Radon decay products average annual exposure

An equilibrium factor of 0.35, derived from measurements made during the local surveys, has been assumed to typify conditions in UK dwellings. This value has been used to convert the average radon concentrations measured in the national survey to potential alpha-energy concentration of radon decay-products. On average, persons in the UK spend 75% of their time in their homes and 15% of their time elsewhere indoors (Brown, 1983). The occupancy factor of 0.75, together with an equilibrium factor of 0.35, results in an annual exposure of 1.3 10"5 J h m"3 (0.0037 Working Level Months,... 

Table II. Annual average exposure to radon decay-products in UK dwellings from the national survey...

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. 

Radon-222 is a product of the nuclear disintegration series of uranium-238 (Figure 21.3) and is continuously generated as uranium in rocks and soil decays. As Figure 21.25 indicates, radon exposure is estimated to account for more than half the 360-mrem average annual exposure to ionizing radiation. 

Swedjemark, G.A., Radon and its decay products in housing - estimation of the radon daughter exposure to the Swedish population and methods for evaluation of the uncertainties in annual averages, Thesis, Department of Radiation Physics, University of Stockholm (1985). 

The average annual effective dose equivalent received by a member of the UK population is currently estimated to be 2150 ySv. Of this total, 87% arises from exposure to radiation of natural origin, the largest single contributor being inhalation of the short-lived decay products of radon. This exposure occurs predominantly in the home. 

For exposure indoors of the general population, it is more useful to work in terms of the concentration of radon than the concentration of decay products. Brown et al. (1986) found the average concentration of 222Rn in Cornish houses to be 300 Bq m-3, and Nero (1988) has estimated that 2% of homes in the USA have similar or higher concentrations. Assuming an annual effective dose equivalent of 80//Sv... 

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. 

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