Experimental Determination of Radon Exhalation Rate: Principles, Methods, Standards, and Application

Radon exhalation rate testing quantifies the amount of radon-222 released per unit time from building materials, soils, and industrial residues into the surrounding air, providing essential data for indoor air quality control, radiation protection, and NORM residue management. This article explains the measurement principles, sample types, standard testing methods, applicable standards such as ISO 11665 and GB/T 14582, and typical applications of radon exhalation rate determination performed by accredited third-party laboratories.

Technical Overview

Radon-222 is an inert radioactive noble gas with a half-life of 3.82 days, produced continuously as a decay daughter of radium-226 within the uranium-238 decay chain. A fraction of the radon atoms generated inside mineral grains—typically 1% to 20%, expressed as the emanation coefficient—escapes into the pore space of the material through recoil and diffusion. This gas then migrates through the interconnected pores by molecular diffusion and pressure-driven advection before being released, or exhaled, at the exposed surface.

The radon exhalation rate is defined as the activity of radon-222 released per unit surface area per unit time, expressed in Bq/(m²·s), or per unit mass per unit time, expressed in Bq/(kg·s), for the mass-specific exhalation rate. Because inhaled radon decay products are recognized by the WHO as the second leading cause of lung cancer after tobacco smoking, controlling radon entry from soil and construction materials is a central objective of indoor radiation protection. Experimental determination of the exhalation rate is normally performed by sealing the sample—or a chamber placed on a flat surface—in an air-tight enclosure, recording the radon concentration growth curve with a calibrated detector, and deriving the exhalation rate from the fitted accumulation model after applying corrections for radon decay, chamber leakage, and back diffusion.

Testing Samples

Building Materials

Concrete blocks and cast slabs, aerated (foamed) concrete, fired clay bricks, fly-ash and slag bricks, cement and mortar specimens, gypsum board, natural dimension stones such as granite and marble, ceramic and porcelain tiles, and both fine and coarse aggregates. Finished or coated products may also be tested to evaluate the retardation effect of paints, sealants, and radon barrier membranes.

Soils and Backfill Materials

Undisturbed or remolded soil samples, sands, gravels, and crushed rock used as sub-slab fill or backfill beneath buildings, as well as soil gas exhalation measured directly in the field for radon potential characterization.

NORM Residues and Industrial By-Products

Coal fly ash, phosphogypsum, blast-furnace and steel slags, red mud from alumina refining, tin and rare-earth processing residues, uranium mill tailings, and pipe scale or sludge from oil and gas production containing naturally occurring radioactive materials.

Finished Surfaces and In-Situ Conditions

Interior walls, floors, basements, tunnel linings, and tailings beach surfaces, where accumulation hoods or chambers are mounted directly onto the surface with air-tight sealing to measure the in-place exhalation flux under real environmental conditions of temperature and humidity.

Testing Items

The experimental determination of radon exhalation rate typically encompasses the following measurable quantities and derived parameters:

• Surface radon exhalation rate, expressed in Bq/(m²·s), for slabs, boards, and in-situ surfaces.
• Mass-specific radon exhalation rate, expressed in Bq/(kg·s) or Bq/(kg·h), for granular and crushed materials.
• Radium-226 activity concentration (Bq/kg) determined by gamma-ray spectrometry, which governs the radon generation potential of the material.
• Radon emanation coefficient (%), derived from the ratio of exhaled radon to total radon generated from radium-226.
• Radon accumulation curve parameters, including the effective decay constant combining radon decay, leakage, and back diffusion.
• Chamber background, blank leakage rate, and measurement uncertainty evaluated in accordance with ISO/IEC 17025 requirements.

Testing Methods

Sealed Accumulation Chamber Method

The sample, or an accumulation hood sealed onto the test surface, is closed in a radon-tight enclosure and the radon concentration is monitored continuously with a pulse ionization chamber, Lucas scintillation cell, or electrostatic-collection silicon detector. In the initial linear regime the surface exhalation rate is calculated as J = V × (C2 − C1) / (A × (t2 − t1)), where V is the effective chamber volume, A the exhaling area, and C1 and C2 the radon concentrations at times t1 and t2. For longer accumulation the full exponential growth model C(t) = (J × A / (λe × V)) × (1 − e^(−λe·t)) + C0 × e^(−λe·t) is fitted, where the effective decay constant λe combines the radon decay constant, chamber leakage, and back diffusion. This approach is described in ISO 11665-7.

Activated Carbon Adsorption Method

A canister of activated carbon is enclosed with the sample or placed beneath a sealed accumulation hood for a defined exposure period of typically two to seven days. The adsorbed radon and its short-lived progeny are subsequently counted by gamma-ray spectrometry using characteristic photopeaks such as 295 keV and 352 keV of lead-214 and 609 keV of bismuth-214. The method is inexpensive, integrates the flux over the exposure window, and requires humidity corrections for the carbon adsorption efficiency.

Flow-Through (Dynamic Ventilation) Method

A carrier gas is passed through the chamber at a known, controlled flow rate, and the exhalation rate is obtained from the steady-state concentration difference between outlet and inlet streams as J = Q × (Cout − Cin) / A. The continuous purge suppresses back diffusion and is particularly suitable for high-exhalation materials such as uranium tailings and NORM residues.

Passive Integrating Detector Methods

Electret ion chambers and solid-state nuclear track detectors, such as CR-39, may be deployed inside sealed accumulation enclosures to integrate the radon exposure over days to weeks, from which the average exhalation rate is derived. These passive techniques are advantageous for field surveys and simultaneous multi-point measurements.

Complementary Radionuclide Analysis

High-purity germanium (HPGe) gamma-ray spectrometry is used to determine the radium-226 content, following the sealing and radon ingrowth equilibration procedures of GB 6566. The result allows calculation of the theoretical maximum exhalation rate and verification of the measured emanation behavior. Soil and surface flux procedures are further standardized in ISO 11665-9 and GB/T 14582, with additional methodological guidance in IAEA Technical Reports Series No. 474.

Applications

Experimental radon exhalation rate data support a wide range of regulatory, engineering, and scientific purposes:

• Indoor air quality assessment and compliance demonstration with indoor radon concentration limits under GB 50325 and GB/T 16146, including the WHO reference level of 100 Bq/m³ for dwellings.
• Certification and conformity evaluation of building materials against radionuclide limitation requirements such as GB 6566, including internal exposure index verification for decorative stones, tiles, and cement products.
• Decommissioning of uranium mining and milling facilities, tailings pile remediation, and long-term monitoring of engineered covers designed to suppress radon release.
• Characterization and beneficial-use evaluation of NORM residues such as phosphogypsum, fly ash, and slag prior to utilization in construction or controlled disposal.
• Geogenic radon potential mapping, soil gas surveys, and radon risk zoning for land-use planning and building-code decisions.
• Research and development of low-radon building materials, radon-resistant membranes and coatings, and sub-slab ventilation or depressurization system design.

FAQ

What is the difference between surface exhalation rate and mass exhalation rate?

The surface exhalation rate (Bq/(m²·s)) describes radon release per unit area of an exposed surface and is directly relevant to walls, floors, and finished products. The mass exhalation rate (Bq/(kg·s)) normalizes release to the total mass of the material and is preferred for granular, crushed, or loose samples where the exhaling geometry is not fixed.

What are typical exhalation rate values for common materials?

Ordinary concrete and clay bricks generally exhibit surface exhalation rates on the order of 0.001 to 0.05 Bq/(m²·s), while some granites, phosphogypsum, and uranium tailings may reach substantially higher values. Representative soils commonly fall in the range of 0.005 to 0.05 Bq/(m²·s), with the global average reported by UNSCEAR at approximately 0.015 Bq/(m²·s).

How long does a radon exhalation rate test take?

A sealed accumulation measurement typically runs from several hours to 24 hours depending on the exhalation intensity and required precision, whereas activated carbon exposures require two to seven days. Including detector calibration verification, quality control, and data evaluation, accredited third-party laboratories commonly issue reports within three to seven working days after testing.

What corrections are applied to the raw measurement?

The fitted concentration growth is corrected for radon radioactive decay, chamber leakage determined from blank tests, back diffusion of radon into the sample, adsorption on chamber walls, background radon, and environmental factors such as temperature and humidity that influence both exhalation and detector response.

How is the exhalation rate related to radium-226 content?

The maximum possible mass exhalation rate equals the radium-226 activity multiplied by the decay constant of radon and the emanation coefficient, i.e., E = λRn × CRa × f. Measuring both quantities therefore allows the emanation coefficient to be calculated and provides an internal consistency check on the experimental result.

What sample preparation is required?

Hydraulic binders and concrete should be cured to the specified age (commonly 28 days) and conditioned to stable temperature and humidity. For surface exhalation measurements on boards and blocks, the lateral faces are sealed with radon-tight film so that only the designated test face exhales; granular materials are dried or conditioned to a defined moisture state, weighed, and sealed inside the accumulation vessel with known free volume.

Which standards govern the experimental determination?

Principal references include ISO 11665-7 for the accumulation estimation of surface exhalation rate, ISO 11665-9 for soil exhalation rate determination, GB/T 14582 for radon measurement in environmental air, GB 6566 for radionuclide limits in building materials, GB 50325 and GB/T 16146 for indoor radon control, and IAEA Technical Reports Series No. 474 for measurements on NORM residues.