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Mapping radon-prone areas using house radon data and geological boundaries
Envimoment International, Vol. 22, Suppl. 1, pp. S7794782,1996 8 1996 National Radiological Protection Board. Published by Elsevier Science Ltd. Printed in the USA. All rights reserved 0160-4120/96 Sl5.00+.00
Pergamon
PI1SO160-4120(96)001?33-3
MAPPING RADON-PRONE AREAS USING HOUSE RADON DATA AND GEOLOGICAL BOUNDARIES Jon Miles National Radiological Protection Board, Chilton, Didcot, Oxon, OX1 1 ORQ, UK
Keith Ball British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK
EI 9508-236 A4(Received 10 August 1995; accepted 5 June 1996)
The results of measurements of radon levels in homes can be grouped by grid square and analysed using a lognormal model to provide accurate estimates of the proportion of homes above a reference level, but this technique cannot provide high resolution at reasonable cost. Geological parameters can provide detailed maps of radon potential, but without assistance from measurements in homes, cannot provide accurate estimates of the levels of radon in homes. An alternative mapping technique is to group house radon measurements by geological unit. Analysis of 36 570 results in an area of 58 x 19 km showed that this technique can provide radon potential maps which are more detailed than grid square maps and more accurate in estimating numbers of homes affected than mapping based only on geology. However, lateral variations in geological formations mean that conclusions drawn for one area cannot necessarily be extrapolated to the same geological formations in adjoining areas. The results also showed that it is important to take superficial deposits into account in mapping radon potential. Published by Elsevier Science Ltd.
INTRODUCTION
The main purpose of radon potential mapping is to map the geographical variation in probability that new or existing buildings will exceed a radon reference level. This information may be used for various purposes, such as to target information campaigns encouraging measurement of radon levels in homes or to allow building regulations to be altered to prevent new buildings from having radon problems. There is a long chain of factors that influence the radon levels found in buildings. Variation in these factors produces the wide range of radon levels measured in buildings. Because these factors are largely independent and are multiplicative, the distribution of radon levels observed in buildings is usually lognormal (Gunby et al. 1993). Differences in the radon levels between buildings are often due to differences in the radon potential of the ground under them, even within small areas. But such differences in radon levels are also
due to differences in the buildings themselves (such as, house type, date of construction, and details of building techniques). This makes it difftcult to map the radon potential independently of influences of the individual buildings. MAPPING TECHNIQUES The National Radiological
Protection Board (NRPB) designated radon Affected Areas in the UK on the basis of house radon data grouped by 5-km grid squares (Miles et al. 1992). The data was interpreted in terms of lognormal distributions, with the geometric mean (GM) and geometric standard deviation (GSD) estimated for each square. Techniques for estimating these parameters where data is sparse have been developed (Miles 1994). These give accurate estimates of the proportion of homes above the UK radon Action Level at this resolution, which is appropriate for many administrative s179
S780
actions. The results can be understood in terms of the local geology (Ball and Miles 1993). If finer resolution of radon-prone areas is required, this technique can be applied to smaller areas, for instance, 1 km grid squares. However, this requires a density of measurements that is higher than the density of homes in rural areas, so the finer resolution can be obtained only in urban areas, and then at significant cost. Geology can give high-resolution maps of radonprone areas as geology has already been mapped in detail in many areas. However, if purely geological indicators such as the radium content of rocks, uranium mineralogy and permeability are used, the estimated number of homes above a threshold is likely to be inaccurate, as the relationship between these parameters and radon potential appears to vary between different geological formations. Since the radon in buildings is largely derived from the radon in soil gas under and around the building, measuring the radon concentration in soil gas provides more relevant information than uranium concentration. It has been shown (Akerblom et al. 1984; Reimer and Gundersen 1989) that for particular rock types, there is a good correlation between radon in soil gas and radon in homes. However, the relationship between these two parameters is different for different rock types, so radon in soil gas cannot be used by itself to predict radon levels in buildings. Measurements of radon levels in homes analysed by grid square can therefore be used to provide accurate estimates of the proportion of homes above a reference level, but cannot provide high resolution. Geological parameters, including the properties of individual rock types, can provide detailed maps, but without assistance from results of measurements in homes cannot provide accurate estimates of radon potential. An alternative mapping technique is to group house radon measurements by geological unit. Where enough results of house measurements are available, this has the potential to provide maps which are both detailed and accurate. Where there are too few houses to provide good data, the technique can be extended using measurements of radon in soil gas under carefully controlled conditions (Ball et al. 1992). DATA
House radon data was provided by surveys carried out by the NRPB on behalf of the Department of Environment. Radon concentrations were measured in 220 000 homes in the UK using passive radon detectors. Each house was measured using two detectors exposed over a three-month period, and the results corrected for
J. Miles and K. Ball
seasonal variations (Kendall et al. 1994). Most of the measurements were performed at the request of the householder, following the announcement of radon Affected Areas. For this exercise, house radon data from the areas of two British Geological Survey (BGS) sheets were used. The survey sheets were the adjacent Northampton and Wellingborough survey sheets, numbers 185 and 186, each covering an area 29 x 19 km. Intensive surveys of radon in homes were carried out in this area, resulting in 25 842 results for the Northampton sheet and 10 728 for the Wellingborough sheet. The exposed bedrock is entirely from the Jurassic Period, ranging from Lower Liassic mudstones through to the Oxford Clay. The rocks comprise an interbedded sequence of mudstones, limestones, and sandstones. The strike of the beds is ENE/WSW with a gentle dip towards the ESE. The older beds outcrop in the NW section with progressively younger beds outcropping towards the SE. Despite the gentle dip and topography, in detail the trace of the lithologies is complex. The bedrock is overlain by unconsolidated deposits of glacial origin comprising ground moraine (Boulder Clay) and fluvio-glacial sands and gravels. Terrace deposits and alluvium are found in the river valleys. The maps used for the geological classification show superficial cover only where it is more than 1 m thick. The thickness or composition of rock types in geological formations may vary laterally. This is particularly common in the Northampton area and the effect of the subtle changes in geology on the house radon values was one of the factors investigated in this paper. The Northampton Sand Formation is up to 21 m thick, the basal part being developed as a phosphatic ironstone which in places is overlain by massive yellow or brown sandstone. Where the sandstone is missing, the formation is commonly 4-8 m thick. The Blisworth Limestone also varies significantly in thickness across this area. The locations of the houses were identified from the postcode. The UK national grid references of the centroids of individual postcodes are commercially available. One postcode covers 15 houses on average, more in towns and fewer in country areas. The grid reference of a house can therefore be obtained with an uncertainty of about a few hundred metres or less in towns, but the uncertainty may be 1 km or more elsewhere. Because the results of measurements are confidential, it was not possible to pass details of the results from the NRPB to the BGS for identification of the underlying geology. Instead, a system was devised to
Mapping radon-prone areas
S781
Table 1. Distributions of results of radon measurements in homes on different bedrocks on two adjacent geological survey sheets. Northampton sheet
Wellingborough
sheet
No. of results
GM
GSD
% above AL
Estimated % above AL
No. of results
GM
GSD
% above AL
Estimated % above AL
Blisworth Limestone
1578
23
2.4
0.8
1.2
1996
35
2.6
3.6
4.5
Upper Estuarine Series
3048
18
2.3
0.4
0.5
1209
28
2.7
3.1
3.0
Lower Estuarine Series
1754
22
2.7
2.3
2.3
532
48
3.0
11.3
12.0
11 260
41
3.0
8.4
8.2
3023
82
3.0
21.5
24.0
20
2.5
1.9
1.8
588
22
2.4
1
Bedrock (no superficial deposits)
Northampton Formation
Sand
738
Upper Lias Clays
0.9
Table 2. The effects of superficial deposits over the Northampton sand formation on the proportion of homes above the action level (AL). Northampton sheet Northampton sand formation
Superficial deposits
No. of results
GM
GSD
% above AL
Estimated % above AL
None
11 260
41.0
3.0
8.4
8.2
73
29.0
2.9
2.7
6
636
39.0
3.1
9.4
10
Boulder clay Glacial sand and gravel
allow the geology and superficial cover of each measurement point to be identified without loss of con-
fidentiality. This process involved three stages: 1) The NRPB sent to the BGS a file of grid references of the postcodes where it has measurements. 2) The BGS attached a geological code to each grid reference and returned the file to the NRPB. 3) The NRPB added the radon concentration for each grid reference to the file, deleted the grid references leaving just geological codes and radon concentrations, sorted the file by geological code, and returned it to the BGS. This allowed the house radon data to be sorted by geological code without any individual results being identifiable. RESULTS AND ANALYSIS
Each set of house radon results for a particular combination of geology and superficial cover was analysed
separately. The mean UK outdoor radon concentration (4 Bq m”) was subtracted from each result, and the GM and GSD of each set were calculated. These parameters were used to estimate the fraction of homes above 200 Bq rnw3on the assumption that the distribution was lognormal. The estimated fraction could then be compared with the actual fraction found above the threshold in each data set. Table 1 shows the results for rock types which appear on both the Northampton and Wellingborough sheets, in areas without superficial cover. The measured percentage of homes above the UK Action Level is, in all cases, close to the percentage estimated using the lognormal model, showing that the data conforms well to this model. The results also show the large variation in the percentage of homes above the UK Action Level between different rocks, from 0.5% to > 20%. The GMs for particular rock types vary by up to a factor of 2.2 between the two geological survey sheets. There are no differences in housing type or age of
J. Miles and K. Ball
S782
housing between the two sheets that could account for this difference, which is therefore attributed to lateral variation in the geological formations. This demonstrates that conclusions about radon potential over individual geological formations cannot necessarily be extrapolated to adjacent areas. This is likely to be particularly true of areas with a high proportion of shelf facies sedimentary rocks. The GSDs for the different rock types vary from 2.3 to 3.0, compared with a range of 2.6 to 4.7 found when the data is grouped by 5 km grid squares instead of by geology. The smaller GSDs found when data is grouped by geology suggest that the data sets are more homogeneous when grouped in this way. The smallest GSDs were found for rock types where the GM varied by less than a factor of 2 between the two geological sheets. This suggests that much of the remaining variation between radon levels in homes when the data is grouped by geology is caused by the lateral variation in the geological formations. Analysis of the geographical variation in radon potential between houses on the same lithology may therefore produce even more detailed and accurate estimates of the fraction of homes exceeding a reference level. Table 2 shows the effects of superficial deposits on one lithoiogy, the Northampton Sand Formation on the Northampton map sheet. A superficial cover of glacial sand and gravel makes little difference to the GM or the number of homes above the Action Level. A capping of Boulder Clay, however, leads to a marked reduction in the number of homes above the Action Level. In this case, the data do not fit the lognormal model well, but could be a combination of two lognormal distributions. It is possible that this is due to variations in the thickness of the boulder clay. In some cases, the foundations of homes will break through the superficial layer, and in others, the clay will form an almost impermeable barrier. It is clearly important to take superficial deposits into account in mapping radon potential. This exercise has shown that grouping house radon data by geological formation and superficial cover can nroduce radon note&al mans which are more detailed
than grid square maps and more accurate in estimating numbers of homes affected than mapping based only on geology. However, lateral variations in geological for-
mations require further study. The type of analysis carried out here was possible only because there were sufficient results of measurements in homes for each combination of geological formation and superficial cover. In areas where there are insufficient homes to allow this analysis (either because the population density is too low or because the geology varies too rapidly), other information such as radon in soil gas may assist in estimating radon potential. This data will require careful interpretation, particularly where impermeable deposits cover permeable rocks, and may only provide a provisional estimate. work described here was partially funded by the Commission of the European Communities under Contract F13P-
Acknowledgment-The CT920064d.
REFERENCES Akerblom, G.; Andersson, P.; Clavensjo, B. Soil gas radon - A source for indoor radon daughters. Radiat. Prot. Dosim. 7: 49-54; 1984. Ball, T.K.; Cameron, D.G.; Colman, T.B. Aspects of radon potential mapping in Britain. Radiat. Prot. Dosim. 45: 211-2 14; 1992. Ball, T.K.; Miles, J.C.H. Geological and geochemical factors affecting the radon concentration in homes in Cornwall and Devon, UK. Environ. Geochem. Health 15: 27-36; 1993. Gunby, J.A.; Darby, S.C.; Miles, J.C.H.; Green, B.M.R.; Cox, D.R. Factors affecting indoor radon concentrations in the United Kingdom. Health Phys. 64: 2-12; 1993. Kendall, G.M.; Miles, J.C.H.; Cliff, K.D.; Green, B.M.R.; Muirhead, C.R.; Dixon, D.W.; Lomas, P.R.; Goodridge, S.M. Exposure to radon in UK dwellings. NRPB-R272. Chilton: National Radiological Protection Board; 1994. Miles, J.C.H.; Green, B.M.R.; Lomas, P.R. Radon affected areas: Derbyshire, Northamptonshire and Somerset. Documents of the NRPB,3,4. London: HMSO; 1992: 19-28. Miles, J.C.H. Mapping the proportion of the housing stock exceeding a radon reference level. Radiat. Prot. Dosim. 56: 207210; 1994. Reimer, G.M.; Gundersen, L.C.S. A direct correlation among indoor radon, soil gas radon and geology in the Reading Prong near Boyertown, Pennsylvania. Health Phys. 57: 155- 160; 1989.
Pergamon
PI1SO160-4120(96)001?33-3
MAPPING RADON-PRONE AREAS USING HOUSE RADON DATA AND GEOLOGICAL BOUNDARIES Jon Miles National Radiological Protection Board, Chilton, Didcot, Oxon, OX1 1 ORQ, UK
Keith Ball British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK
EI 9508-236 A4(Received 10 August 1995; accepted 5 June 1996)
The results of measurements of radon levels in homes can be grouped by grid square and analysed using a lognormal model to provide accurate estimates of the proportion of homes above a reference level, but this technique cannot provide high resolution at reasonable cost. Geological parameters can provide detailed maps of radon potential, but without assistance from measurements in homes, cannot provide accurate estimates of the levels of radon in homes. An alternative mapping technique is to group house radon measurements by geological unit. Analysis of 36 570 results in an area of 58 x 19 km showed that this technique can provide radon potential maps which are more detailed than grid square maps and more accurate in estimating numbers of homes affected than mapping based only on geology. However, lateral variations in geological formations mean that conclusions drawn for one area cannot necessarily be extrapolated to the same geological formations in adjoining areas. The results also showed that it is important to take superficial deposits into account in mapping radon potential. Published by Elsevier Science Ltd.
INTRODUCTION
The main purpose of radon potential mapping is to map the geographical variation in probability that new or existing buildings will exceed a radon reference level. This information may be used for various purposes, such as to target information campaigns encouraging measurement of radon levels in homes or to allow building regulations to be altered to prevent new buildings from having radon problems. There is a long chain of factors that influence the radon levels found in buildings. Variation in these factors produces the wide range of radon levels measured in buildings. Because these factors are largely independent and are multiplicative, the distribution of radon levels observed in buildings is usually lognormal (Gunby et al. 1993). Differences in the radon levels between buildings are often due to differences in the radon potential of the ground under them, even within small areas. But such differences in radon levels are also
due to differences in the buildings themselves (such as, house type, date of construction, and details of building techniques). This makes it difftcult to map the radon potential independently of influences of the individual buildings. MAPPING TECHNIQUES The National Radiological
Protection Board (NRPB) designated radon Affected Areas in the UK on the basis of house radon data grouped by 5-km grid squares (Miles et al. 1992). The data was interpreted in terms of lognormal distributions, with the geometric mean (GM) and geometric standard deviation (GSD) estimated for each square. Techniques for estimating these parameters where data is sparse have been developed (Miles 1994). These give accurate estimates of the proportion of homes above the UK radon Action Level at this resolution, which is appropriate for many administrative s179
S780
actions. The results can be understood in terms of the local geology (Ball and Miles 1993). If finer resolution of radon-prone areas is required, this technique can be applied to smaller areas, for instance, 1 km grid squares. However, this requires a density of measurements that is higher than the density of homes in rural areas, so the finer resolution can be obtained only in urban areas, and then at significant cost. Geology can give high-resolution maps of radonprone areas as geology has already been mapped in detail in many areas. However, if purely geological indicators such as the radium content of rocks, uranium mineralogy and permeability are used, the estimated number of homes above a threshold is likely to be inaccurate, as the relationship between these parameters and radon potential appears to vary between different geological formations. Since the radon in buildings is largely derived from the radon in soil gas under and around the building, measuring the radon concentration in soil gas provides more relevant information than uranium concentration. It has been shown (Akerblom et al. 1984; Reimer and Gundersen 1989) that for particular rock types, there is a good correlation between radon in soil gas and radon in homes. However, the relationship between these two parameters is different for different rock types, so radon in soil gas cannot be used by itself to predict radon levels in buildings. Measurements of radon levels in homes analysed by grid square can therefore be used to provide accurate estimates of the proportion of homes above a reference level, but cannot provide high resolution. Geological parameters, including the properties of individual rock types, can provide detailed maps, but without assistance from results of measurements in homes cannot provide accurate estimates of radon potential. An alternative mapping technique is to group house radon measurements by geological unit. Where enough results of house measurements are available, this has the potential to provide maps which are both detailed and accurate. Where there are too few houses to provide good data, the technique can be extended using measurements of radon in soil gas under carefully controlled conditions (Ball et al. 1992). DATA
House radon data was provided by surveys carried out by the NRPB on behalf of the Department of Environment. Radon concentrations were measured in 220 000 homes in the UK using passive radon detectors. Each house was measured using two detectors exposed over a three-month period, and the results corrected for
J. Miles and K. Ball
seasonal variations (Kendall et al. 1994). Most of the measurements were performed at the request of the householder, following the announcement of radon Affected Areas. For this exercise, house radon data from the areas of two British Geological Survey (BGS) sheets were used. The survey sheets were the adjacent Northampton and Wellingborough survey sheets, numbers 185 and 186, each covering an area 29 x 19 km. Intensive surveys of radon in homes were carried out in this area, resulting in 25 842 results for the Northampton sheet and 10 728 for the Wellingborough sheet. The exposed bedrock is entirely from the Jurassic Period, ranging from Lower Liassic mudstones through to the Oxford Clay. The rocks comprise an interbedded sequence of mudstones, limestones, and sandstones. The strike of the beds is ENE/WSW with a gentle dip towards the ESE. The older beds outcrop in the NW section with progressively younger beds outcropping towards the SE. Despite the gentle dip and topography, in detail the trace of the lithologies is complex. The bedrock is overlain by unconsolidated deposits of glacial origin comprising ground moraine (Boulder Clay) and fluvio-glacial sands and gravels. Terrace deposits and alluvium are found in the river valleys. The maps used for the geological classification show superficial cover only where it is more than 1 m thick. The thickness or composition of rock types in geological formations may vary laterally. This is particularly common in the Northampton area and the effect of the subtle changes in geology on the house radon values was one of the factors investigated in this paper. The Northampton Sand Formation is up to 21 m thick, the basal part being developed as a phosphatic ironstone which in places is overlain by massive yellow or brown sandstone. Where the sandstone is missing, the formation is commonly 4-8 m thick. The Blisworth Limestone also varies significantly in thickness across this area. The locations of the houses were identified from the postcode. The UK national grid references of the centroids of individual postcodes are commercially available. One postcode covers 15 houses on average, more in towns and fewer in country areas. The grid reference of a house can therefore be obtained with an uncertainty of about a few hundred metres or less in towns, but the uncertainty may be 1 km or more elsewhere. Because the results of measurements are confidential, it was not possible to pass details of the results from the NRPB to the BGS for identification of the underlying geology. Instead, a system was devised to
Mapping radon-prone areas
S781
Table 1. Distributions of results of radon measurements in homes on different bedrocks on two adjacent geological survey sheets. Northampton sheet
Wellingborough
sheet
No. of results
GM
GSD
% above AL
Estimated % above AL
No. of results
GM
GSD
% above AL
Estimated % above AL
Blisworth Limestone
1578
23
2.4
0.8
1.2
1996
35
2.6
3.6
4.5
Upper Estuarine Series
3048
18
2.3
0.4
0.5
1209
28
2.7
3.1
3.0
Lower Estuarine Series
1754
22
2.7
2.3
2.3
532
48
3.0
11.3
12.0
11 260
41
3.0
8.4
8.2
3023
82
3.0
21.5
24.0
20
2.5
1.9
1.8
588
22
2.4
1
Bedrock (no superficial deposits)
Northampton Formation
Sand
738
Upper Lias Clays
0.9
Table 2. The effects of superficial deposits over the Northampton sand formation on the proportion of homes above the action level (AL). Northampton sheet Northampton sand formation
Superficial deposits
No. of results
GM
GSD
% above AL
Estimated % above AL
None
11 260
41.0
3.0
8.4
8.2
73
29.0
2.9
2.7
6
636
39.0
3.1
9.4
10
Boulder clay Glacial sand and gravel
allow the geology and superficial cover of each measurement point to be identified without loss of con-
fidentiality. This process involved three stages: 1) The NRPB sent to the BGS a file of grid references of the postcodes where it has measurements. 2) The BGS attached a geological code to each grid reference and returned the file to the NRPB. 3) The NRPB added the radon concentration for each grid reference to the file, deleted the grid references leaving just geological codes and radon concentrations, sorted the file by geological code, and returned it to the BGS. This allowed the house radon data to be sorted by geological code without any individual results being identifiable. RESULTS AND ANALYSIS
Each set of house radon results for a particular combination of geology and superficial cover was analysed
separately. The mean UK outdoor radon concentration (4 Bq m”) was subtracted from each result, and the GM and GSD of each set were calculated. These parameters were used to estimate the fraction of homes above 200 Bq rnw3on the assumption that the distribution was lognormal. The estimated fraction could then be compared with the actual fraction found above the threshold in each data set. Table 1 shows the results for rock types which appear on both the Northampton and Wellingborough sheets, in areas without superficial cover. The measured percentage of homes above the UK Action Level is, in all cases, close to the percentage estimated using the lognormal model, showing that the data conforms well to this model. The results also show the large variation in the percentage of homes above the UK Action Level between different rocks, from 0.5% to > 20%. The GMs for particular rock types vary by up to a factor of 2.2 between the two geological survey sheets. There are no differences in housing type or age of
J. Miles and K. Ball
S782
housing between the two sheets that could account for this difference, which is therefore attributed to lateral variation in the geological formations. This demonstrates that conclusions about radon potential over individual geological formations cannot necessarily be extrapolated to adjacent areas. This is likely to be particularly true of areas with a high proportion of shelf facies sedimentary rocks. The GSDs for the different rock types vary from 2.3 to 3.0, compared with a range of 2.6 to 4.7 found when the data is grouped by 5 km grid squares instead of by geology. The smaller GSDs found when data is grouped by geology suggest that the data sets are more homogeneous when grouped in this way. The smallest GSDs were found for rock types where the GM varied by less than a factor of 2 between the two geological sheets. This suggests that much of the remaining variation between radon levels in homes when the data is grouped by geology is caused by the lateral variation in the geological formations. Analysis of the geographical variation in radon potential between houses on the same lithology may therefore produce even more detailed and accurate estimates of the fraction of homes exceeding a reference level. Table 2 shows the effects of superficial deposits on one lithoiogy, the Northampton Sand Formation on the Northampton map sheet. A superficial cover of glacial sand and gravel makes little difference to the GM or the number of homes above the Action Level. A capping of Boulder Clay, however, leads to a marked reduction in the number of homes above the Action Level. In this case, the data do not fit the lognormal model well, but could be a combination of two lognormal distributions. It is possible that this is due to variations in the thickness of the boulder clay. In some cases, the foundations of homes will break through the superficial layer, and in others, the clay will form an almost impermeable barrier. It is clearly important to take superficial deposits into account in mapping radon potential. This exercise has shown that grouping house radon data by geological formation and superficial cover can nroduce radon note&al mans which are more detailed
than grid square maps and more accurate in estimating numbers of homes affected than mapping based only on geology. However, lateral variations in geological for-
mations require further study. The type of analysis carried out here was possible only because there were sufficient results of measurements in homes for each combination of geological formation and superficial cover. In areas where there are insufficient homes to allow this analysis (either because the population density is too low or because the geology varies too rapidly), other information such as radon in soil gas may assist in estimating radon potential. This data will require careful interpretation, particularly where impermeable deposits cover permeable rocks, and may only provide a provisional estimate. work described here was partially funded by the Commission of the European Communities under Contract F13P-
Acknowledgment-The CT920064d.
REFERENCES Akerblom, G.; Andersson, P.; Clavensjo, B. Soil gas radon - A source for indoor radon daughters. Radiat. Prot. Dosim. 7: 49-54; 1984. Ball, T.K.; Cameron, D.G.; Colman, T.B. Aspects of radon potential mapping in Britain. Radiat. Prot. Dosim. 45: 211-2 14; 1992. Ball, T.K.; Miles, J.C.H. Geological and geochemical factors affecting the radon concentration in homes in Cornwall and Devon, UK. Environ. Geochem. Health 15: 27-36; 1993. Gunby, J.A.; Darby, S.C.; Miles, J.C.H.; Green, B.M.R.; Cox, D.R. Factors affecting indoor radon concentrations in the United Kingdom. Health Phys. 64: 2-12; 1993. Kendall, G.M.; Miles, J.C.H.; Cliff, K.D.; Green, B.M.R.; Muirhead, C.R.; Dixon, D.W.; Lomas, P.R.; Goodridge, S.M. Exposure to radon in UK dwellings. NRPB-R272. Chilton: National Radiological Protection Board; 1994. Miles, J.C.H.; Green, B.M.R.; Lomas, P.R. Radon affected areas: Derbyshire, Northamptonshire and Somerset. Documents of the NRPB,3,4. London: HMSO; 1992: 19-28. Miles, J.C.H. Mapping the proportion of the housing stock exceeding a radon reference level. Radiat. Prot. Dosim. 56: 207210; 1994. Reimer, G.M.; Gundersen, L.C.S. A direct correlation among indoor radon, soil gas radon and geology in the Reading Prong near Boyertown, Pennsylvania. Health Phys. 57: 155- 160; 1989.