There are fears that hundreds more inmates and staff have been exposed to unsafe levels of cancer-causing gas, with the Ministry of Justice now investigating 42 prison and probation sites, The Independent can reveal.
HMP Dartmoor in Devon was closed in 2024 after levels of radon 10 times higher than the recommended limit were recorded in one area. The government is currently spending around £4m a year of taxpayers’ money on the empty prison because it is locked into a lease signed after high levels of the toxic gas were discovered.
At least 16 prisons have measured “above action” levels of the gas by officials, including HMP Exeter, Lincoln, Lindholme, and Channings Wood, and The Independent revealed last month that unsafe levels were recorded at 33 prison and probation sites in 2024.
Now, in response to a freedom of information (FOI) request, the HM Prison and Probation Service has admitted that this figure has risen to 42 sites.
Radon is a colourless, odourless radioactive natural gas, formed when small amounts of uranium in rocks and soils decay, and can build up in indoor areas. It is known to increase the risk of lung cancer, with more than 1,100 deaths from the disease attributed to radon each year in the UK, according to the UK Health Security Agency (UKHSA).
Law firm Kesar and Co is leading a legal action by former prisoners and prison staff over their potentially harmful exposure to radon at HMP Dartmoor. Solicitor Mladen Kesar said the firm now has instructions from people who believe they have been affected by radon at six prisons in total; HMP Exeter, Channings Wood, Lindholme, Portland, The Verne and Dartmoor.
Documents disclosed under FOI laws show that high levels of radon were measured at HMP Exeter in 2020, with parts of D wing measuring over nine times the legal limit. Further measurements from the end of 2024, into 2025, show levels at more than 900 bq/m³ in some parts of the prison.
By law, mitigation measures must be put in place to reduce radon exposure in workplaces if levels are shown to exceed 300 bq/m³, averaged annually. For sleeping areas, such as prison cells, this is lowered to 200 bq/m³, according to UKHSA guidance.
Mr Kesar said: “In Exeter, there is evidence that some areas have very high levels of radon, and yet the prison has not been evacuated. It’s fully operational as if nothing is happening. The same with the others. There is no sense of urgency or responsibility.
“Dartmoor has been evacuated, but in these other 41 sites, people are potentially at risk. If a deadly virus was discovered there, you would think they would just empty the prison and get everyone safe, but with radon, somehow, they are not really in a rush despite it being the second biggest cause of lung cancer.”
Gemma Abbott, legal director at the Howard League for Penal Reform, said: “When the state holds people in custody, it has a legal – and moral – imperative to ensure that they are safe.
“Radon is known to increase the risk of lung cancer and the Ministry of Justice has shown a worrying lack of urgency in addressing the risks, just as it has failed to fix the tens of thousands of prison places that do not meet lawful fire safety standards.
“Over the last seven months, we have made repeated requests for copies of detailed risk assessments for prisons which have higher radon readings than the law allows, but so far the government has refused to provide them.
“Being locked up for up to 23 hours in an unsafe cell must be terrifying. What do ministers have to hide?"
A Ministry of Justice spokesperson said: “We routinely monitor levels of naturally occurring radon gas across the HMPPS estate and take action to mitigate risks wherever elevated levels are identified.”
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