Radon detectors measure the radioactive decay of radon gas by counting alpha particles, the tiny bursts of energy radon releases as it breaks down. Some detectors capture physical evidence of these particles on film or charcoal for later lab analysis, while others use electronic sensors to count particles in real time and display a concentration reading. The technology varies, but every radon detector relies on the same basic principle: radon and its decay products emit radiation that can be captured, counted, and converted into a concentration level.
Passive Detectors: Lab-Analyzed Results
Passive radon detectors have no electronics or power source. They collect evidence of radon over a set exposure period, then get mailed to a lab for analysis. Three main types dominate home testing.
Alpha Track Detectors
These are the most common long-term radon tests. A small chip of special plastic called CR-39 sits inside a tiny diffusion chamber. When radon seeps into the chamber and decays, the alpha particles it releases strike the plastic and leave microscopic damage marks. After the exposure period (typically 90 days to a year), the plastic chip goes to a lab where it’s soaked in a chemical solution that enlarges those damage marks into visible pits, or “tracks.” Technicians count the tracks under a microscope. More tracks means more radon.
Charcoal Canisters
Activated charcoal has an enormous surface area packed with tiny pores. Radon atoms stick to these pores through weak molecular attraction, accumulating over a short exposure window of 2 to 7 days. At the lab, technicians don’t measure the radon directly. Instead, they measure the gamma radiation given off by radon’s decay products using a specialized detector. Since those decay products exist in a predictable ratio with radon itself, the lab can calculate the original radon concentration from the gamma signal.
Charcoal canisters are the cheapest and most widely available home test kits, but they have a notable weakness: humidity. Moisture competes with radon for space on the charcoal’s surface. In high-humidity environments, standard charcoal canisters can produce errors of 12 to 18% compared to reference instruments. Some newer designs use moisture-absorbing filters that keep accuracy within about 1 to 3% even at 90% relative humidity.
Electret Ion Chambers
An electret is a disk of material that holds a permanent electrical charge, similar to how a magnet holds a magnetic field. In this detector, the charged disk sits inside a small ion chamber. When radon drifts in and decays, the alpha particles knock electrons off nearby air molecules, creating charged particles called ions. Those ions gradually neutralize the charge on the disk. After the test period, a technician measures how much the disk’s voltage dropped. The bigger the drop, the higher the radon concentration during the test.
Active Detectors: Real-Time Readings
Active (also called continuous) radon monitors use electronic sensors and provide ongoing concentration data, often updating every hour. These are the plug-in devices you can buy for home use or that a professional inspector brings for a short-term test.
Silicon Photodiode Sensors
Most consumer-grade digital radon monitors use a solid-state silicon detector paired with a small diffusion chamber. Air drifts into the chamber through a filter that blocks dust but lets radon gas pass. When radon decays inside the chamber, alpha particles strike the silicon sensor, generating a tiny electrical pulse. A microprocessor counts those pulses over time and converts the count into a radon concentration displayed on screen. Because radon levels fluctuate hour to hour, these devices typically average readings over 24 hours or longer to produce a stable number.
Pulsed Ionization Chambers
Professional-grade continuous monitors often use a pulsed ionization chamber. Radon diffuses through a filter into the chamber, where its alpha particles ionize oxygen and nitrogen molecules in the air. The resulting positive and negative ions are pulled toward oppositely charged surfaces inside the chamber, creating measurable electrical pulses. A microprocessor logs each pulse, and the device is calibrated so a specific number of pulses per hour corresponds to a known radon concentration. These monitors record timestamped data, which helps professionals spot unusual spikes or patterns that might indicate a ventilation issue or test interference.
Where and How to Place a Detector
Placement matters more than most people expect. You should test in the lowest lived-in level of your home, whether that’s a finished basement, a ground-floor bedroom, or a den. The device should sit on a flat surface at least 20 inches above the floor, because radon concentrations near the floor can be unrepresentative. Keep it at least 1 foot from any exterior wall, and 3 feet from exterior walls that have doors or windows. It should also be at least 4 inches from any other object and away from drafts, heat sources, and humidity (so not next to a washer, dryer, or bathroom).
Short-term tests (2 to 7 days) give a snapshot. Long-term tests (90 days or more) give a seasonal average that’s more representative of your actual exposure. If you’re doing a short-term test, closed-house conditions are standard: keep windows and exterior doors shut as much as possible for 12 hours before and during the test.
Why Seasonal Timing Matters
Radon levels inside a home are not constant. They follow a seasonal pattern driven by how tightly a house is sealed and how air pressure differences pull soil gas indoors. A study analyzing 13 years of indoor radon data found that January concentrations averaged about 204 Bq/m³, while July averaged roughly 130 Bq/m³, a difference of nearly 40%. Monthly averages exceeded the EPA action level in every month except June, July, and August.
Homes with basements show the most dramatic seasonal swings, with winter readings significantly higher than summer. Homes without basements show much less variation. Testing in winter gives you closer to a worst-case reading. Testing in summer risks underestimating your exposure. You can test any time of year, but if your result comes back borderline during warm months, a follow-up winter test or a long-term test that spans multiple seasons will give you a clearer picture.
Understanding Your Results
Radon concentration is measured in picocuries per liter (pCi/L) in the United States, or becquerels per cubic meter (Bq/m³) internationally. One pCi/L equals 37 Bq/m³.
The EPA’s action level is 4 pCi/L (148 Bq/m³). At or above that number, the EPA recommends installing a mitigation system to reduce radon levels. Between 2 and 4 pCi/L, the EPA still suggests considering mitigation, because there is no known safe level of radon exposure. Below 2 pCi/L is generally considered low risk, though it’s not zero risk.
If a short-term test comes back at 4 pCi/L or higher, the standard next step is either a second short-term test to confirm or a long-term test that captures seasonal variation. Two elevated short-term results, or one elevated long-term result, is enough to justify mitigation.
Passive vs. Active: Which Type to Use
- Charcoal canisters and alpha track kits cost $15 to $30 including lab analysis. Charcoal kits are best for a quick initial screening. Alpha track detectors are better for a long-term average over several months. Neither gives you real-time feedback.
- Continuous digital monitors range from about $100 to $250 for consumer models. They let you watch trends over time, see how opening windows or running ventilation affects levels, and get a rolling average without mailing anything to a lab. The tradeoff is higher upfront cost.
- Professional continuous monitors are what inspectors use during real estate transactions. They log hourly data with tamper indicators, providing court-defensible results for a home sale. A professional test typically costs $150 to $300.
For most homeowners, a cheap charcoal canister is a reasonable first step. If the result is near or above 4 pCi/L, following up with a long-term alpha track test or investing in a continuous monitor gives you the confidence to decide on mitigation.