A radon test measures the concentration of radon-222, a radioactive gas, in the air inside your home. Radon is colorless and odorless, so testing is the only way to know whether it’s present. The gas forms naturally underground when radium in soil and rock decays, and it seeps into homes through cracks in foundations, gaps around pipes, and other openings. Radon is responsible for an estimated 21,000 lung cancer deaths in the United States every year, making it the leading cause of lung cancer among non-smokers.
What the Test Actually Measures
Radon tests detect the concentration of radon-222 gas and its radioactive decay products in indoor air. As radon breaks down, it releases alpha particles, tiny bursts of energy that can damage lung tissue when inhaled over time. The result is reported in picocuries per liter (pCi/L), a unit that reflects how much radioactive material is present in a given volume of air.
Different test devices capture this measurement in different ways. Some use activated charcoal that absorbs radon from the air, which is then analyzed in a lab. Others use alpha track detectors, small strips of material that record the marks left by alpha particles over a longer period. Both approaches are measuring the same thing: how much radon is accumulating in your living space.
Short-Term vs. Long-Term Tests
Short-term test kits measure radon over a period of 2 to 90 days. These give you a quick snapshot and are commonly used during real estate transactions or as a first screening. Long-term kits measure radon for more than 90 days and provide a more accurate picture of your home’s average exposure, since radon levels fluctuate with the seasons, weather patterns, and how you use your home.
Several factors cause those fluctuations. Radon levels tend to be higher in winter, when homes are sealed up and less ventilation dilutes the gas. Changes in barometric pressure can push more radon out of the soil and into your basement. Even home renovations, like adding insulation to reduce energy costs, can raise radon concentrations by reducing airflow. A single short-term test might catch your home on a high day or a low one, which is why long-term testing gives a more reliable number.
Where to Place the Test
The EPA recommends placing the test device in the lowest livable level of your home, typically the basement or ground floor. The room should be one you use regularly: a family room, living room, bedroom, or den. Kitchens, bathrooms, laundry rooms, and hallways are not good choices because of the humidity, drafts, or lack of consistent air patterns in those spaces.
Set the device at least 20 inches above the floor, away from exterior walls, drafts, and sources of high heat or humidity. Leave it undisturbed for the full testing period. Moving it or opening windows during the test can skew your results.
What the Results Mean
The EPA’s action level is 4 pCi/L. If your test comes back at or above that number, the recommendation is to install a radon mitigation system, typically a vent pipe and fan that pulls radon from beneath your foundation and releases it outside. Because there is no known safe level of radon exposure, the EPA also suggests considering mitigation for levels between 2 and 4 pCi/L.
For context, the World Health Organization sets its reference level at 300 Bq/m³ for homes, which is roughly equivalent to about 8 pCi/L. The U.S. threshold is more conservative, reflecting a goal to reduce risk as much as practically possible.
Radon in Well Water
Radon testing isn’t limited to air. If your home uses well water, radon can dissolve into the groundwater and enter your home when you shower, run the dishwasher, or use a faucet. The primary risk isn’t from drinking the water (the associated stomach cancer risk is very low) but from breathing in the gas after it escapes into your indoor air. As a rough rule, every 10,000 pCi/L of radon in your water adds about 1 pCi/L to your indoor air.
The recommended action level for radon in groundwater is 4,000 pCi/L. If levels fall between 4,000 and 10,000 pCi/L, an activated charcoal filtration system can reduce them. Above 10,000 pCi/L, an aeration system is the better option, as it agitates the water to release radon before it reaches your taps.
How Often to Retest
A single test gives you a baseline, but it’s not a permanent answer. The EPA recommends retesting even if your initial result was below 4 pCi/L, particularly if your living habits change. Moving into a previously unused basement, finishing a lower level, or making energy-efficiency upgrades can all shift your home’s radon profile. In real estate transactions, buyers often request a new test if the most recent one is more than two years old.
If you’ve had a mitigation system installed, periodic retesting confirms it’s still working properly. Fans wear out, seals degrade, and foundation cracks can develop over time. A quick short-term test every couple of years is a simple way to verify your system is keeping levels where they should be.