Radon levels in almost one in five homes tested in the South Hams are too high, according to survey results.
The district council offered free home radon tests to 17,000 people
Radon is a naturally-occurring gas which can cause lung cancer in high concentrations.
The district council offered free tests to 17,000 people because they live in high-risk areas.
The risk threshold is 200 units, but in more than 30 cases levels were more than 1,000, and in one case almost 9,000 units were registered.
Environmental Health Officer Pete Smith said homeowners could protect themselves by getting builders in to carry out specialised work.
He said: "We are talking about a lifetime risk in the majority of cases, and as long as people remedy the situation in the next year they are cutting down the risks."
Installations designed to protect homes from radon typically cost between £500 and £1,500.
Common systems involve creating an impermeable barrier between the earth and the property and venting the gas.