On 13 May 2026, the Health and Safety Executive (HSE) formally authorised a Crown Censure against HM Prison and Probation Service (HMPPS) after an investigation found that staff and prisoners at HMP Dartmoor had been exposed to dangerous levels of radon gas for a prolonged period. The censure represents one of the most significant regulatory findings ever made against the prison service in England and Wales, and comes as hundreds of former staff and inmates pursue legal action over resulting health conditions, including lung cancer.

What Is Radon Gas and Why Is It Dangerous?

Radon is a naturally occurring radioactive gas that forms from the decay of uranium in soil and rock. It is colourless and odourless, meaning it cannot be detected without specialist equipment. When radon accumulates in enclosed spaces, it becomes a serious health hazard. Breathing in radon over time is the second leading cause of lung cancer in the United Kingdom, responsible for more than 1,000 deaths per year. Unlike other environmental risks, exposure to radon typically only becomes apparent years or decades later, when a diagnosis of lung cancer is made.

All buildings carry some level of radon exposure risk, but the Health and Safety Executive sets action levels above which employers must take steps to reduce exposure. In a workplace context, including a prison, exceeding those levels constitutes a breach of health and safety law. The consequences for prisoners, who have no ability to leave the building and no choice over where they are housed, are particularly acute.

Radon at HMP Dartmoor: What Happened?

HMP Dartmoor, a Victorian-era prison on the edge of Dartmoor in Devon, was closed to inmates in August 2024 after monitoring revealed radon concentrations far in excess of the legal limits. In some parts of the prison, radon levels were found to be 14 to 15 times above the permitted threshold. Evidence disclosed during the HSE investigation showed that elevated readings had been recorded as far back as 2020, with further abnormal levels identified again in 2023.

The failure to act promptly on the 2020 data is at the heart of the HSE’s findings. During the period when readings exceeded legal limits, prison officers were working full shifts inside the affected areas, and prisoners were held in the building around the clock. Over 680 inmates were transferred to other facilities when the prison finally closed in August 2024. At the time of writing, HMP Dartmoor remains empty, with the Ministry of Justice paying approximately four million pounds per year to maintain the unused site while remediation work is investigated.

For information on which prisons inmates from HMP Dartmoor may have been transferred to, and how family members can stay in touch during a transfer, see our guide on how to find a prisoner in the UK.

The HSE Crown Censure Explained

On 13 May 2026, the HSE announced that it had authorised a Crown Censure against HMPPS. A Crown Censure is a formal regulatory finding used when the evidence would justify a criminal prosecution but a government body, as a Crown organisation, cannot be prosecuted under criminal law in the usual sense. It carries no financial penalty but represents a serious official censure on the public record, equivalent in severity to a criminal conviction.

The HSE’s investigation concluded that prisoners and staff had been exposed to radon significantly above the specified dose limits for a prolonged period of time. The regulator determined that HMPPS had failed in its duty of care as an employer and had not acted with appropriate urgency when the scale of the problem first became apparent. The Crown Censure was accepted by HMPPS without challenge.

HSE Chair Sarah Newton described the findings as deeply troubling and called for urgent action to ensure the same failures were not repeated at other affected sites. The HSE has since confirmed that 42 prison sites across England and Wales have recorded radon readings above action levels, though HMP Dartmoor remains the most severely affected.

Wider Impact Across UK Prisons

The Dartmoor case has drawn attention to a much wider problem across the prison estate. HM Prison and Probation Service has confirmed that 42 sites have recorded unsafe radon concentrations, including HMP Bedford, HMP Exeter, HMP Leicester, HMP Lincoln, HMP Stafford, and HMP Portland, among others. The extent of the problem has raised concerns among prison officers’ unions, who argue that staff have been routinely exposed to a known carcinogen without adequate protection or warning.

Radon monitoring in workplaces is a legal requirement under the Ionising Radiations Regulations 2017. Critics argue that the prison service was slow to implement these requirements consistently across its ageing estate, many parts of which date back to the Victorian era and are built on granite or other radon-prone geology. The situation at HMP Dartmoor, which sits on Dartmoor granite, was considered predictable by independent experts.

If you have a family member currently held in a UK prison and want to understand their rights regarding health and safety, our guide on how to contact a prisoner explains the communication options available while legal and welfare concerns are addressed.

Legal Action and Compensation Claims

Around 500 former prisoners and staff have signed up to a group legal action against the Ministry of Justice, seeking compensation for health conditions they believe were caused or worsened by radon exposure at HMP Dartmoor and other affected prisons. The claims include cases of lung cancer, other respiratory conditions, and general health deterioration attributable to prolonged radioactive exposure.

Legal representatives for the claimants have argued that HMPPS knew about elevated radon levels well before the 2024 closure and failed to act. The HSE Crown Censure issued in May 2026 is expected to strengthen those civil claims significantly, as it establishes on the public record that the regulatory threshold was breached and that HMPPS bears institutional responsibility for the exposure.

Solicitors handling the group action have indicated that individual compensation awards, if successful, could range from tens of thousands to several hundred thousand pounds per claimant, depending on the severity of the resulting health condition. The litigation is expected to continue for several years.

Frequently Asked Questions

What is the HSE Crown Censure against HMPPS for?

The HSE Crown Censure was issued on 13 May 2026 because HM Prison and Probation Service exposed staff and prisoners at HMP Dartmoor to radon gas at levels significantly above the legal dose limits for a prolonged period of time without taking prompt remedial action.

Is HMP Dartmoor still open?

No. HMP Dartmoor has been closed to prisoners since August 2024 following the discovery of radon concentrations up to 15 times above the legal limit. As of May 2026, the prison remains empty while the Ministry of Justice investigates remediation options. The government continues to pay approximately four million pounds per year to maintain the unused site.

How many UK prisons have unsafe radon levels?

HM Prison and Probation Service has confirmed that 42 prison sites across England and Wales have recorded radon readings above the action level set by the Health and Safety Executive. HMP Dartmoor is the most severely affected site and the only one that has been closed as a result of radon contamination.

Can prisoners and staff claim compensation for radon exposure?

Yes. Around 500 former prisoners and staff have joined a group legal action against the Ministry of Justice seeking compensation for health conditions linked to radon exposure at HMP Dartmoor. The HSE Crown Censure issued in May 2026 is expected to support those civil claims by confirming that safety thresholds were breached.

What is a Crown Censure?

A Crown Censure is a formal regulatory sanction issued by the Health and Safety Executive when a government body or Crown organisation has breached health and safety law in a way that would normally result in criminal prosecution. Because Crown bodies cannot ordinarily be prosecuted in the criminal courts, the censure is the equivalent mechanism and forms an official record of the finding.