A prison has had to close down more than 180 cells after the radioactive gas radon was detected.
The leak at HMP Dartmoor is understood to have led to the transfer of 194 cons.
Odourless and colourless, Radon comes from decaying uranium in rocks and soils and causes around 1,000 lung cancer deaths a year.
It is thought to occur more often in areas high in granite, such as Dartmoor, Devon.
Labour revealed the figures. Shadow prisons minister Ruth Cadbury said the gas was first detected in 2020. She added: “Yet measures weren’t put in place until 2022. Ministers need to explain why it has taken over two years for them to act.”
'I ventured into Alcatraz after dark and was terrified by what I saw and heard'Justice minister Edward Argar insisted the Tories are “investing in the prison estate to ensure jails remain safe”.
The Prison Service said: “A number of prisoners were relocated as a precautionary measure after routine testing revealed higher than normal levels of radon.”