Michael Sheen toxic waste documentary the tip of the iceberg

John Browne, a lawyer specialising in industrial disease claims at Slater and Gordon, said cases involving historical contamination were difficult to investigate – but not impossible.

“Time passing makes cases more difficult to prove,” he said.

“Witnesses’ memories fade and locating [eye]witnesses becomes increasingly challenging and difficult.”

However, he said scientific, medical and forensic evidence could help establish what happened and whether exposure to hazardous substances contributed to illness.

The key question is not simply what the law was at the time, but what was known about the risks, he said, adding: “Communities affected by contamination deserve transparency and clear answers.”

Public Health Wales said its Welsh cancer intelligence and surveillance unit had collected and analysed “every single case of cancer” since the 1970s.

It added: “Based on the evidence currently available, there is no indication that historical environmental pollution is responsible for an increased risk of cancer, or any other disease or illness, in these areas.”

Natural Resources Wales said the Environment Agency, its predecessor, investigated the acid tar lake in 2006 and 2008 but tests concluded it did not meet the legal definition of contaminated land.

“Therefore, responsibility for the site returned and remains with Wrexham County Council as regulator.”

Wrexham council said, since these investigations 20 years ago, ongoing monitoring had not identified any significant changes and there was no statutory requirement for remediation.

The Welsh government said addressing contaminated land was one of its key priorities.

“A survey of local authorities carried out by Natural Resources Wales in 2015 identified more than 9,000 potential sites requiring further investigation.

“While not all of these sites will pose a risk to human health or the environment, this figure demonstrates the scale of the issue facing regulators in Wales.

“We now intend to develop a Wales-wide strategy to progressively restore post-industrial land, so that communities are no longer exposed to potential risks.”

The UK government referred the BBC to the Welsh government when asked to comment.