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A UN report will urge an independent investigation into the potential health impacts of the UK's largest opencast coalmine, BBC Wales has learnt.
Residents living alongside Ffos-y-Fran near Merthyr Tydfil have led a long campaign, alleging that they are affected by air and noise pollution.
The UN's special rapporteur on hazardous substances and wastes Baskut Tuncak met campaigners in Merthyr Tydfil as part of an official visit to the UK in January. He has said that the case raised "many concerns" about the UK's approach to regulation.
His role is to assess and advise governments about their efforts to protect the human rights of communities at risk of pollution, such as the rights to life, health and adequate housing.
In an interview with BBC Wales, he said the plight of the community surrounding Ffos-y-Fran was "top of the list" in terms of the "many pressing issues" he had encountered.
"The first observation that came to mind was how incredibly close this community is to a massive open pit coalmine," Mr Tuncak said.
"I heard allegations of very high rates of childhood asthma and cancer clusters within the community. But despite those allegations I didn't hear any evidence of a strong intervention by the government to investigate or any strong reaction by the companies concerned to investigate themselves."
His official report will be handed to the UN's Human Rights Council in Geneva in September. The UK government said it will respond after this happens.
Work to reclaim more than 11 million tonnes of coal over the course of 17 years started at the Ffos-y-Fran site in 2007, despite numerous court battles, a public inquiry, petitions and protests. The closest houses are less than 40m (132ft) away from the site, which is the size of some 400 football pitches.
Residents were told that modern mining methods would mean they would not be affected by pollution, while the operation would restore 1,000 acres of land which was once riddled with old mine shafts and was used to dump spoil.
In the meantime, the Welsh Government has imposed a 500m buffer-zone between any new opencast mines in Wales and the communities around them but this doesn't apply retrospectively to Ffos y Fran.
Terry Evans, chairman of local campaign group Residents Against Ffos y Fran, lives closest to the site - 37m from the boundary - and says he is affected by "absolutely phenomenal amounts of dust".
"On a fine day when the wind is blowing from an easterly direction you just see it coming over us - it's frightening."
In previous years he said the mine's operator sent window cleaners and gave out "envelopes of money to get our houses clean".
Local campaigners Chris and Alyson Austin of Merthyr Tydfil Friends of the Earth, who live a few hundred yards away, talk of having their lives ruined. He said;
"I find it difficult to put the washing out on the clothes line or enjoy my garden. On hot days we can't even open the windows. And all that dust goes straight into our lungs," Mr Austin added, "with the elderly and children particularly affected."
"We've explored every avenue, every opportunity that was available to us,"
"You go to the MP - it's not a Westminster issue, you go to the AM - they say it's for the local authority, you go to the local authority and they say speak to the mining company. It's 'send the fool further' all the time."
They claim their complaints have not been listened to and calls for an investigation into the health implications dismissed.
Five hundred locals attempted to take court action as a group, but their application was refused by the High Court as they were deemed unable to afford it.
Ms Austin is now attempting to take a case to the European Court of Human Rights.
She said Mr Tuncak had "hit the nail right on the head".
"We've spent so much time dealing with the council, dealing with the company, dealing with Welsh Government and nobody is interested in resolving this issue. There are times when we've just sat here in tears."
Mr Tuncak said he found the way the community's concerns had been handled by the authorities to be inadequate, with "various layers of government shifting responsibility".
"I heard some dismissal of certain allegations as being more linked to lifestyle choices or other factors among this working class community, but what I didn't see was a solid investigation by the authorities to look into these claims.
"Ultimately it falls to the UK central government," he said.
Mr Tuncak explained that while Welsh ministers in Cardiff Bay had devolved authority over environmental issues such as air pollution, "they often don't have the resources to effectively monitor and enforce compliance".
"And even though they themselves have transferred certain authorities to local councils, that is also not often followed with commensurate financial resources to ensure adequate enforcement that the community has a right to expect."
The result for people living near Ffos-y-Fran had been "a decade of frustration", as they "tried to engage with processes and have access to justice".
Haf Elgar, acting director of Friends of the Earth Cymru said the case had highlighted a "flawed system" of environmental regulation in the UK and wider issues around "lack of access to justice for communities like those living in Merthyr".
She said an independent investigation into the specific circumstances surrounding Ffos y Fran should be initiated by the UK Government "as soon as possible".
"The local community has been blighted by noise and dust from Ffos y Fran opencast coal site for ten years now. It’s a disgrace that this hasn’t been resolved already and that it’s taken a UN visit to draw attention to the problems they face.
"We agree with the UN Special Rapporteur that an urgent independent inquiry is needed – the residents have not been able to get the quality of life they deserve through any other means. And this is more than a local issue, it raises concerns about access to justice for communities faced with environmental problems in the UK.
"Ultimately the only way to stop this from happening again is an end to opencast coal in Wales and the pollution it inevitably causes.”
Mr Tuncak's official report is due to be presented to the UN's Human Rights Council in Geneva in September. His 15-day fact-finding mission to the UK was at the invitation of the Westminster government.
The Welsh Government said it supported councils to undertake regular air quality assessments and focus their monitoring and actions in those areas thought to be at the highest risk of non-compliance. It said it would respond to a consultation on air quality shortly.