Radon, the cancer-causing gas that seeps out through the ground, may be prevalent in areas previously regarded as safe, according to a British Geological Survey study.
The area includes a stretch from Lyme Bay, in the southwest, to Hull in the northeast; Bristol; and parts of Wales.
Radon is a gas produced by the radioactive decay of uranium, which is naturally present in most rocks. It disperses quickly in the air but can collect in poorly ventilated homes, where it increases the risk of lung cancer.
The traditional danger areas have been Cornwall and Devon, where most affected houses are, Northamptonshire, Derbyshire and parts of Somerset and Scotland. Granite is one of the main culprit rocks.
But survey scientists have used measurements of radon levels in homes around the country, taken by the National Radiological Protection Board, to draw up a map matching these home radon levels to the geological units of the country, rather than to an arbitrary grid system.
Geochemists Donald Appleton and Keith Ball said that they had discovered that areas of the country with carboniferous limestone can release as much radon as the granite areas of Cornwall.
Although the limestone contains less uranium it is more dispersed, so more radon reaches the atmosphere.
The new "action level" areas, where more than a tenth of homes have radon levels exceeding 200 Bequerels per cubic metre, also include the Gower peninsula in Wales and land stretching to the east and west of Merthyr Tydfil.
Current maps of radon danger areas are inaccurate, Dr Appleton said. By the time the data has been averaged, smoothed and aligned to parish boundaries a lot of the crucial information, that pinpoints one small area as high, and the next as low level has been lost. BGS and NRPB have been collaborating to improve the maps.
The study is part of a review of natural contamination in Great Britain, written by Dr Appleton, which includes studies of methane, carbon dioxide and oil seeps.
Dr Appleton said that the risk from carboniferous limestone decreases towards the north of Britain because the proportion of shale, which is safe, increases. Radon risk can also be depressed by many factors such as what is deposited on top of the rocks. Houses can be altered to protect against the gas.