ISO 11665-4:2021

Overview

ISO 11665-4:2021 specifies an integrated measurement method for radon-222 (Rn-222) in air using passive sampling and delayed analysis. It is part of the ISO 11665 series on measuring radioactivity in the environment. The standard covers unattended sampling periods from a few days up to one year and applies to average radon activity concentrations above 5 Bq/m3. It provides practical guidance for easy-to-use, low-cost passive sensors and the conditions of use for reliable determination of the average activity concentration (Bq/m3).

Key topics and technical requirements

• Principle: Continuous passive collection of radon by diffusion or accumulation, followed by laboratory or delayed analysis of an accumulated physical quantity (exposure), then conversion to average radon activity concentration by dividing exposure by sampling duration.
• Sensor types covered (Annexes):
  • SSNTD (solid-state nuclear track detector) - latent track counting
  • Electret detectors - electret voltage discharge measurement
  • Activated charcoal - adsorption and gamma spectrometric analysis of decay products
• Sampling scope: Uninterrupted sampling for periods from days to one year; sensors may be in open (free convection) or closed (accumulation chamber) configurations.
• Performance and metrology:
  • Guidance on sampling conditions and installation, minimum detectable concentrations (> 5 Bq/m3), and representative placement of sensors.
  • Detection, calibration and measurement procedures, including influence quantities, calibration factors and uncertainty evaluation (aligned with GUM principles).
  • Definitions and procedures for standard uncertainty, decision threshold, detection limit, and confidence interval limits.
• Reporting: Required contents for a compliant test report, including exposure result, sampling duration, calibration data, and uncertainty statements.

Practical applications and who uses this standard

ISO 11665-4:2021 is intended for:

• Environmental and radiological testing laboratories performing integrated radon surveys.
• Public health and regulatory bodies establishing monitoring programs and reference-level compliance (complements WHO/EU guidance on indoor radon).
• Building professionals and radon mitigation contractors assessing average indoor radon levels for remediation decisions.
• Researchers and surveyors conducting long-term or large-area radon mapping using low-cost passive methods.

The standard is particularly useful for cost-effective population exposure assessments, national radon surveys, and pre-/post-mitigation verification when continuous active instruments are impractical.

Related standards (applicable)

• ISO 11665-1 - Origins of radon and measurement methods
• Other parts of ISO 11665 series (radon in water, spot/continuous methods)
• ISO 11929 - Characteristic limits for ionizing radiation measurements
• ISO/IEC 17025 - Laboratory competence and calibration
• IEC 61577-1 - Radon measurement instrumentation principles

Keywords: ISO 11665-4:2021, radon-222, passive sampling, integrated measurement, SSNTD, electret detector, activated charcoal, average activity concentration, Bq/m3, radon monitoring.