The National Radiological Protection Board's new Radon Atlas pinpoints areas at risk in one kilometre (0.062 mile) squares.

In some parts of Devon and Cornwall, more than 30% of homes have readings above the level where action is recommended.

More than 15,000 properties have been found to be above this level in Cornwall, with 5,500 in Devon.

Lung cancer

North and south-east Cornwall are particularly affected, with Gunnislake, Liskeard, Looe, Callington and Camelford among the places highlighted.

In Devon, the west of the county and the Dartmoor area have the worst problems.

Health studies from around the world link exposure to radon and an increased likelihood of lung cancer, especially for smokers.

The gas enters buildings from the ground, exposing occupants to radiation.

Small amounts of radon are present in all buildings and the atmosphere outdoors.

In houses where radon is above the action level, owners are recommended to take protective measures, costing between £500 and £1,000.

The most common measure involves installing an air pump in a property to vent the gas into the atmosphere.