A number of inmates have been moved from Dartmoor Prison in Devon after "higher-than-normal" levels of radon gas were detected.
The Prison Service said prisoners from the jail had been relocated. HMP Dartmoor has had to close down more than 180 cells and carry out prisoner transfers after the radioactive gas radon was detected.
Radon is a colourless and odourless radioactive gas formed by decaying uranium found in rocks and soils.
Prolonged exposure can cause illness and regular testing is conducted at buildings like Dartmoor Prison.
Radon, an odourless and colourless gas, is produced by decaying radioactive materials in rocks and soils, and is responsible for around 1,000 lung cancer deaths a year in the UK. It occurs more often in areas with high concentrations of granite, such as Dartmoor.
A Prison Service spokesperson said: "A number of prisoners have been relocated as a precautionary measure after routine testing revealed higher-than-normal levels of radon.
"This is a temporary measure while work to permanently reduce radon levels is completed and there are no safety implications to staff or prisoners who remain on site."