§ Mr. Waldegrave
The largest source of exposure to naturally occuring radioactivity in England and Wales is from the build-up of radon gas in houses. For the country as a whole, the mean annual dose is 1 milliSievert per year (mSv/y). Higher than average doses arise because of geological factors, particularly the natural uranium content of the ground, and the permeability of the rock and soil to radon. The National Radiological Protection Board has been carrying out national and regional studies to identify houses where radon gives rise to high annual doses of radioactivity. The results are not necessarily representative of the housing stock as a whole in the area, as they represented a deliberate attempt to identify houses with high radon concentrations. But the areas identified by the NRPB with the highest mean indoor exposure to radon products are:
Location and County Grid Square Number of dwellings surveyed Mean annual dose (mSv/y) Eastern part of Sedgemoor, Somerset ST 43 1 56 North-eastern edge of Dartmoor, Devon SX 88 4 41 Coast area of Mount's Bay, Cornwall SW 52 2 40 North-eastern edge of Dartmoor, Devon SX 78 11 38 Coas area near Tintagel Head, Cornwall SX 09 1 37 West of St. Austell, Cornwall SW 95 21 35 Northern part of Dartmoor, Devon SX 68 1 31 Northern edge of Dartmoor, Devon SX 79 2 28 West of Truro, Cornwall SW 74 6 27 South of Camborne, Cornwall SW 63 13 27 Eastern edge of Dartmoor, Devon SX 77 11 25
Location and County Grid Square Number of dwellings surveyed Mean annual dose (mSv/y) Northen part of Bodmin Moor, Cornwall SX 18 6 23 Southern edge of Dartmoor, Devon SX 65 10 22 Central part of Dartmoor, Devon SX 67 6 22 South-western edge of Dartmoor, Devon and Cornwall SX 46 2 21 Northern edge of Dartmoor, Devon SX 69 5 20 Coast area north of St. Agnes Head, Devon SW 75 14 19 Eastern edge of Dartmoor, Devon and Cornwall SX 47 6 18 North of Penzance, Cornwall SW 43 6 17 South-eastern edge of Dartmoor, Devon SX 76 10 16
The fewer the number of dwellings surveyed, the less reliance should be given to the result.
Radon gives rise to doses of alpha radiation. The presence in the ground of minerals with radioactive content can also give rise to naturally occurring gamma radiation, but the doses received are much lower: the highest in England and Wales being less than 1 mSv/y.