Air pollution in cities has reached toxic levels and could cost governments 'enormous' amounts, according to the World Health Organisation.
The warning comes ahead of a new report detailing the amount of deaths caused by poor air quality to be released next month.
Dr Maria Neira, the WHO's head of public health, said the world is confronting one of the "biggest public health issues" it has faced.
Exposure to air pollution has now been linked to cardiovascular disease, she said, as well a catalogue of other illnesses.
It comes after figures released last year by the WHO suggested that seven million, or one in eight, premature deaths were linked to air pollution. Dr Neira called on governments to take responsibility for tackling the deadly threat
The WHO's conclusions are based on data collected on 2,000 world cities, showing many populations are exposed to levels of air pollution exceeding health standards.
Figures recently revealed that air pollution limits for the whole of 2016 had already been breached in London.