Year : 2017 | Volume
: 40 | Issue : 3 | Page : 107--109
Radon in workplaces: An update
Ex. RSSD, BARC, Mumbai, Maharashtra, India
Ex. RSSD, BARC, Mumbai, Maharashtra
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Pushparaja. Radon in workplaces: An update.Radiat Prot Environ 2017;40:107-109
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Pushparaja. Radon in workplaces: An update. Radiat Prot Environ [serial online] 2017 [cited 2018 Dec 11 ];40:107-109
Available from: http://www.rpe.org.in/text.asp?2017/40/3/107/225582
As we know, all human beings are exposed to background radiation emanating from various natural sources such as cosmic radiation and primordial radionuclides such as K-40, Rb-87 and decay-product radionuclides from uranium and thorium series. Radon-222 and Radon-220 (thoron) are gaseous, alpha-emitting radionuclides of radon with short half-lives of 3.82 days and 55.6 s, respectively from uranium and thorium series. The parent radionuclides of radon-222 are radium-226 (Ra-226) and that of radon-220 is Ra-224. The daughter products of radon and thoron are alpha-, beta-, and gamma-emitting particulates which get airborne and constitute inhalation hazard. Lung is the target organ for the radon exposure. It is recognized that the long-term exposure to radon increases the risk of lung cancer.
Exposure of humans to radon gas and its progeny constitute the most important source of natural background radiation. As per the UNSCEAR-(2010), it constitutes about half of the annual average dose received from the natural background radiation. There is a large variation in the dose depending on the location and the concentration of the radionuclides in the soil and in the air. Elevated levels of radon-222 are observed in uranium mines and in other work areas where uranium concentrate and Ra-226-bearing materials are handled. Radon-220 is present in thorium/thorium concentrate handling/storage facilities.
The average outdoor Rn-222 level varies from 5 to 15 Bq/m 3. The radon gets diluted quickly in air to very low concentrations.
Indoors, particularly, in mines, caves, and water treatment facilities, the concentrations can be higher and vary from 10 to 10,000 Bq/m 3 depending on the emanation rate and the ventilation status (WHO). As per a study reported around Indian Jaduguda uranium mines, the environmental radon concentration varied from 4 to 157 Bq/m 3.
Units of Radon Progeny Concentration
Working Level (WL) is the historical unit to express the radon concentration in air in the mining industry. One WL corresponds to a radon concentration of 100pCi/L or 3700 Bq/m 3. Radon is assumed to be present in equilibrium with its short-lived daughters, i.e., Po-218, Pb-214, Bi-214 and Po-214, and Tl-210. 1 WL of Rn-220 corresponds to 275 Bq/m 3 of thoron in equilibrium with its short-lived daughters, namely, Po-216, Pb-212, Bi-212, Po-212, and Tl-208.
Ventilation rate disturbs the state of equilibrium of radon/thoron with its daughters. The ratio of measured radon/thoron concentration to the measured concentration of the daughters in the air gives equilibrium factor, F. The equilibrium factor helps in radon/thoron exposure management as recommended by International Commission on Radiological Protection (ICRP). The ICRP uses a default value of 0.4 for the equilibrium factor for radon. This means 100 Bq/m 3 of measured radon concentration gives 40 Bq/m 3 of equilibrium equivalent radon (EEC) concentration in the air.
The concentration of radon daughters is also expressed in terms of potential alpha energy concentration (PAEC) in the air. Potential alpha energy is the total energy emitted ultimately by the radon/thoron progeny during their decay up to a stable lead isotope. The alpha energy exposure is the time integral of the concentration in the air over the time for which an occupational worker is exposed. One WL is equal to 1.3 × 108 MeV/m 3 of air. In SI unit, one WL is 2.1 × 10−5 J / m 3 (the use of this classical unit is now being discouraged). The SI unit of the exposure is J. h. m -3.
1 Bq/m 3 of radon at equilibrium with its daughters gives PAEC of 3.47 × 104 MeV/m 3. Similarly, 1 Bq/m 3 of thoron gives PAEC of 4.72 × 105 MeV/m 3.
Exposure of personnel, breathing air containing Radon-222, at a concentration level of 1 WL for 170 h, receives a cumulative exposure of 1 WL month (WLM). In SI units, a WLM is 3.54 × 10−3 J. h. m −3.
1 WLM = 6.37 × 105 Bq. h. m −3 (radon); 1 WLM = 4.68 × 104 Bq. h. m −3 (thoron) and the dose received due to 1 WLM of thoron daughters exposure is 1.9 mSv (ICRP-50), whereas the effective dose from exposure of 1 WLM of radon daughters is 13 mSv (ICRP-2015).
ICRP Publication-32 (Limits for Inhalation of Radon Daughters by Workers, 1981, superseded later by ICRP-65) updated the recommendations and limits of previous recommendations of ICRP 24 (Radiation Protection in Uranium and other Mines, 1977, superseded later by ICRP 47 in 1986) for inhalation of radon daughters by workers. Epidemiological data of the uranium miners were considered for the recommendations. Occupational limits were derived based on the annual effective dose limit of 50 mSv for the miners.
It was understood by the time that the dose to the respiratory tract was nonuniform, and using the regional lung dose concept, ICRP recommended an exposure limit from radon daughters of 4.8 WLM a year for the protection of miners to meet the dose limit of 50 mSv. This works out to 1500 Bq/m 3 of equivalent equilibrium concentration of radon for 2000 working hours. The limit was endorsed by ICRP-47.
ICRP publication 39 recommended an action level for radon exposure in existing situations, below which it was considered not relevant to introduce any protective measures. The action level was equilibrium equivalent concentration of 200 Bq/m 3, corresponding to an effective dose equivalent of 20 mSv/y. However, for new exposure situations, it was felt that the exposure of the highly exposed individuals needs to be governed by the lower bound limit of EEC concentration of 100 Bq/m 3 a year which is equivalent to 10 mSv/y.
All the previous recommendations were superseded by ICRP-65 (Protection against Radon at Home and at Work) in 1993, for workers as well as the members of the public. The publication considered epidemiological approach (data from miners) to derive limits rather than the dosimetric approach followed in the earlier recommendation. This was considered as the direct method with less uncertainty as compared to the dosimetric approach and considered as most suitable to assess and control radon exposure.
Using the estimated risk of lung cancer based on the epidemiological data of adult male miners, the ICRP-65 recommended the lifetime lung cancer risk of 2.8 × 10−4 WLM −1, for general population.
The dose conversion factors are then obtained by comparing the estimated lung cancer risk per unit exposure with the total detriment per unit of effective dose provided by the ICRP-60. The dose conversion factor is used to derive the radon concentration from the effective dose limits. The dose conversion factors used in ICRP-65 were 5 mSv/WLM for workers and 4 mSv/WLM for the public.
ICRP-65 recommends the action levels to manage the exposures at work and at dwellings. The action level is defined as the radon concentration at which intervention (protective actions) is recommended to reduce the exposure in dwellings or in workplace. No distinction is made between an existing and a new dwelling. Miners are considered as occupationally exposed workers and dose limits apply.
The action levels for radon in dwellings (existing and new) are:
Choose annual effective dose in the range: 3–10 mSv corresponding to 200–600 Bq/m 3 of radon, occupancy – 7000 h, and equilibrium factor of 0.4.
The action levels for radon in workplaces are for workers who are occupationally exposed to radiation are:
Action level not to exceed an annual dose of 3–10 mSv corresponding to radon concentration in workplace to 500–1500 Bq/m 3, occupancy – 2000 h, and equilibrium factor of 0.4.
Current Position of Icrp on Radon
Updated guidance on the management of occupational radon exposure, the challenges, and the radiological protection against radon exposure was provided by the ICRP in the publication no. 126 (Radiological Protection against Radon Exposure, ICRP-126, IAEA, 2014).
The current ICRP recommendations classify radon in dwellings and workplaces as “existing exposure situation” and the reference levels (in place of action levels) in terms of individual dose provided by the ICRP-103 to be used for optimization and control of radon exposure. Exposures above the reference levels are not allowed to occur. The upper value of 10 mSv per year for an individual dose is retained as reference level. The reference levels in terms of corresponding radon concentration works out to be 600 Bq/m 3 for dwellings and 1500 Bq/m 3 for workplaces.
Subsequent to 2007, based on the availability of epidemiological and dosimetric data, the ICRP accepted the recommendations of the task group assigned to study the risk of exposure to radon. The task group concluded that the absolute risk of lung cancer due to radon needs to be multiplied by 2.
Dosimetric studies by several investigators over the years have given different values for radon risk estimates. Based on the new findings, the Main Commission issued a Statement in November 2009 which gives the most recent position of ICRP on radon. The lifetime lung cancer risk attributable to exposure to radon as 5 × 10−4 WLM −1 (based on dosimetric approach), whereas the value provided in ICRP-65 was 2.8 × 10−4 WLM −1. A new assessment of the dose conversion factor has resulted in the values as 12 and 9 mSv/WLM for workers and public, respectively (Marsh et al.: Dose conversion factors for radon, in: Health Physics Vol. 99, No. 4 [October. 2010], p. 511-516).
Using this risk conversion factor, an average effective dose of 20 mSv corresponds to 1.7 WLM of radon exposure, a significant reduction in the exposure limit as compared to earlier 4.8 WLM. For a single-year dose limit of 50 mSv, the limit for exposure to radon works out to be 4.2 WLM.
It also recommends calculation of dose coefficients for radon and its progeny following the approach used by ICRP for other radionuclides, using reference biokinetic and dosimetric models. The dose coefficients is expected to replace the dose conversion conventions given in ICRP-65.
The ICRP continues to recommend annual dose of 10 mSv as the reference level for radon exposure in workplaces.
Current Iaea Stand-Bss, Gsr Part 3, 2014
The IAEA, keeping in view of the international harmonization, recommended that national regulators should set a value not exceeding an annual average activity concentration of 1000 Bq/m 3 as a reference level for radon which may be considered as an entry level for applying occupational radiation protection requirements in planned exposure situations. Optimization of the protection, ALARA, is further emphasized.
Lung cancer risk from exposure to radon is found to be twice the earlier estimated risk. The acceptable exposure limits need to be reduced considerablyThe exposure to radon is continued to be treated as special with 10 mSv as the reference level. The recommended radon exposure range of 3–10 mSv annual dose is much higher as compared to the doses received in other nuclear fuel cycle operationsThe recommended annual dose range of 3–10 mSv is higher than the general annual average background radiation dose encountered worldwideLung is the only major target organ for the exposure from radon and radon progeny. The effective half-life is 30 min for Rn-222 daughters. In that respect, it is special and can be treated separately. The lung weighting factor will provide the necessary input to calculate effective dose from radon exposureNational regulatory authorities are empowered to set country's own national reference levels for optimization of radon exposures, preferably as a part of national radon action plan for existing exposure situationsIt should be strongly recommended to ensure optimized protection strategy for radon to maintain the exposures below 3 mSv/yIt may be very difficult to control exposure to radon at around 1.7 WLM in mining sector, corresponding to the average annual dose limit of 20 mSvEnforcement of the adequate protective measures in the mines is the only ethical way to ensure workers' safetyIAEA-BSS (2014) recommends an annual average radon reference level of 1000 Bq/m 3 for entry point for occupational exposures for globally harmonized monitoring and record keeping. However, the entry point for reference level for occupational radiation protection requirements should rather be set at the lower level corresponding to around 3 mSv/y, i.e., at 300 Bq/m 3 rather than 1000 Bq/m 3 of radon. This will be in line with the exposure scenarios in other NFC facilities.