Radon measurement is the process of detecting and quantifying radon gas levels inside a building, typically expressed in picocuries per liter (pCi/L) in the United States or becquerels per cubic meter (Bq/m³) internationally. Because radon is colorless, odorless, and the second leading cause of lung cancer, measurement is the only way to know whether your home has a problem. The EPA recommends taking action if your home tests at or above 4 pCi/L (150 Bq/m³), and suggests considering mitigation even between 2 and 4 pCi/L, since there is no known safe level of exposure.
How Radon Is Measured
Radon measurement captures how much radioactive radon gas is present in a given volume of air. One becquerel equals one radioactive decay per second, so a reading of 100 Bq/m³ means 100 radon atoms are decaying every second in each cubic meter of air. The U.S. uses picocuries per liter instead: 10 Bq/m³ converts to roughly 0.27 pCi/L. Both units tell you the same thing, just on different scales.
Radon seeps into homes from the soil through cracks, joints, and gaps in the foundation. It accumulates most in areas with reduced ventilation, especially basements. Because levels fluctuate with weather, season, and how tightly a home is sealed, a single snapshot reading can be misleading. That’s why testing protocols, placement, and duration all matter.
Types of Radon Detectors
Radon detectors fall into two broad categories: passive devices that collect data without power, and active monitors that take continuous electronic readings.
Charcoal Canisters
These are the most common short-term test kits. You open a sealed container of activated charcoal and leave it in place for 2 to 7 days. Radon in the air adsorbs onto the charcoal granules. After the testing period, you reseal the canister and mail it to a lab, where technicians count the gamma radiation emitted by radon’s decay products to calculate the concentration. Charcoal canisters are inexpensive (typically $10 to $40) but have a notable limitation: the charcoal continuously adsorbs and releases radon, so the result isn’t a true average over the full exposure period. High humidity can also affect accuracy, which is worth knowing if you’re testing in a damp basement.
Alpha Track Detectors
Alpha track detectors use a small piece of special plastic inside a container. Radon drifts into the container and decays, releasing alpha particles that strike the plastic and leave microscopic damage marks. After deployment (usually 90 days or more for a long-term test), the plastic is treated with a chemical solution that makes the damage visible under a microscope. A lab counts the track density to determine radon concentration. These detectors cost roughly $25 to $60 and give a better picture of average levels because they integrate exposure over months.
Continuous Radon Monitors
Electronic monitors sample the air and log radon levels at regular intervals, often hourly. Professional-grade versions are used during real estate inspections and can cost $150 to $700 when deployed by a certified tester. Consumer-grade digital monitors, priced between $100 and $250, let you track levels in real time at home. Recent research has found that lower-cost continuous sensors perform well even in humid basement conditions, producing results comparable to more expensive research instruments.
Short-Term vs. Long-Term Testing
Short-term tests run from 2 to 90 days and give you a quick read. They’re useful as a first screening step, especially during a home purchase when time is limited. Long-term tests run for more than 90 days and capture seasonal variation in radon levels, giving you a much more reliable picture of your actual year-round exposure. The CDC notes that the longer the test, the better the results reflect both your home’s radon levels and your daily habits (how often you open doors, run ventilation, and so on).
If a short-term test comes back at or above 4 pCi/L, a common next step is either a second short-term test to confirm or a long-term test to get a more representative average before investing in a mitigation system.
Closed-House Conditions
For a short-term test to produce valid results, you need to maintain what’s called closed-house conditions. All exterior doors and windows must be shut for at least 12 hours before testing begins, and they must stay closed for the entire testing period. Normal entry and exit through doors is fine. You don’t need to seal your home or stop living normally. You just can’t leave windows open or run whole-house fans that pull in outside air, because that dilutes radon and produces artificially low readings.
Where to Place the Detector
Placement directly affects accuracy. The general rule is to test on the lowest level of your home where you spend four or more hours per day. For many people, that’s the main floor rather than an unfinished basement you only visit to do laundry. But if you exercise, work, or spend significant time in your basement, test there, since higher breathing rates increase your exposure risk over time.
Keep the detector off the floor and away from the ceiling. Hanging it or placing it on a table at breathing height works well. Avoid kitchens, bathrooms, spots near open windows, and areas with strong airflow, all of which can skew readings. Crawl spaces and utility rooms aren’t ideal either, since they don’t represent the air you actually breathe throughout the day.
Understanding Your Results
The EPA’s action level is 4 pCi/L (150 Bq/m³). At or above this threshold, the agency recommends installing a radon mitigation system, which typically involves a vent pipe and fan that draw radon from beneath the foundation and exhaust it outside. Most countries worldwide have adopted similar action levels. Between 2 and 4 pCi/L, the EPA still encourages homeowners to consider mitigation, acknowledging that any radon exposure carries some lung cancer risk.
Keep in mind that radon levels naturally fluctuate. A single reading of 3.8 pCi/L one week could be 4.5 pCi/L the next, depending on weather, barometric pressure, and ventilation patterns. That’s why results near the action level often warrant a follow-up test, ideally a long-term one, before making a decision.
DIY Kits vs. Professional Testing
DIY test kits are the most accessible option. Short-term charcoal kits run $10 to $40, long-term alpha track kits cost $25 to $60, and digital monitors for ongoing home use range from $100 to $250. Lab fees and shipping may be extra for mail-in kits. These are perfectly adequate for routine home screening.
Professional radon testing costs $150 to $700, depending on your home’s size and location. A certified professional brings calibrated continuous monitors, follows standardized protocols, and delivers a detailed report. This is the route most commonly required during real estate transactions, where accuracy and documentation matter. Look for testers certified through the National Radon Proficiency Program (NRPP), which requires adherence to national measurement standards and a professional code of ethics.
What Affects Measurement Accuracy
Several factors can throw off a radon reading. Humidity is the most common culprit: charcoal canisters are particularly sensitive to moisture, which is why some professionals choose electret ion chambers instead when testing in damp basements. Ventilation matters too. Opening windows during a test dilutes radon and produces a falsely low result. Even barometric pressure shifts can temporarily raise or lower radon entry from the soil.
The best way to minimize these effects is to follow closed-house conditions, place the detector correctly, and choose a testing duration that smooths out short-term fluctuations. A 90-day test with an alpha track detector or a continuous digital monitor running over several weeks will always be more reliable than a 48-hour charcoal canister, especially if your home sits near the action level.