Radon decay products effect

In order to compare exposures to radon decay-products with those to other forms of ionising radiation, it is useful to assess the effective dose equivalent expressed in sieverts (Sv). A conversion coefficient of 15 Sv per J h m"3, equivalent to 5.5 mSv per WLM, has been recommended (UNSCEAR, 1982). With this conversion factor, the... 

The average concentration of radon in outdoor air in the UK is 2.6 Bq m"3. Comprehensive data on the equilibrium factor in outdoor air in the UK is not available. Assuming equilibrium, the average exposure to radon decay products received by a member of the UK population during the 10% of time spent in the open is 0.0036 WLM, an annual effective dose equivalent of 0.02 mSv. 

Rudnick, S.N., W.C. Hinds, E.F. Maher, and M.W. First, Effect of Plateout, Air Motion and Dust Removal on Radon Decay Product Concentration in a Simulated Residence, Health Phvs. 45 463-470... 

Method. Figure 3 shows the equipment used by us for loading a culture medium with radon and decay products. Air was circulated in a closed system, driven by a membran pump (MP). The system consisted of a Ra-226 solution (Ra), a security bubble flask with water (H20), a membran bacteria filter (MF) and a second bubble flask containing 100 ml RPMI 1640 culture medium (CM). This medium contains 100 IE/ml penicilin and streptomycin and 0.75% L-glutamin. Foetal calf serum, an essential part of blood cultures, must not be added, else the airstream would develop foam. Furthermore we added a small amount of Pb(N03)2 and Bi(N03)2, about 10 ng of each, as "carriers" for the radon decay products to avoid a "wall effect". 

Measures to reduce radon concentrations have been studied in an old house in which the radon decay-product concentration initially exceeded 0.3 Working Level (WL). Some of the measures were only partially successful. Installation of a concrete floor, designed to prevent ingress of radon in soil gas, reduced the radon decay-product concentration below 0.1 WL, but radon continued to enter the house through pores in an internal wall of primitive construction that descended to the foundations. Radon flow was driven by the small pressure difference between indoor air and soil gas. An under-floor suction system effected a satisfactory remedy and maintained the concentration of radon decay products below 0.03 WL. 

Before standards for indoor exposure to radon can be formally established, work is necessary to determine whether remedies are feasible and what is likely to be involved. Meanwhile, the Royal Commission on Environmental Pollution (RCEP) in the UK has considered standards for indoor exposure to radon decay products (RCEP, 1984). For existing dwellings, the RCEP has recommended an action level of 25 mSv in a year and that priority should be given to devising effective remedial measures. An effective dose equivalent of 25 mSv per year is taken to correspond to an average radon concentration of about 900 Bq m 3 or an average radon decay-product concentration of about 120 mWL, with the assumption of an equilibrium factor of 0.5 and an occupancy factor of 0.83. 

Figure 4 shows the rapid increase in the concentration of radon decay-products in the sitting room on removal of the polythene sheets. This was effected in three stages. Initially, the sheets were removed along the front wall of the room and for a distance of 1.5 m across the floor. A sharp increase occurred immediately. An hour later the remaining polythene, except that covering the fire place, was removed. The concentration was still rising at 21 00. 

Figure 9. Radon decay-product concentrations and ventilation rates showing the effect of covering the fire-places.

The highest radon production rates occurred in rooms with suspended wooden floors through which air could pass readily. Simple ventilation of the underfloor space with a 30 W fan was effective in reducing radon decay-product concentrations in these rooms, but not below the reference value in all rooms. More rigorous application of this technique with multiple extraction points and a larger fan could clearly serve as an effective remedy. 

Polythene and mylar sheeting laid over the suspended wooden floor reduced the concentrations of radon decay products, but not below the reference level. This technique was only partially successful because of the difficulty of effecting good seals to the walls, indicating the care needed to remedy high radon levels with membrane barriers. 

Electrostatic precipitators were effective in reducing the radon decay-product concentration below the reference level, but analysis of the room aerosol indicated that the reduction was largely offset by an increase in lung dose per unit exposure. 

A new concrete floor incorporating barriers to radon transport from the subsoil appeared to be only partially successful in reducing radon decay-product concentrations. It was shown that the Venturi effect of the wind across two chimney stacks caused pressure-driven flow of radon from the ground. Covering the fireplaces to eliminate this effect resulted in concentrations below the reference level. 

Another concept relating to the decay products is that of the "unattached" fraction. Although it is now known that the decay product atoms are really attached rapidly to ultrafine particles (0.5 to 3 nm in diameter), there is a long history of an operationally defined quantity called the "unattached" fraction. These decay products have much higher mobilities in the air and can more effectively deposit in the respiratory system. Thus, for a long time the "unattached" fraction has been given extra importance in estimating the health effects of radon decay products. Typically most of the "unattached" activity is Po-218 and the value of unattached frac-... 

It is possible to remove radon decay products from indoor air by filtration. The effects of air cleaning on dose levels are described by Jonassen (1987). However, there are major uncertainties in the effectiveness of air cleaning to remove the decay products because the particles are also removed. When the particles are removed, the "unattached fraction increases and although there are fewer decay products, they are more effective in depositing their dose of radiation to the lung tissue. Thus, there will. be much lower dose reduction than there is radioactivity reduction. It, therefore, may be more protective of health to control the radon rather than its decay products. 

Collier CG, Strong JC, Humphreys JA, et al. Carcinogenicity of radon/radon decay product inhalation in rats—effect of dose, dose rate and unattached fraction. Int JRadiatBiol2005 81(9) 631—47. 

The second report, "Interim Protocols for Screening and Follow-up Radon and Radon Decay Product Measurements" (EPA 1987a), outlines the recommendations for making reliable, cost effective radon measurements in homes (Ronca-Battista et al. 1988). 

Figures 5.2 and 5.3 show the effective dose per unit exposure of the tracheo-bronchial (bronchial and bronchiolar) and pulmonary or alveolar region as a function of aerosol particle diameter calculated for the radon decay products Po, " Pb, and " Po and the thoron decay products Pb, Bi and Po. A tissue weighting factor of the lung and the radiation weighting factor of 0.12 and 20, respectively, are taken into account. The effective dose from a radioactive mixture can be deduced by adding the effective doses of each decay product.

Fig. 5.4. Effective dose by the inhalation of radon decay products as a function of the particle diameter at different relative cancer sensitivity distributions between the bronchial, wbb, bronchiolar, Wbb, and alveolar, wai, region of the thoracic lung (wr = 20, U) = 0.12). v = 0.75 (basal cells + secretory cells).

The dose conversion factor, DCF, in effective dose per exposure unit is calculated by taking into account the dose function of the particle diameter (Figure 5.4) and the radon decay product characteristics. The dose conversion factors for living and work places with typical activity size distributions, as a function of the unattached fraction, fp, are illustrated in Figures 5.13 and 5.14, respectively. The value of the dose conversion factor, DCFae, for fp = 0 represents the dose contribution of the radon product aerosols. The values of dose conversion factor in Figure 5.14 are based on aerosol conditions which are typical for many workplaces. 

The radon concentration indoors in terms of radiation dose exposure is expressed in WL (Working Level, the radiation level of 100 pCi per litre or 3700 Bq per m of Rn in equilibrium with its decay products). Effects of radon are given in terms of WLM (Working Level Months), which is the exposure at 1 WL for one working month, or 170 h. Since there are 365 X 24 = 8760 h per year and 80% of them (7000 h) are spent indoors, the annual time of exposure is 7000/170 = 41 working months . A world-wide representative value adopted by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 1977) is that 1 pCi per litre or 37 Bq per m is the average Rn concentration indoors, with an equilibrium factor (ratio of Rn decay product concentration to their concentration in radioactive decay equilibrium with Rn gas) of 0.5. This corresponds to an average concentration of 0.005 WL or 5 mWL. The annual indoor exposure is then 0.005 WL x 41 WM = 0.205 WLM. 

National Radiological Protection Board, NRPB (1990). Human exposure to radon in homes. Doc. NRPB 1, 17-32. National Research Council Committee on the Biolt ical Effects of Ionizing Radiations, NRC (1988). Health Risks of Radon and Other Interrudly Deposited Alpha Emitters (BEIRIV). National Academy Press, Washington, DC. Nero, A.V. (1988). Estimated risk of lung cancer from exposure to radon decay products in U.S. homes A brief review. Atmos. Environ. 22, 2205—2211. 

Possibly the most cost-effective method involves radon absorption on a small charge of activated charcoal that can be placed in an inexpensive polyethylene bag. The gamma-emitting radon decay products are measured by a scintillometer with an efficient collecting geometry, preferably approaching 4 TT. 

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