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New dose coefficients for radon progeny: Impact on workers and the public
Published 5 February 2018
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- Radon is a naturally occurring radioactive gas. The inhalation of radon and its progeny is recognised as a cause of lung cancer. However tobacco smoking is the main cause of lung cancer both globally and in Australia.
- The International Commission on Radiological Protection (ICRP) has recently re-evaluated its estimates of lung cancer risk for radon progeny and doubled its estimate of risk from exposure.
- Dose coefficients are used to calculate the radiation dose from inhalation of radon progeny. The ICRP has stated its intent to use biokinetic and dosimetric models to provide revised radon progeny dose coefficients. Implementing the new ICRP dose coefficients in Australia will increase the radon progeny inhalation doses assessed for workers and members of the public.
- Studies carried out in Australia indicate that radon progeny inhalation doses assessed for workers in the uranium mining industry and in show caves will increase by a factor of two or more from current assessments.
- These studies indicate that the default values of new ICRP dose coefficients would be suitable for regulatory and radiation protection purposes in Australian underground mines and show caves.
- Surveys have shown that radon levels in Australian homes are low. The overall risk for lung cancer from radon in the Australian population is very small.
- However, there is evidence that smoking leads to a strong enhancement of the radon-related lung cancer risk. The best way of reducing the total lung cancer risk, as well as the lung cancer risk from exposure to radon, is to avoid tobacco smoking.
- In 2017, ARPANSA published its Guide for Radiation Protection in Existing Exposure Situations for optimisation and consideration of remedial actions for exposure to radon in homes and workplaces.
This advisory note provides an explanation of the new approach by the ICRP on how the risk of lung cancer risk from radon progeny have changed and are applied in the Australian context. The audience target are regulators, radiation safety officers, hygienists and industry sectors that need to mitigate radon.
Radon-222 (radon) is a naturally occurring radioactive gas (half-life 3.8 days) formed through the decay of radium-226 in the uranium-238 decay series. When radon in air decays, it forms a number of short-lived radioactive decay products, known as radon progeny, which include polonium-218, lead-214, bismuth-214 and polonium-214.
The inhaled short-lived radon progeny are particles that can deposit within the respiratory tract. Two of these short-lived progeny emit alpha particles, and the energy deposited by these alpha particles represents the major contributor to radiation exposure that can lead to health effects. The inhalation of radon and its progeny have been recognised as a cause of lung cancer by the International Agency for Research on Cancer (IARC 1988).
The ICRP provides guidance and recommendations on radiation protection of people and the environment from exposure to ionising radiation. It has developed the International System of Radiological Protection used worldwide as the common basis for radiological protection standards.
In its 1993 recommendations, the ICRP defined a factor for the conversion of radon progeny exposure to inhalation dose that was based on epidemiological studies, mainly involving the follow-up of disease in underground uranium mine workers, combined with the risk derived from epidemiological studies of Japanese atomic bomb survivors (ICRP 1993).
The ICRP has reviewed recent epidemiological studies of the association between lung cancer and exposure to radon and its progeny, and it has doubled its estimate of risk from exposure to radon (ICRP 2010). Based on this change, together with other considerations, the ICRP in Publication 126 (ICRP 2015) stated that:
“Converting exposures to radon progeny into doses requires the use of dose conversion factors. In the past, the dose conversion factors for radon progeny were based on epidemiological studies. The Commission is now proposing to calculate effective dose coefficients following exposure to radon progeny using ICRP reference biokinetic and dosimetric models with specified radiation and tissue weighting factors.”
The ICRP assessment was that residential and miner epidemiological studies provide consistent estimates of the risk of lung cancer, with significant associations observed at average annual concentrations of approximately 200 Bq m-3.
In particular, the European pooled analysis of the risk of lung cancer from residential radon exposures reported an excess relative risk of 16% per 100 Bq m-3 average radon over a 30 year period. The ICRP considered this value to be a reasonable estimate of the risk associated with relatively low and prolonged radon exposures in homes. This is about double the previous value recommended by the ICRP.
The World Health Organization (WHO) estimates around 4% of lung cancer deaths worldwide are attributed to exposure to radon found in homes and workplaces (WHO 2017).
New dose coefficients for radon progeny are published in Publication 137, Occupational Intakes of Radionuclides: Part 3 (ICRP 2018). The new dose coefficients are consistent with the estimates of lung cancer risk. The new ICRP approach will apply the ICRP reference biokinetic and dosimetric models to the inhalation of the radon progeny, consistent with the approach for other radionuclides. This approach uses the ICRP Human Respiratory Tract Model and information about the radon progeny aerosol characteristics in the workplace.
For the calculation of doses following inhalation of radon and radon progeny in mines and most buildings, the ICRP recommends a dose coefficient of 3 mSv per mJ h m-3 (approximately 10 mSv per WLM). The ICRP considers this dose coefficient to be applicable to the majority of circumstances with no adjustment for aerosol characteristics.
For indoor workplaces where workers are engaged in physical activities, and for workers in tourist caves, the ICRP recommends a dose coefficient of 6 mSv per mJ h m-3 (approximately 20 mSv per WLM).
Radon occurs everywhere on Earth. It occurs both inside and outside, underground and above ground, and at work and at home. Radon levels may build up in locations where there is naturally high uranium or radium in the environment and there is restricted or poor ventilation. In Australia, underground operations such as uranium mines and show caves meet these conditions. Radon gas can also enter our houses from the ground through cracks in floors and walls, and through building products. Consequently, radon levels are usually higher in basements and underground cellars. The focus of this Advisory is on underground uranium mines, tourist caves and homes, however there are likely to be other environments in Australia that will need to consider management of radon, such as mineral spring bathing houses, office buildings and other underground mining activities.
Exposure to radon in underground uranium mines is a planned exposure situation. The Radiation Protection Series Code for Radiation Protection in Planned Exposure Situations (ARPANSA 2016) sets out the requirements in Australia for the protection of occupationally exposed persons, the public and the environment in planned exposure situations.
The new ICRP approach allows for site-specific radon progeny dose coefficients that take account of the local conditions. Based on measurements of radon progeny aerosol characteristics in an Australian underground uranium mine, site-specific dose coefficients for operational work areas would be slightly increased from the new ICRP dose coefficients, depending on the site-specific aerosol conditions (ARPANSA 2018). Given the uncertainties in the assessing individual worker exposure and the complexity of determining site-specific factors, it is considered that the new ICRP dose coefficients will be suitable for regulatory and radiation protection purposes in Australian underground mines.
If estimates of worker doses warrant more detailed assessment, the use of site-specific dose coefficients may be considered by the operator based on best available technical information in consultation with the relevant radiation regulatory authority.
Radiation doses to workers in uranium mines in Australia are typically low and when other exposure pathways are considered, the increase in the total effective dose for a uranium mine worker due to the new ICRP dose coefficients is expected to be about a factor of two or less.
There is always a need to remain vigilant on radiation exposures of uranium mining workers. The changed dose assessments represent an improvement in the overall risk assessment. However, the implications for workers health are minimal at the current exposure levels in Australian mines.
Exposure to radon in show caves is an existing exposure situation. Australian guidance in ARPANSA’s Radiation Protection Series Guide for Radiation Protection in Existing Exposure Situations (ARPANSA 2017) is based on the relevant requirements in the IAEA General Safety Requirements GSR Part 3 (IAEA 2014). The Australian guide establishes a strategy for the protection of workers against exposure to radon in workplaces, including the establishment of a reference level of 1000 Bq m-3 to deal with workplaces with elevated levels of radon.
An Australia-wide survey of radon levels in underground show caves found that about one fifth of the caves measured had yearly average radon levels exceeding 1000 Bq m-3 (ARL, 1996). These caves were located in south-eastern Australia (New South Wales, Victoria and Tasmania). At the time of the survey, the radiation doses to the tour guides in these caves from exposure to radon were assessed to be low (Solomon et al 2005). A reassessment of these doses using the new ICRP dose coefficients would lead to inhalation doses exceeding 20 mSv per year for some show cave tour guides in the original study.
Although the total number of show cave tour guides in Australia is very small, the updated radon progeny dose estimates are a significant radiation protection and health issue for the affected individuals. Consultation with the affected workers, employers and relevant regulators is needed to better understand the current situation and, if required, to devise strategies to minimise potential health risk while maintaining worker livelihoods.
While the radon levels in these show caves are relatively high, short-term exposure to members of the public during a guided tour of the cave represents negligible health concern.
Exposure to radon in homes is an existing exposure situation. The Australian guidance establishes a strategy for protection of people against exposure to radon in homes, including the establishment of a reference level of 200 Bq m-3 to deal with homes with elevated levels of radon (ARPANSA 2017).
Based on a 1990 nationwide survey of Australian homes, the average annual radon concentration ranged from 6 Bq m-3 in Queensland to 16 Bq m-3 in the Australian Capital Territory, with an overall average for Australian homes of 11 Bq m-3 (ARL 1991). The global average indoor value of 40 Bq m-3 (UNSCEAR 2008). While the average level of radon in Australian homes is low, it is estimated that about one in a thousand of Australian homes could have levels of radon exceeding 200 Bq m-3, the level at which remedial actions should be considered.
At the time of the Australian radon survey, the radon contribution to the background radiation dose for the Australia population was estimated to be 0.2 mSv per year (Webb 1999). Applying the new ICRP dose coefficients, the estimates of the average radiation dose from exposure to radon in Australian homes would increase by factor of more than two, from 0.2 mSv per year to 0.5 mSv per year, a value that is similar to the contribution of terrestrial radiation to the background radiation dose.
For more information on radon levels in Australian homes, contact the radiation regulatory authority in your state or territory.
It is estimated that there will be more than 12 000 new lung cancer cases in Australia in 2017, representing about 10% of all new cancer cases and nearly 20% of all cancer deaths (AIHW, 2017). Based on lung cancer data for 2010, it was estimated that about 80% of the lung cancer cases were attributable to tobacco smoke (Pandeya, 2015). Radon levels in most homes within Australia are low, below the levels where associations between radon exposure and lung cancer has been demonstrated with certainty. With the new ICRP dose coefficient and the low levels of radon in homes within Australia, it is estimated that a few percent of lung cancer cases might be due to indoor radon exposure in homes.
The ICRP noted that a synergy between radon and smoking has been demonstrated in a number of studies of radon exposure and risk of lung cancer (ICRP 2010). Due to the dominant effect of tobacco use on lifetime risk of lung cancer, the excess absolute risk of lung cancer attributable to a given level of radon is much higher among tobacco smokers than among non-smokers. For smokers, the additional risk from radon exposure is small relative to the risk from tobacco smoking. The best way of reducing the risk of developing lung cancer is never to start smoking, or to quit smoking as soon as possible.
For the major part of the non-smoking Australian population, the risk for lung cancer associated with radon exposure is very small or manageable.
The ICRP recommends management of radon exposure based on optimisation, with a reference level of approximately 10 mSv per year effective dose, corresponding to an annual residential indoor radon concentration of about 200 Bq m-3. This is consistent with the value recommended in Australia’s national Guide for Radiation Protection in Existing Exposure Situations (ARPANSA 2017) for optimisation and consideration of remedial actions. Various methodologies are available to prevent radon gas from penetrating into homes or to increase ventilation in homes so that the radon concentration in air decreases. The WHO Handbook on Indoor Radon (WHO 2009) provides guidance on options for radon prevention for new homes and for radon mitigation in existing homes.
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