This means keeping all windows and exterior doors closed except for normal entry and exit. Don't run whole-house fans. Heating and air conditioning can operate normally. The goal is to prevent outdoor air from diluting the reading and to simulate typical winter conditions, when radon levels are highest and ventilation is lowest.

Long-term tests and continuous monitors don't require closed-house conditions because they capture the full range of living conditions over their measurement period.

When to Test

Certain situations make testing particularly important:

• Before buying a home. A short-term test is standard practice in many real estate transactions. If results are elevated, negotiate mitigation as a condition of sale or adjust the offer to account for the cost.
• If you've never tested. Every home with ground contact should be tested at least once, regardless of geography. High-radon zones are well documented, but elevated levels occur in homes outside those zones regularly enough that geography alone isn't a reliable predictor.
• After foundation work. Any work that disturbs or modifies the foundation, crack repair, waterproofing, sump pump installation, basement finishing, can change radon entry pathways. Retest after the work is complete.
• After finishing a basement. Converting an unfinished basement to living space changes both the occupancy pattern and the ventilation dynamics. A previously untested or low-reading basement may need attention once people are spending time there regularly.
• After weatherization or air sealing. Tightening a home's envelope reduces air exchange. This is good for energy efficiency but can increase radon accumulation. If you've had significant air sealing work done, blown insulation, new windows, spray foam in the rim joist, retest.

Interpreting Your Results

Once you have a number, compare it against the thresholds above. Here is what the ranges mean in practice:

Below 2 pCi/L (74 Bq/m³): Low. This is within or near the SBM-2008 "slight concern" range. No immediate action needed, but periodic retesting every few years is reasonable, conditions change as buildings age and settle.

2–4 pCi/L (74–148 Bq/m³): Moderate. The EPA considers this the range where mitigation should be "considered." The WHO and SBM-2008 standards would flag this as concerning. If a long-term test confirms levels consistently in this range, mitigation is a sound investment, both for health and for resale value.

Above 4 pCi/L (148 Bq/m³): Elevated. The EPA action level. Mitigation is recommended. The higher the level, the more urgently you should act, but even at 4-8 pCi/L, this is not an emergency that requires leaving the home. It is a problem that should be addressed within months, not ignored indefinitely.

Above 10 pCi/L (370 Bq/m³): High. Mitigation should be a priority. At these concentrations, the annual radiation dose to the lungs is substantial, and every month of delay adds to the cumulative exposure.

If your result is borderline, near any of these thresholds, test again. Ideally, test in a different season. A short-term test that reads 3.5 pCi/L in summer might read 6 pCi/L in winter when the house is sealed tight and the stack effect is pulling soil gas upward. The long-term alpha track test resolves this ambiguity by averaging across seasons.

Mitigation: Reducing Radon Levels

If testing confirms elevated radon, the good news is that mitigation works. Proven systems reduce radon levels by 95–99%, often bringing homes from well above 4 pCi/L down to below 1 pCi/L. The technology is mature and reliable.

Sub-Slab Depressurization

This is the most common and most effective residential radon mitigation method. A contractor drills a hole through the basement slab or installs a suction point beneath it, connects a PVC pipe, and runs the pipe up through or alongside the building to exhaust above the roofline. A small, continuously running fan in the pipe creates negative pressure beneath the slab, drawing radon-laden soil gas out before it can enter the home.

Effectiveness: 95–99% reduction in most installations.

Cost: $800–2,500 installed, depending on foundation type, accessibility, and local labor rates. Most installations fall in the $1,000–1,500 range.

The system runs continuously and uses about as much electricity as a 75-watt light bulb. Fan replacement every 5–10 years is the only ongoing maintenance, and replacement fans cost $100–200. A properly installed system is quiet, unobtrusive, and effectively permanent.

Variations include sub-membrane depressurization for crawlspaces (the same principle, with a sealed vapor barrier over the soil instead of a concrete slab) and drain tile suction for homes with existing perimeter drain systems.

Sealing Cracks and Openings

Sealing visible cracks in the foundation, gaps around pipes, and openings around sump pits reduces radon entry points. However, sealing alone is not a reliable mitigation strategy. The pressure differential between the soil and the indoor air will find pathways even through intact concrete, radon can pass through hairline cracks and porous concrete that you can't see or seal effectively. Sealing is a useful supplement to active depressurization, not a replacement for it.

Ventilation Improvements

Increasing air exchange dilutes indoor radon by replacing some of the contaminated indoor air with outdoor air. Opening windows works but is impractical in cold climates and defeats the purpose of an insulated building envelope. Mechanical ventilation, supply fans, exhaust fans, or balanced systems, can reduce radon but is generally less effective and less predictable than sub-slab depressurization.

HRV and ERV Systems

Heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) exchange indoor air for outdoor air while recovering most of the thermal energy from the outgoing airstream. An HRV recovers heat; an ERV recovers both heat and moisture. These systems reduce radon by increasing the air exchange rate while minimizing the energy penalty of ventilation.

HRVs and ERVs are particularly well suited for tight, energy-efficient homes where natural air exchange is minimal. They address radon as part of a broader indoor air quality strategy, also diluting VOCs, CO2, moisture, and other indoor pollutants. For homes with moderately elevated radon (2–4 pCi/L), an HRV or ERV may be sufficient on its own. For higher levels, they work well as a complement to sub-slab depressurization.

Costs at a Glance

After Mitigation: Verify and Monitor

A mitigation system is only as good as the post-installation test confirms it to be. After installation, wait at least 24 hours (48 is better), then run a short-term test to verify the system is working. Levels should drop dramatically, typically by 90% or more. If the reduction is less than expected, the contractor should evaluate the system for leaks, insufficient suction points, or fan sizing issues.

Follow up the initial verification test with a long-term alpha track test over the next heating season. This confirms that the system maintains low levels across the full range of conditions your home experiences.

Ongoing monitoring is simple: check the system's manometer or indicator gauge periodically to confirm the fan is running and maintaining suction. Most systems include a U-tube manometer, a small, liquid-filled gauge on the pipe that shows pressure differential. If the liquid levels are equal, the fan isn't pulling and needs attention. A continuous digital monitor provides an additional layer of assurance and will alert you to any change in conditions.

Retest every two to five years even with a functioning system. Building settlement, new cracks, changes to the foundation, and aging fan motors can all affect performance.

Common Questions

My neighbor tested low, do I still need to test?

Yes. Radon levels can vary dramatically between adjacent homes. Differences in foundation type, soil contact area, crack patterns, ventilation, and microscale geology mean that one home can test at 1 pCi/L while the house next door reads 8 pCi/L. Your neighbor's result tells you nothing about your home.

Is radon only a problem in basements?

Basements and ground-floor slabs typically have the highest concentrations because they're in direct contact with the soil. But radon can migrate to upper floors, especially in homes with strong stack effect, the natural upward airflow caused by warm air rising. Apartments on upper floors of multi-story buildings generally have lower radon levels, but ground-floor and first-floor units can be affected, particularly over crawlspaces or in slab-on-grade construction.

Does a new home need testing?

Absolutely. New construction can have high radon levels. Some new homes are built with radon-resistant features, a gravel layer beneath the slab, sealed vapor barriers, a passive pipe roughed in for future fan installation, but these features don't guarantee low levels. They make mitigation easier and cheaper if needed, which is valuable, but testing is still necessary to know where you stand.

How long does mitigation take?

A professional sub-slab depressurization installation typically takes half a day to a full day. The system works immediately upon activation, there's no waiting period for it to take effect. You can verify the reduction within 48 hours using a short-term test or continuous monitor.

Next Steps

If you've never tested your home for radon, a short-term test kit is the simplest starting point. Order one online or from your state radon program, place it in the lowest occupied level, and mail it back after the exposure period. You'll have a number within two weeks, and that number will either put your mind at ease or point you toward a straightforward fix.

For a definitive measurement, follow up with a long-term alpha track detector over three to twelve months. This gives you the annual average that actually represents your exposure.

Radon is part of the broader picture of indoor air quality. For related testing, mold, VOCs, formaldehyde, and particulates, see the indoor air quality testing guide. The healthy home checklist covers all the factors that affect health in the home. For professional help, a certified building biologist can conduct a full evaluation. For the standards referenced throughout this guide, see the SBM-2008 standard overview and the building biology FAQ.

You cannot see radon, smell it, or feel it. But you can measure it in a weekend and fix it in a day. That is one of the rare pieces of good news in indoor air quality, the most dangerous gas in most homes is also the easiest to solve.