A radon detector is a device that measures the concentration of radon gas in indoor air, typically reporting results in picocuries per liter (pCi/L). Radon is a naturally occurring radioactive gas that seeps into buildings from the ground, and because it’s colorless and odorless, a detector is the only way to know if it’s present. The EPA recommends taking action if your home’s radon level reaches 4 pCi/L or higher.
How Radon Detectors Work
All radon detectors rely on the same basic physics: radon is radioactive, and as it decays, it releases alpha particles. Detectors capture or count these particles to calculate how much radon is in the air. The differences between detector types come down to how they detect those particles and whether they give you a single reading after days or weeks, or continuous updates in real time.
Detectors fall into two broad categories: passive and active. Passive detectors have no electronics or power source. They collect a sample over a set period and then get sent to a lab for analysis. Active detectors are electronic monitors that measure radon continuously and display results on a screen or send them to a phone app.
Passive Detectors: Test Kits
Passive detectors are the most affordable option and what most people use for an initial screening. They come in two main varieties.
Charcoal canisters are small containers filled with activated charcoal that adsorbs radon gas from the surrounding air. You leave the canister open in your home for a set exposure period (typically 2 to 7 days), seal it, and mail it to a lab. The lab measures the radioactive decay products trapped in the charcoal to determine your radon level. One limitation is that humidity can affect the charcoal’s ability to capture radon, though newer designs use a diffusion barrier to reduce this problem and extend the useful exposure window by several days.
Alpha track detectors use a small piece of plastic film. As radon decays nearby, alpha particles strike the film and leave microscopic damage marks. After the exposure period, a lab chemically treats the film to make those tracks visible and counts them. More tracks mean more radon. These detectors aren’t sensitive enough for exposures of a week or less, so they’re used for longer-term testing, usually 90 days to a full year.
Both types typically cost under $30 for a kit, including the lab analysis fee. They give you a single average reading for the entire exposure period rather than showing how radon levels change hour to hour.
Active Detectors: Digital Monitors
Electronic radon monitors (sometimes called continuous radon monitors) measure radon in real time and update their readings at regular intervals, often hourly. They cost more upfront, usually between $100 and $300 for consumer-grade models, but they’re reusable and let you track radon trends over time. Most connect to a smartphone app for easy data logging.
These devices use a few different sensor technologies. Pulsed ionization chambers draw air through a small chamber and detect the electrical charge created when alpha particles ionize the air inside. They respond quickly, typically within one to two hours. Silicon diffusion chambers, used in popular brands like Airthings, rely on a silicon chip that registers alpha particle hits as tiny electrical signals. Air enters the detector passively through structural filters that block unwanted particles. This diffusion process means response times are slower, roughly 8 to 16 hours, but these sensors tend to have less interference from thoron (a related gas that can skew readings).
Research-grade instruments use solid-state alpha spectrometers that actively pump air through the detector and can distinguish between different radioactive isotopes, virtually eliminating thoron interference. These are primarily used by professionals and cost significantly more.
Consumer digital monitors generally have sensor lifespans around 10 years. Some battery-powered models can run for 2 to 3 years on a single charge, making them low-maintenance once set up.
Short-Term vs. Long-Term Testing
Radon levels in a home fluctuate with the seasons, weather patterns, and how often you open windows. This means the length of your test matters as much as the type of detector you use.
Short-term tests (2 to 10 days) are useful as a quick screening tool. They’re the standard approach in real estate transactions when time is limited. However, their accuracy drops at moderate radon levels, with roughly a 50% success rate at correctly classifying homes near the action level. Winter tests tend to be more representative of annual exposure than summer tests, because closed-house conditions in cold months concentrate radon indoors. The tradeoff is that winter readings can overestimate your year-round average.
Long-term tests (90 days to 12 months) capture seasonal variation and give a much more reliable picture of actual exposure. International health organizations recommend testing for at least 3 months, and preferably a full year, before making decisions about mitigation. If a short-term test comes back elevated, a follow-up long-term test can confirm whether the problem is persistent or was a temporary spike.
A digital monitor sidesteps some of this dilemma because it tracks radon continuously. You can see both short-term spikes and long-term averages on the same device.
Where to Place a Radon Detector
Placement directly affects the accuracy of your results. The EPA recommends testing in the lowest livable level of your home, meaning a finished basement counts but a dirt-floor crawl space does not. Choose a room you or your family actually use: a bedroom, living room, den, or playroom. Avoid kitchens, bathrooms, laundry rooms, and hallways, where ventilation, humidity, or air movement can distort readings.
Set the detector at least 20 inches above the floor, away from exterior walls, drafts, windows, and heat sources. A bookshelf or side table in the middle of a room works well. Don’t disturb the device during the testing period, and keep windows and exterior doors closed as much as practical (normal entry and exit is fine).
Understanding Your Results
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, which typically involves a vent pipe and fan that pulls radon from beneath your foundation and releases it outdoors. Most countries worldwide use a similar threshold.
Readings between 2 and 4 pCi/L are worth monitoring. There is no known safe level of radon exposure, and the EPA notes that even levels below 4 pCi/L carry some risk. If your initial test falls in this range, a long-term follow-up test can help you decide whether mitigation makes sense for your situation.
Choosing the Right Detector
Your choice depends on your goal. If you need a quick, inexpensive answer about whether your home has a radon problem, a charcoal canister kit from a hardware store is the simplest starting point. If you want ongoing monitoring with real-time data, a digital monitor is worth the investment, especially if you live in a high-radon area or have already installed a mitigation system and want to verify it’s working.
For the most reliable results from any device, look for detectors listed on the National Radon Proficiency Program’s approved device list. Professional radon measurement technicians are required to use NRPP-approved devices, have them tested biennially in certified radon chambers, and submit proof of calibration annually. Consumer devices don’t face the same requirements, but choosing one from a recognized manufacturer with published accuracy data gives you more confidence in your readings. Research evaluations have found that consumer-grade active sensors differ from professional instruments in both absolute accuracy and their ability to track rapid changes in radon levels, so treat consumer monitor readings as a reliable trend indicator rather than a laboratory-precise measurement.