Devon residents are to get advice on how to protect themselves from radon gas after the radioactive material was above recommended levels in 850 houses.
A series of road shows will offer advice on reducing radon levels
A series of roadshows in the South Hams area this week will outline what action to take if the gas level is high.
Radon is a naturally occurring odorous gas which comes from the decay of uranium found in most soils. High concentrations can cause lung cancer.
The council says out of 4,700 tested homes a fifth were above action levels.
The indoor level of radon set by the National Radiological Protection Board (NRPB) is 250 Becquerels.
The government recommends action is taken to reduce levels over this amount.
Cathie Pannell's house in South Brent is one of those tested which recorded a level of 4000 Becquerels.
Ms Pannell is advising everyone to have the test done so they know what their levels are.
She said: "I know that literally a house next door can be totally different and that just because my house happens to have a high reading, doesn't mean to say that my neighbours are necessarily the same."
The road shows will demonstrate how levels can be reduced, including improved ventilation, sealing cracks and gaps in solid floors and installing a radon sump system.
Sump installations create an impermeable barrier between the earth and the property and venting the gas and cost between £500 and £1,500.