Port Talbot is listed as the most polluted area in the UK, according to a new report.
It suggested dozens of towns and cities suffer from unsafe levels of air pollution and urged the UK Government has been urged to take action.
Thirty-two areas in Britain are exceeding limits of PM2.5 deemed safe by the World Health Organisation (WHO).
The tiny particles, which come from sources like transport, industry, coal plants and burning wood, fuels or waste, are linked to diseases including stroke, heart disease, lung cancer and disease, and respiratory infections.
Levels of PM2.5 particles in the air in Port Talbot were recorded as 18 micrograms per cubic metre, nearly double the WHO recommended limit of 10.
But Cardiff, Chepstow, Newport and Swansea are also failing, or close to failing, WHO standards.
The report added that seven million people a year are dying due to poor air quality, and nine out of 10 people worldwide are exposed to levels of air pollution that are dangerous to their health.
Clean air campaigners called for the UK Government, which has faced legal action over its failure to meet legal targets on another air pollutant nitrogen dioxide, to take steps to stop people breathing dirty air.