Measuring Radon

Passive detectors require no electrical power and accumulate a signal over the entire deployment period, yielding a single time-averaged concentration. They are deployed for days to months, mailed to a laboratory for analysis, and provide results as Bq/m³ or pCi/L for the measurement window.

Active detectors (continuous radon monitors, CRMs) require power, measure radon concentration continuously with resolution from minutes to hours, and provide time-series data showing how radon varies throughout the day, week, or season. They are used for diagnostics, real-time feedback during mitigation installation, and occupied-hours exposure assessment in workplaces.

WHO recommends long-term passive measurements for estimating annual average exposure and short-term active monitoring for investigating temporal variation and building-specific radon dynamics.

Alpha-track detectors contain a small piece of radiation-sensitive plastic (CR-39 polycarbonate or LR-115 cellulose nitrate) enclosed in a diffusion chamber. Alpha particles from radon and its progeny inside the chamber strike the plastic and create microscopic damage tracks.

After exposure (typically 90 days to 12 months), the detector is returned to a laboratory where the plastic is chemically etched. Etching enlarges the damage tracks so they can be counted under a microscope or optical scanner. Track density (tracks per cm²) is proportional to average radon concentration over the deployment period.

Advantages: no power needed, stable over long periods, insensitive to humidity and temperature variations, low cost per deployment. Disadvantages: no temporal information (single average), requires laboratory analysis, 2–4 week turnaround time. Alpha-track detectors are the standard for long-term residential measurements globally.

An electret is a piece of electrically charged Teflon. When placed inside a small chamber that allows radon to diffuse in, ionization from radon decay reduces the electret's surface voltage. The voltage drop is proportional to integrated radon exposure.

The electret is read before and after exposure using a voltage reader. The system can be configured for short-term (2–7 days) or long-term (1–12 months) measurements by changing chamber volume and electret sensitivity. Results are calculated from voltage change, exposure time, chamber calibration factor, and background correction.

Advantages: immediate readout (no lab delay), reusable electrets reduce per-test cost for frequent users. Disadvantages: electrets lose charge with humidity, requiring correction or humidity control; voltage drift can occur; more user training required than alpha-track. Common in schools and workplace screening.

Activated charcoal adsorbs radon from air. A canister containing charcoal is exposed for 2–7 days, sealed, and sent to a laboratory where radon progeny gamma emissions (from lead-214 and bismuth-214) are counted using a gamma spectrometer or scintillation counter.

Because radon desorbs from charcoal over time and progeny decay, analysis must occur within a few days of sealing. Longer delays require decay correction. Charcoal sensitivity decreases at high humidity as water vapor competes for adsorption sites, requiring humidity correction or desiccant barriers.

Advantages: inexpensive, simple deployment, short measurement period for rapid screening. Disadvantages: humidity-sensitive (can under-read by 15–50% at high RH), time-sensitive analysis, higher variability than long-term detectors. Best used for initial screening or real-estate transaction timelines, not for compliance decisions near reference levels.

CRMs actively sample air and detect alpha particles from radon or its progeny, providing radon concentration readings every hour (or faster). Common technologies: ion chamber (measures ionization from alpha decay), silicon photodiode (counts alphas from plated-out progeny Po-218 and Po-214), or scintillation cell (alpha hits on ZnS scintillator produce light pulses counted by PMT).

Devices display current radon level, short-term average (e.g., 24-hour), long-term average (e.g., 30-day running mean), and often log hourly data for download. High-end models include sensors for temperature, pressure, and humidity to identify measurement conditions and potential tampering.

Advantages: real-time feedback, reveals diurnal patterns and weather-driven variations, detects HVAC and ventilation effects, suitable for post-mitigation verification and occupied-hours workplace assessment. Disadvantages: higher cost ($150–$2000+), requires power, calibration drift, potential tampering in unattended deployments. Essential for diagnostics and continuous workplace monitoring.

Radon gas vs progeny: some detectors measure radon gas directly (scintillation cells, some CRMs), while others detect alpha emissions from deposited progeny (electrostatic collection CRMs, charcoal gamma counting). All are calibrated to report radon gas concentration in Bq/m³ or pCi/L, but the measurement pathway differs.

Equilibrium assumptions: detectors measuring progeny assume a progeny-to-radon equilibrium factor (F). Standard calibration assumes F ≈ 0.4. If actual F differs significantly (e.g., in highly filtered environments or outdoors), reported concentrations may be biased. Long-term passive detectors average over many F fluctuations, making this less critical.

WHO notes that devices should be calibrated under conditions (humidity, temperature, radon level, progeny equilibrium) matching their intended use. Certification programs (NRPP in North America, PHE/UKHSA in UK, national schemes in EU) validate performance against reference radon chambers.

US EPA protocols: residential measurements use closed-building conditions (windows and doors closed 12 hours before and during test, except for normal entry/exit) to reduce variability and provide reproducible, conservative estimates. Schools and workplaces measure under normal occupied conditions.

International standards: ISO 11665 series covers radon measurement in air (parts 1–11), soil gas (part 11), water (ISO 13164), and building materials (ISO 11665-7). WHO recommends long-term measurements of at least 3 months, preferably 12 months, for estimating annual average exposure.

Spain CSN IS-47: workplace measurements must be ≥3 months between 1 October and 31 May, or full year in quarterly periods. Continuous monitoring ≥1 week required for occupied-hours assessment in time-limited occupancy buildings above 300 Bq/m³.