|Year : 2019 | Volume
| Issue : 1 | Page : 1-4
Radon in dwellings and workplaces: An update on current regulations
EX. Editor, Radiation Protection and Environment, and Former Head, Radiation Hazards Control Section, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
|Date of Web Publication||3-Jun-2019|
EX. Editor, Radiation Protection and Environment, and Former Head, Radiation Hazards Control Section, Bhabha Atomic Research Centre, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Pushparaja. Radon in dwellings and workplaces: An update on current regulations. Radiat Prot Environ 2019;42:1-4
| General|| |
Uranium (238U), with a half-life of 4.5 × 109 years, and thorium (232Th) are the primordial radionuclides present in the earth and rocks. 238U decays to form a series of radionuclides, with the decay chain ending with stable206Pb.226Ra (half-life – 1600 years) is one of the radiotoxic radionuclides in the 238U series. These naturally occurring radionuclides form a continuous source of gaseous progeny radionuclides – radon (222Rn) and thoron (220Rn from 232Th series). Radon, with a half-life of 3.82 days, is the gaseous radionuclide, which is the major natural source of radiation exposure of the world's population. Radon is ubiquitous and its concentrations in the environment, including dwellings, vary from place to place depending on the level of its parent radionuclide, 226Ra. According to the WHO, indoor radon exposure poses a significant public health hazard. Within the system of protection, exposure to unmodified naturally occurring radionuclides has characteristics of the existing exposure situations.
In uranium mines, the 226Ra levels are higher, and hence, the radon and its short-lived alpha-emitting progeny constitute the major source of internal exposures of the underground mine workers in the absence of adequate control measures. Radon levels in the mine environment depend on the ventilation rate; low ventilation rates result in higher radon levels due to poor dilution. Radon levels in the overground mines are lower as compared to the underground mines.
| Early Years|| |
The exposure of the mine workers to radon and its progeny has been correlated with the induction of lung cancer in several groups of mine workers. This led to the exposure-limiting guides, starting with the recommended concentration limit of 10 picoCi of radon per liter of air, in 1940. Ultimately, the International Commission on Radiological Protection (ICRP), in 1959, brought out recommendations on maximum permissible concentration in air. The United States Public Health Service introduced a unit “Working Level (WL),” which relates to the ultimate alpha energy that could be released by the radon daughters, and the cumulative exposure to the radon daughters is expressed as WL Month (WLM). These developments, including the methods of measurement, control measures, and protective equipment, are presented in the ICRP publication 24 (1977).
Taking into account the ICRP-60 recommendations, and based on the lung cancer risk estimates of epidemiological studies using the risk projection models for lung cancer, the ICRP made important and specific recommendations for protection against radon (222Rn) at home and at work, which were published in the publication 65 (1993), with good number of relevant references. In general, the ICRP recommends that exposure to radon needs to be excluded from the system of protection and treated separately by a general defined system based on the knowledge and judgment.
For protection purposes, the ICRP-65 adopted a nominal probability coefficient (fatality) for public and for workers of 8 × 10−5 per (mJ.h.m-3). The corresponding value in the historical unit of WLM was 3 × 10−4.
There are uncertainties with respect to the equilibrium factor; F in workplaces which is assumed to be 0.4. Equilibrium factor gives the degree of radioactive equilibrium radon and its short-lived progeny. The equilibrium factor also may be significantly different from 0.4.
The ICRP-65 also provided conventions for conversion of radon exposure to effective dose, to accommodate exposures from multiple sources. The conversion has been effected by direct comparison of ICRP-60-recommended detriment values associated with a unit effective dose (5.6 × 10−5 per mSv for workers and 7.3 × 10−5 per mSv for general public) and a unit radon exposure of 8 × 10−5 per (mJ.h.m-3). Or in terms of detriment, an exposure to radon progeny of 1 mJ.h.m-3 is equivalent to an effective dose of 1.43 mSv for workers and 1.10 mSv for members of the public. The corresponding numbers, called conversion conventions, for the exposure of 1 WLM equals to 5.06 mSv for workers and 3.88 mSv for members of the public. The numbers are rounded to 5 mSv/WLM at work and 5 mSv/WLM at home. For occupational limit of 20 mSv/year, the conversion coefficient corresponds to 4 WLM.
The derived air concentration (DAC) for radon in workplaces as recommended by the ICRP is 3000 Bq/m3, assuming 2000 h occupancy and 0.4 as equilibrium factor. However, the Commission cautions not to use the exposure limit and DAC as the primary basis for control. “The whole of the commission's system of protection for practices should be applied, with emphasis on optimization of protection.”
The ICRP-65 also provided action levels for radon concentrations for dwellings and workplaces for instituting remedial measures. The ICRP recommended an action level within the range of 500–1500 Bq/m3 both for home and workplace, and the corresponding range of annual effective dose is 3–10 mSv. The ICRP system of protection should be applied when simple countermeasures do not reduce radon concentrations.
Assuming yearly occupancy of 7000 h at home (indoors) and 2000 h at work, and equilibrium factor of 0.4, the action levels are in the range of 200–600 Bq/m3 and 500–1500 Bq/m3 corresponding to an annual effective dose in the range of 3–10 mSv at dwellings and workplaces, respectively (ICRP, 2007). Authorities need to choose values within this range.
The exposures in mines are considered as planned exposure situations. There are other exposure pathways from the inhalation of silica-bearing dust and external exposure to beta/gamma radiation from uranium and its progeny radionuclides. Due to the ventilation in the mines and the dynamic nature of the radiological conditions, the assessment of exposures of workers to radon and its progeny is challenging. A well-planned monitoring program is necessary to assess the situation during the operations.
There is a recent detailed update (ICRP-115, 2010) on the relation of lung cancer to radon exposure. The ICRP reviewed the recent epidemiological studies of the association between lung cancer and radon exposure. The studies are pertaining to the case–control studies on residential exposures to radon and cohorts of underground miners exposed to low levels of radon. The review indicated the association of lung cancer risk to annual average concentrations of about 200 Bq/m3 and cumulative occupational exposure levels of about 50 WLM.
Based on the review and combined analysis of the data, the ICRP recommended the use of nominal probability coefficient of 5 × 10−4 per WLM (14 × 10−5 per mJ h m−3) for radon and radon progeny-induced lung cancer. This risk factor is about twice the value of 2.8 × 10−4 per WLM (8 × 10−5 per mJ h m−3) assumed in the ICRP-65. This is an important update for radiological protection connected with radon.
Based on the epidemiological findings, the ICRP recommends a reference level of 300 Bq/m3 as an input for the public health policy related to indoor radon.
The ICRP, in 2014, brought out a publication (ICRP-126) entitled “Radiological Protection Against Radon Exposure” considering the finding of the publications ICRP-115 and ICRP-103. The ICRP emphasizes that national authorities should establish a graded and integrated approach for protection against radon exposures in all buildings. The authorities can set their own national reference levels taking into account the economic and societal circumstances and apply optimization to effect substantial reduction in radon exposure to ensure radiological protection and safety of the personnel and environment. Such a national reference level can be set as low as reasonably achievable in the range of 100–300 Bq/m3 at home and at most workplaces which are not considered as occupational.
In 1986, the US Environmental Protection Agency (EPA)recommended an action level of 4pCi/L in homes, which corresponds to 150 Bq/m3. In view of the latest information, the WHO (2009) proposes a reference level of 100 Bq/m3 to minimize the health hazards due to indoor radon exposure. However, a not-to-exceed level of 300 Bq/m3 (approx. 10 mSv/year) is also proposed.
It is the responsibility of the individual owners to take action against radon at homes if the measured concentration levels are higher than a value specified by the national regulatory authority (which is considered a typical value for the country as whole and which is within the specified range).
Radon in workplaces
For the protection of workers in occupational settings such as uranium/thorium mines, the requirements of planned exposure situations apply, including classification of areas, individual/collective monitoring, work area monitoring, use of personal protective equipment, a dose limit of 20 mSv/year. Some specific or complementary provisions for control of the exposures may be made. This includes:
- Derive and keep a dose reference level, not exceeding 10 mSv/year. Calculated using actual exposure parameters such as occupancy and equilibrium factor
- Regulatory body may establish an appropriate reference level in workplaces which does not exceed an internationally harmonized annual average activity concentration of 222Rn of 1000 Bq/m3, as recommended by IAEA and ICRP to define globally as the entry point for occupational protection requirements (ICRP-103, Para 298)
- Radon measurement protocols in the work areas
- Use of local ventilation wherever required
- Control on the occupancy period of the workers.
ICRP Publication 137 (2017)
ICRP Publication 137 (2017), in general, provides data on individual elements and their radioisotopes, including information on chemical forms encountered in the workplaces, principal radioisotopes, decay schemes, parameter values of reference biokinetic model, data on monitoring techniques, etc., For the first time, in line with other radionuclides, this Occupational Intakes of Radionuclides Part 3 provides effective dose coefficients for inhalation and ingestion of radon and its airborne particulate radon progeny individually, and as a group of progeny radionuclides of 222Rn, 220Rn, and actinon (219Rn; half-life – 4.0 s). The progeny radioisotopes contribute most of the radiation dose and not the radon gas itself.
The effective dose coefficients are expressed in units of mSv per Bq h m–3 and mSv per WLM. Effective dose coefficients are provided for inhalation of short-lived 222Rn and 220Rn in indoors and mines [Table 1].
|Table 1: Effective doses from inhalation of radon and its progeny in workplaces|
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Effective dose coefficients in Sv/Bq following inhalation of radon gas alone is 4.4 × 10−10 Sv/Bq and for thoron is 1.8 × 10−10 Sv/Bq. The equilibrium effective dose rate for continuous chronic exposure to unit concentration of 222Rn is 1.8 × 10−7 mSv per Bq h m−3.
The effective dose coefficients in Sv/Bq for inhaled radon and thoron progeny radionuclides in different types of workplaces are also given in the ICRP-137. The same dose coefficients apply to radon exposure in homes. The ICRP 137 does not specifically address public exposures.
In view of the consistency observed between the coefficients obtained by dosimetric calculations and conversion coefficients (based on epidemiological studies), the ICRP gives the following recommendations, based on the revised nominal risk coefficient and the available data taking into account realistic average breathing rate.
- For inhalation of radon and radon progeny in underground mines and buildings,
3 mSv per mJ h.m−3 (approx. 10 mSv/WLM)
- For indoor workplaces where reference workers are engaged in substantial physical activities, and for workers in tourist caves,
6 mSv per mJ h.m−3 (approx. 20 mSv/WLM)
- Dose coefficient for the inhalation of thoron, for indoor workplaces,
1.5 mSv per mJ h.m−3 (approx. 5 mSv/WLM of exposure)
- Dose coefficient for radon exposure in homes is around 3.7 mSv per mJ h.m−3 or 13 mSv/WLM of exposure.
| Comments|| |
The recommendations look apparently stringent for adopting in indoor workplaces and in uranium mines. This stringent protection standards for radon was expected because of the recommendations in the ICRP publications – ICRP-115 (2010) and ICRP-126 (2014). However, to ensure adequate radiological protection of the workers, it is necessary that the national authorities accept these recommendations in the regulations and enforce the same in a phased manner. The reference level of 1000 Bq/m3 or the dose reference level of 10 mSv/year may be taken as the entry point to establish requirements for planned exposure situations.
Effective dose coefficients provided in the ICRP-137 will be useful for the calculation of effective dose following inhalation of radon gas alone, exposure to radon/thoron progeny and for confirming any overexposure situations by bioassay. For the purpose of operational radiation protection, the present system of using progeny radionuclides of 222Rn and 220Rn as a group, for calculating effective dose, may be adequate.
However, instead of using default values which are generally conservative, use of measured values for the dose-related parameters such as site-specific equilibrium factor (F), measurement of unattached fraction (fp), aerosol distribution, and radon daughter measurement techniques and procedures, will facilitate removing inconsistencies and differences, if any, among different measurements, at different workplaces.
| References|| |
International Commission on Radiological protection (IAEA), Radiation protection in uranium and other mines, ICRP Publication 24, 1977, Ann. ICRP (1), IAEA, Vienna.
International Commission on Radiological Protection (ICRP), Protection against Rn-222 at home and at work, ICRP Publication 65, 1993, Ann. ICRP 23(2).
The 2007 recommendations of the international commission on radiological protection. ICRP publication 103. Ann ICRP 2007;37:1-332.
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ICRP publication 126: Radiological protection against radon exposure. Ann ICRP 2014;43:5-73.
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