IRPA Commentary on Radon Dose Coefficients

One of the topics on IRPA 's Horizon Scan, which monitors issues which could have a significant impact on radiation protection practice, is that of radon dose coefficients (DCs) – sometimes termed 'Dose Conversion Factors (DCFs)'. The Inter Agency Committee on Radiation Safety (IACRS1) has issued an overview on this topic, which will be of interest to many IRPA members. UNSCEAR and ICRP have produced a supporting document with additional technical details.

The key messages can be summarised as follows:

  1. ICRP recommends DCs for all radionuclides for the purposes of quantifying dose for protection purposes. UNSCEAR uses its own DCFs for estimating exposure levels for comparison to other sources of radiation exposure.
  2. For exposure to radon, assessments in recent decades have used factors published by the respective organisations as follows:
    • UNSCEAR used 5.7 mSv per working level month (WLM) for public and worker exposure
    • ICRP recommended 4 mSv per WLM for public at home, and 5 mSv per WLM for workplace exposure.
  3. Both organisations have recently reviewed the latest scientific information on the risks of radon exposure. Whilst noting the considerable uncertainties associated with this information, the organisations reached differing conclusions:
    • UNSCEAR concluded that the totality of recently assessed evidence is compatible with its previous assessments, and hence that it is appropriate to continue the use of the factor of 5.7 mSv per WLM for estimating radon exposure levels in its dose assessments for public and workers.
    • Based on an updated review of epidemiological data which gave substantially higher risk estimates, and taking account of epidemiological and dosimetric approaches with their associated uncertainties, ICRP recommends a single rounded DC value for use in most circumstances of occupational exposure of 10 mSv per WLM. ICRP has also indicated that this value is applicable to exposures in homes.
  4. An October 2019 IAEA Technical Meeting on the Implications of the new DCs for radon recommended the use of a DC of 10 mSv per WLM (from ICRP Publication 137) as the default for workplaces unless a different DC is justified by specific aerosol characteristics.
  5. For protection purposes, ICRP has noted that "If circumstances of occupational exposure warrant more detailed consideration and reliable alternative data are available, site-specific doses can be assessed using methodology provided by ICRP."
  6. Taking account of the above recommendations and uncertainties, IACRS notes that no changes are necessary to the International Basic Safety Standards recommendations on the use of radon Reference Levels expressed in terms of Bq/m3.
  7. IACRS also notes that national authorities need to define DCs for all relevant nuclides as an integral part of radiation protection regulation. Authorities generally base their DCs on ICRP recommendations. National authorities therefore need to decide if, and when, to update their radon DCs, taking account of the latest ICRP recommendations. IACRS noted that "The new DCF for radon could be implemented immediately, or It may be practical to do so after the full set of new DCFs for occupational exposures is available to ensure a consistent approach. All updated DCFs for occupational exposures should be published within a year."
  8. The use of the new ICRP recommendation for the radon DC will increase the assessed dose for radon by a factor of around two. Where assessed radon workplace exposures are significant, employers will need to review their protection measures to ensure that protection remains optimised and doses remain within limits.

There will be other implications of this new information on radon DCs. For example, the use of the new ICRP DC for assessing contributions to national exposure, such as in pie charts, will also increase the relative contribution of radon by around a factor of two.

1 IACRS is comprised of the following organisations: eight inter-governmental organisations, including the seven which sponsor the International Basic Safety Standards (IAEA, WHO, PAHO, ILO, OECD/NEA, EC, FAO) and UNSCEAR, plus five non-governmental bodies (ICRP, ICRU, IEC, IRPA, ISO).