Indoor air radon testing quantifies the activity concentration of radon-222, an invisible radioactive gas recognized by the World Health Organization as a leading cause of lung cancer, in residential, office, and public building environments. Accredited third-party laboratories apply standardized techniques such as track-etch dosimetry, activated charcoal adsorption, and continuous electronic monitoring in accordance with GB/T 14582, GB/T 18883, GB 50325, and the ISO 11665 series, with results reported in becquerels per cubic meter (Bq/m³).
Technical Overview
Radon-222 is a chemically inert, colorless, and odorless radioactive noble gas produced in the uranium-238 decay chain. With a half-life of 3.82 days, it emanates continuously from soil, bedrock, groundwater, and mineral-based construction materials. Because it exists as a free gas under normal conditions, radon migrates through the ground and enters enclosed spaces where pressure differences driven by the stack effect, wind, ventilation systems, and temperature gradients pull soil gas through foundation cracks, expansion joints, service penetrations, floor-wall interfaces, and untreated concrete slabs. Additional contributions come from degassing of tap water and from radium-bearing building materials such as certain stone aggregates and phosphogypsum-based products.
The principal health hazard arises not from radon itself but from its short-lived decay products, principally polonium-218 and polonium-214, which emit alpha particles. When inhaled, these progeny attach to respirable aerosols and deposit on the bronchial epithelium, delivering highly localized alpha radiation dose to lung tissue. According to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), radon accounts for roughly half of the average natural background radiation dose received by the general public, and the World Health Organization (WHO) identifies radon as the second most important cause of lung cancer after tobacco smoke.
Indoor radon testing therefore measures the radon-222 activity concentration in air, expressed in becquerels per cubic meter (1 Bq/m³ corresponds to one decay per second per cubic meter, equal to 0.027 pCi/L). Measured values are compared against applicable reference levels: the WHO recommends a national reference level of 100 Bq/m³ with an absolute upper bound of 300 Bq/m³; GB/T 18883-2022 limits the annual average radon concentration in indoor air to 300 Bq/m³; GB 50325-2020 sets an acceptance limit of 150 Bq/m³ for newly completed civil building projects; and the United States EPA action level is 148 Bq/m³ (4 pCi/L).
Standards and Reference Documents
- GB/T 14582-1993: Standard methods for measuring environmental radon concentration, defining track-etch, activated charcoal, double-filter membrane, balloon, and scintillation cell techniques.
- GB/T 18883-2022: Indoor air quality standard, specifying the annual average radon limit of 300 Bq/m³ and associated sampling requirements.
- GB 50325-2020: Standard for indoor environmental pollution control of civil building engineering, governing radon acceptance testing of completed buildings.
- ISO 11665 series: Measurement of radioactivity in the environment, covering integrated passive methods (Part 4), continuous measurement (Part 5), spot measurement (Part 6), soil gas (Part 11), building material exhalation (Part 12), and related procedures.
- IEC 61577 series: Requirements and tests for radon and radon decay product measuring instruments.
- ASTM E1465 and ASTM E2121: Radon-resistant design of new low-rise residential buildings and installation of radon mitigation systems in existing buildings.
- ISO/IEC 17025: General requirements for the competence of testing and calibration laboratories.
Testing Samples
Residential and Office Indoor Air
Ambient air is sampled in habitable rooms such as living rooms, bedrooms, and enclosed offices. Sampling points are distributed according to room area, typically one to three points for rooms under 50 m², three to five points for 50–100 m², and at least five points above 100 m². Detectors and probes are positioned in the breathing zone, 0.5–1.5 m above the floor, at least 0.5 m from interior walls, and away from windows, doors, supply diffusers, and direct airflow. Short-term measurements require closed-building conditions, generally 12 hours of closed doors and windows before and during screening and at least 24 hours for engineering acceptance testing.
Newly Completed Civil Buildings (Engineering Acceptance)
For acceptance under GB 50325-2020, air is sampled in finished buildings prior to handover, with rooms selected in proportion to the total number of rooms and emphasis on the lowest floors and rooms in direct contact with soil. Class I civil buildings, including residences, hospitals, schools, kindergartens, and elderly-care facilities, are subject to the strictest radon limit because they house sensitive or long-duration occupants.
Special and Underground Spaces
Basements, civil air-defense shelters, tunnels, underground garages, wine cellars, commercial caves, thermal spas using geothermal water, and mining or NORM-industry workplaces exhibit elevated geogenic radon sources and reduced ventilation, and they require dedicated measurement campaigns with appropriate duration and instrumentation.
Ancillary Matrices for Source Diagnosis
When indoor concentrations are elevated, complementary samples may be collected to identify the source: soil gas at depth for geogenic radon potential assessment, building material specimens or in-situ surfaces for radon exhalation rate determination, and water samples where groundwater is suspected as a carrier.
Testing Items
Core and supplementary measurands reported for indoor radon evaluation include:
- Radon-222 activity concentration in indoor air, expressed in Bq/m³, as a short-term value, a 24-hour continuous value, or a long-term annual average.
- Potential alpha energy concentration (PAEC) of short-lived radon progeny, expressed in µJ/m³, together with the equilibrium equivalent concentration (EEC).
- Equilibrium factor between radon and its decay products, used to convert measured gas concentration to dose-relevant progeny concentration.
- Radon exhalation rate from building material surfaces, expressed in Bq/(m²·s).
- Soil gas radon concentration for geogenic potential surveys, typically in kBq/m³.
- Radon activity concentration in drinking or groundwater, expressed in Bq/L.
Testing Methods
Solid-State Nuclear Track Detectors (Track-Etch Method)
Passive dosimeters containing CR-39 or LR-115 track detectors are housed in filtered diffusion chambers that admit radon while excluding progeny, moisture, and thoron. Alpha particles from radon decay leave latent damage tracks that are enlarged by chemical etching and counted microscopically. Track density is proportional to the integrated radon exposure over deployment periods of one to twelve months, making the method, specified in GB/T 14582 and ISO 11665-4, the preferred technique for annual average assessment and large-scale building surveys, with detection limits of a few Bq/m³ over multi-month exposures.
Activated Charcoal Adsorption
Open canisters of activated charcoal adsorb radon passively for two to seven days. The adsorbed radon equilibrates with gamma-emitting progeny lead-214 and bismuth-214, which are quantified by gamma spectrometry using characteristic lines at 295, 352, and 609 keV. The method provides rapid, low-cost screening results but is sensitive to humidity and saturation, so it is limited to short-term measurements and is not suitable for annual averaging.
Scintillation Cell (Lucas Cell) Grab Sampling
A known volume of air is drawn into a zinc-sulfide-coated scintillation cell. After a defined delay allowing progeny ingrowth, alpha scintillations are counted, yielding an instantaneous radon concentration as described in GB/T 14582 and ISO 11665-6. Grab sampling is widely used for spot checks, instrument intercomparisons, and calibration work.
Continuous Electronic Radon Monitors
Active instruments pump air continuously through detection modules based on pulse ionization chambers, scintillation flow cells, or electrostatic collection of positively charged polonium-218 ions onto silicon semiconductor detectors. Alpha spectroscopic discrimination of the 6.0 MeV polonium-218 and 7.7 MeV polonium-214 lines suppresses thoron interference. These monitors, covered by ISO 11665-5 and IEC 61577, log concentrations at hourly or finer resolution and are the standard instruments for GB 50325 acceptance deployments of at least 24 hours, as well as for studying temporal variation and ventilation effects.
Double-Filter Membrane Method
Air is drawn at a known flow rate through an inlet filter that removes ambient progeny, passes through a decay volume where newly formed polonium-218 is generated, and is collected on an outlet filter that is subsequently alpha counted. The method yields a short-interval concentration within tens of minutes. The balloon method operates on the same decay-volume principle using a collapsible bag. Both techniques are established in GB/T 14582 for instantaneous field measurements.
Electret Ion Chamber
Passive chambers fitted with charged PTFE electrets measure the voltage drop produced by ionization from radon alpha decays. The reduction in surface charge, corrected for temperature, humidity, and gamma background, is proportional to the integrated radon exposure. Selectable electret sensitivities allow deployments ranging from a few days to several months, with immediate on-site readout.
Quality assurance for all methods includes calibration in reference radon chambers traceable to national metrology institutes, instrument performance verification against IEC 61577, deployment of field blanks and duplicate detectors, spiked-sample checks, and full uncertainty budgets. Accredited third-party laboratories operating under ISO/IEC 17025 report results with stated minimum detectable activities, commonly below 10 Bq/m³.
Applications
Indoor air radon testing services support regulatory compliance, risk management, and healthy building objectives across multiple sectors:
- Acceptance testing of newly completed residential and public civil building projects in accordance with GB 50325.
- Indoor air quality assessment and annual average radon evaluation under GB/T 18883.
- Pre-purchase, pre-lease, and post-renovation screening of apartments and houses.
- Monitoring of schools, kindergartens, hospitals, and elderly-care facilities serving sensitive populations.
- Radiation protection surveys in basements, underground civil air-defense works, tunnels, mines, spas, and NORM-industry workplaces.
- Geogenic radon potential mapping using soil gas surveys per ISO 11665-11.
- Evaluation of radon exhalation from building materials per ISO 11665-12 and of radon barrier materials per ISO 11665-13.
- Verification of mitigation systems, including active soil depressurization installed per ASTM E2121, and post-remediation validation measurements.
- Support for green building and healthy building certification schemes and for epidemiological exposure studies.
FAQ
The following questions address the most common technical and practical concerns raised by building owners, contractors, and facility managers regarding indoor radon measurement.
What level of indoor radon is considered safe?
No threshold concentration is considered completely risk-free, so the objective is to keep concentrations as low as reasonably achievable. The WHO reference level is 100 Bq/m³, with 300 Bq/m³ as the maximum recommended level; GB/T 18883-2022 sets the annual average limit at 300 Bq/m³, and GB 50325-2020 applies a 150 Bq/m³ acceptance limit for new civil buildings. Mitigation is generally recommended whenever sustained concentrations approach or exceed these values.
How long does an indoor radon test take?
Short-term screening measurements typically run from two to seven days under closed-building conditions, while confirmation and exposure assessment rely on long-term integrated measurements of three months to one year that capture seasonal variation. Engineering acceptance testing requires continuous monitoring for at least 24 hours, and 48-hour deployments are common to improve statistical confidence.
Where and how should detectors be placed?
Detectors should be installed in the lowest regularly occupied floor, in frequently used living or sleeping rooms, 0.5–1.5 m above the floor, at least 0.5 m from walls, and away from windows, doors, direct sunlight, air supply diffusers, and humid areas such as kitchens and bathrooms. Passive detectors should remain undisturbed for the entire deployment period.
Which measurement method should be selected?
Method selection depends on the objective: continuous electronic monitors are appropriate for engineering acceptance and time-resolved diagnostics; track-etch dosimeters provide the most representative annual average exposure; activated charcoal and electret chambers suit rapid screening; and grab methods support spot verification. Accredited third-party laboratories can advise on the optimal combination for a given building.
What happens if the measured radon concentration exceeds the limit?
Exceedances should first be confirmed with a repeat or long-term measurement, since radon levels fluctuate with ventilation and season. Confirmed elevations are addressed through mitigation measures such as active soil depressurization, sealing of foundation entry routes, mechanical supply or exhaust ventilation, and radon-resistant construction details in new buildings per ASTM E1465. Retesting is performed shortly after mitigation and again on an annual basis initially.
How often should indoor radon be retested?
Residential and commercial buildings are generally retested every five years, and additionally after major renovation, changes to heating or ventilation systems, structural settlement, or seismic events. Because radon concentrations vary seasonally, pairing a winter measurement with a summer measurement provides a more reliable annual picture.