The steel town of Port Talbot has been named as the most polluted place in the country.
It comes as data from the World Health Organisation (WHO) showed that 47 towns and cities across the country matched or exceeded limits of air pollution, with the Welsh town coming top of the pile.
Researchers looked at fine particle emissions – called PM 2.5 – which travel deeply into people’s respiratory systems and can lead to health problems.
Many of the UK's main cities exceed the WHO limit of 10 micrograms per cubic metre, including London, Manchester, Liverpool and Nottingham.
Port Talbot, home of the Port Talbot steelworks, recorded 18 micrograms per cubic metre in the last recorded data from 2015, the same as Marseille in France, Singapore and the Ecuadorian capital Quito, and higher than Belgian capital Brussels (16).
How polluted are the world's biggest cities?
Beijing, China - 73 (in 2016)
Buenos Aires, Argentina - 12 (in 2016)
Kampala, Uganda - 104 (in 2013)
Los Angeles, United States - 13 (in 2015)
Madrid, Spain - 11 (in 2015)
Moscow, Russia - 20 (in 2009)
New York, United States - 8 (in 2015)
Paris, France - 14 (in 2015)
Sydney, Australia - 8 (in 2016)
Tokyo, Japan - 17 (in 2014)
The figures show varying levels of pollution in Port Talbot over time, with the figure at 16 in 2013, down to 10 in 2014 and on the rise again in 2015.
Scunthorpe and Salford recorded the second-worst levels of pollution in the UK, with 15 micrograms per cubic metre recorded, followed by Gibraltar and Thurrock on 14.
Manchester and Swansea were on 13, and cities including Leeds, Leicester, Liverpool, Plymouth, Sheffield and York recorded 12, one above London which showed 11 micrograms per cubic metre in 2015.
This figure represents a drop from 17 in 2013, while in Carlisle the levels have gone up from eight in 2013 to 12 in 2015.
Cities on the WHO limit of 10 include Birmingham, Brighton, Bristol, Newcastle, Portsmouth and Southampton.
The most polluted city in 2015 according to the WHO data is Muzaffarpur in India, with a figure of 197 micrograms per cubic metre, although this figure is under revision.
Below that is Pasakha in Bhutan (150), Delhi in India (123) and greater Cairo in Egypt (117).
UK towns exceeding WHO pollution limit
Port Talbot: 18 micrograms per cubic metre
Royal Leamington Spa: 12
UK towns level with WHO pollution limit