In January 1990 NRPB recommended that the Action Level for radon in existing homes should be 200 Bq M-3 averaged over a year (NRPB, 1990). Parts of the UK with a 1% probability or more of present or future homes being above the Action Level are regarded as Affected Areas: such areas were identified from radiological evidence.
Radon Affected Areas are defined using maps of mean radon concentrations in houses which are based on data from the radon surveys carried out by the NRPB. The maps are based on the Ordnance Survey 10 km Irish grid square as shown in Figure 2.7 below:
Figure 2.7 Average indoor radon concentrations in Northern Ireland by 10km of the Irish grid from cumulative results Source (NRPB, 1993)
The map in Figure 2.8 shows a map of Northern Ireland and indicates that areas such as Ards, Armagh, Down and Strabane have elevated radon levels. Particular attention has been paid to the Down area with additional radon measurements carried out by the NRPB. The results of the survey have led to the Down area being declared a Radon Affected Area within Northern Ireland. Figure 2.8 shows the Southeast corner of Northern Ireland based on the Ordnance Survey 5 km Irish grid square and gives a more accurate break down of the Affected Area.
Figure 2.8 Average indoor radon concentrations in the Southeast corner of Northern Ireland by 5km squares of the Irish grid from cumulative results Source (NRPB,1993)
The NRPB using information based on the results of the surveys and using statistical observations on the distribution of radon concentrations have improve the estimates of the number of houses above the Action Level in a grid square, in part by using data from neighbouring squares in a data smoothing exercise.
The smoothing techniques are used to improve the estimate of the fraction of the housing stock exceeding the Action Level in each grid square. First, however, the outdoor radon concentration is subtracted from each result as this improves the fit of the results to the log-normal distribution. The values of the geometric mean (GM) are smoothed between adjacent squares to remove any anomalies that might be caused by small numbers of results in some squares.
The average value of the geometric standard deviation (GDV) over the area of interest is used in estimating the fraction of the housing stock above the Action Level. For the county, region or district, a final Affected Area map is produced showing areas with <1%, 1-3%, 3-10%, 10-30% and >30% of homes above the Action Level. Figure 2.9 below shows these probabilities for Northern Ireland.
Figure 2.9 Percentage of homes with radon levels greater than 200 Bq m-3 in Northern Ireland Source (NRPB, 1994)
Measurements of radon levels in dwellings in Northern Ireland have indicated that there are no areas where more than 30% of dwellings are above the action level.
The Building Regulation (Northern Ireland) 1994 have adopted the 5 km Irish grid to show the location of the affected radon zones. See Figure 2.10.
Figure 2.10 Building Control map of radon affected zones Source (Building Regulations (NI), 1994)
Zone A -1% to 3% probability of radon concentration exceeding 200 Becquerels per cubic metre.
Zone B – 3% to 10% probability of radon concentration exceeding 200 Becquerels per cubic metre.
Zone C – Greater than 10% probability of radon concentration exceeding 200 Becquerels per cubic metre.
The above Building Control map is based on the NRPB map (Figure 2.9) and therefore follows the advice of this organisation.