Equivalent radon concentration

The variations in the background, the sensitivity to moisture, the alpha activity of the chamber itself and the influence of recombination were discussed by Hultqvist. The standard deviation due to counting statistics was estimated to be about 3 % (in a few measurements 6 %). The calibration was made by counting each alpha particle by a proportional counter specially designed at the Department for this purpose. The statistical uncertainty of the calibration of the equivalent radon concentration was estimated to be 12 %. 

From the measured activity concentrations the ratios of the free activity to the total activity fj = cjf/cj (j = 1>2), the corresponding values of fp and the equilibrium equivalent radon concentration c q and F, respectively, were calculated. 

Then the unattached fraction was calculated in each measurement and was found to be between. 05 and. 15 without aerosol sources in the room and below. 05 in the presence of aerosol sources. The effective dose equivalent was computed with the Jacobi-Eisfeld model and with the James-Birchall model and was more related to the radon concentration than to the equilibrium equivalent radon concentration. On the basis of our analysis a constant conversion factor per unit radon concentration of 5.6 (nSv/h)/(Bq/m ) or 50 (ySv/y)/(Bq/m3) was estimated. 

The fraction of unattached daughters (fp), the equilibrium factor (F) and the activity median diameter (AMD) are plotted in Figure 6 for all the measurements. The AMD is derived from the aerosol measurements. These three parameters are important in the dosimetric models. At the top of Figure 6 the effective dose equivalent is plotted, computed with two models called the J-E (Jacobi-Eisfeld) and J-B (James-Birchall) models in the NEA-report (1983, table 2.9, linear interpolation between AMD=0.1 and 0.2 ym). The figure also shows the effective dose equivalent calculated from the equilibrium equivalent radon concentrations with the NEA dose conversion factor (NEA,1983, table 2.11). 

It is to be noted that the conversion factor of 20 mSv effective dose equivalent per 200 Bq m 3 equilibrium equivalent radon concentration, considered by the ICRP (ICRP, 1984) in recommending an Action Level for remedying high indoor radon concentrations, corresponds to a dose rate of about 40 pGy per y per Bq m 3 radon gas concentration. 

In 1955, the International Commission on Radiological Protection set a maximum permissible occupational concentration of 3.7 x 103 Bq m-3 (10-10 Ci l-1), for continuous exposure, equivalent to 1.1 x 104 Bq m-3 (3 x 10-10 pCi P1) for a 40-h working week. Subsequently, when it was realised that the critical dose to the lung was from inhalation of decay products, not radon itself, the permissible concentration was defined in terms of the concentration of decay products. The current recommended limit (ICRP, 1986) for a working period of 2000 h per year is 1.5 x 103 Bq m 3 equilibrium equivalent radon concentration (a term defined in Section 1.8 below). 

The S.I. unit for the PAEC is J m-3, and the unit for the time integrated PAEC, or dosage, is Jh m-3. The equivalent radon concentration is that concentration of radon, with decay products in equilibrium, which has the same PAEC as have the decay products actually present. In indoor air, the equivalent concentration is often about half the actual radon concentration. High in the atmosphere, the equivalent and actual radon concentrations are the same. 

The PAEC can be readily calculated once the activities of the individual radionuclides have been determined from measurements. Direct measurements of the concentrations of all short-lived decay products of Rn are difficult and limited. They are estimated from considerations of equilibrium (or disequilibrium) between Rn and its decay products. An equilibrium factor F is defined that permits the exposure to be estimated in terms of the PAEC from the measurement of radon gas concentration. This equilibrium factor is defined as the ratio of the actual PAEC to the PAEC that would prevail if all the decay products in each series were in equilibrium with the parent radon. However, it is simpler to evaluate this factor in terms on an equilibrium equivalent radon concentration, EEC. This quantity, EEC, represents the activity concentration of the radon gas that would have to exist in complete equilibrium with the decay products if the short-lived decay products had the same PAEC as in the nonequilibrium mixture. The units of EC are Bqm . ... 

Table I. Radon Concentrations and Radiation Dose Rate Equivalent to Epithelial Cells of the Occupants of Residential and Plant Buildings In and Near the Savannah River Plant...

MeV. WL-R = 100% x WL/radon concentrations (pCi/1). The dose conversion factor of 0.7 rad/working level month (WLM) (Harley and Pasternack, 1982) was used to calculate the mean absorbed dose to the epithelial cells and a quality factor (OF) of 20 was applied to convert the absorbed dose to dose equivalent rate. For example, from the average value of (WL) obtained from the arithmetic mean radon concentrations measured in the living area during winter and summer in South Carolina (Table I), the calculated dose equivalent rate is 4.1 rem/yr, e.g.,... 

A survey of the radon concentrations in a representative sample of more than 2000 dwellings in the UK has been completed and provisional results are now available. On average, concentrations are 29% lower in bedrooms than in living areas. The mean radon concentration weighted for room occupancy is 22 Bq m 3. Assuming an equilibrium factor of 0.35 and a mean occupancy of 75%, the mean annual exposure in UK homes is assessed as 0.08 Working Level Months (WLM) and the mean annual effective dose equivalent as 0.43 mSv. 

It is noted that the ICRP has assumed a higher conversion coefficient between annual effective dose equivalent and radon concentration (ICRP, 1984) in recommending an action level for remedial measures in homes, i.e. 1 mSv y"1 per 10 Bq m"3 of equilibrium equivalent radon gas concentration (9 mSv per WLM). If this conversion coefficient were applied to our regional survey data, we would estimate, from the distribution parameters given in table 3, that about 15% of the residents of certain areas of Devon and... 

Indoor air radon concentrations measured in a randomly selected sample of 220 Irish houses have been found to range from about 20 Bq/nr to as high as 1740 Bq/nr with a median value of 61 Bq/nr. Using current dose estimation methods the estimated effective dose equivalents due to radon daughter inhalation in these houses are 1.6 mSv/year (median value) and 46 mSv/year (maximum value). 

The results for the first phase of the national survey are also presented in histogram form in Figures 1 and 2 together with the annualised effective dose equivalents estimated using the factors given in Table I. It is evident from the data that in the majority of households surveyed the radon concentrations and associated doses are low. In a small percentage of cases however individual households have been found with very high radon... 

Figure 7. Effective dose equivalent per hour and per unit radon concentration (AJ-B, 7 J-E) as a function of the equilibrium factor. The full lines are calculated with the mean values of the 72 measurements (Xa -. 37/h, XVent . 41/h, P -. 53, A.M.D. —. 15 lJm) and changing attachment rates.

Figure 9. Effective dose equivalent per hour and per unit radon concentration (A J-B, V J-E), equilibrium factor ( ) and unattached fraction (o, right ordinate) versus the attachment rate. The curves are calculated as in Figure 7.

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. 

This concentration is equivalent to a partial pressure of radon of 6.6x10 atmospheres. Yet this value is an elevated radon concentration. If all of the decay products formed by the decay of the radon remain in the air, then there would also be 10 pCi/1 of Po-218, Pb-214, etc. Such a mixture would be said to be in secular equilibrium. From the monitoring of uranium mines, an equilibrium mixture of these decay products at 100 pCi/1 is called a working level (WL). Thus, a 10 pCi/1 equilibrium mixture represents 0.1 working level. 

If the equilibrium factor is 0.5, then the NCRP recommended action level is equivalent to 8 pCi/1 or twice the current EPA guidline of 4 pCi/1. It is important to note that NCRP has made its recommendation on the decay product not the radon concentration. There is currently some uncertainty as to the precise level at which action should be taken. There is general agreement that remedial action is clearly needed above 20 pCi/1 and at levels of 100 pCi/1 and higher, immediate action is required. 

In a passive detector developed by the National Radiological Protection Board (Wrixon et al., 1988), the etched pits in the detectors are filled with scintillator fluid. After exposure to radon, the detector is irradiated with an alpha source, and the resulting scintillations counted with a photo-multiplier tube. In this way, track density over 1 cm2 of detector can be measured in a few seconds. Passive detectors used in the UK National Survey were sensitive down to 20 kBq m-3 h of accumulated exposure, equivalent to a radon concentration of 5 Bq m-3 measured over 4000 h exposure. 

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. 

In the radon surveys the primary quantity determined is the indoor air mean radon activity concentration. From a radiological health perspective it is the dose arising from the inhalation of radon daughters that is of interest. The conversion from radon exposure to annualised effective dose equivalent for the survey was carried out using the factors given in Table I which are similar to those being used in other European surveys. The occupancy and equilibrium factors given in this table are assumed mean values for Irish... 

In the uranium mining industry, the Working Level (WL) is defined as a concentration of decay products having PAEC equal to that of decay products in equilibrium with 100 pCi l-1 (3.7 x 103 Bq m-3) of radon. The Working Level Month (WLM) is defined as exposure to decay products equivalent to 1 WL for 170 h, this being the nominal number of hours worked per month in a mine. The WLM is still used in discussions of the epidemiology of lung cancer in relation to exposure to radon and its decay products. 

---