Air pollution in cities has reached such toxic levels that the world is confronting one of the "biggest public health issues" it has faced, the World Health Organisation (WHO) says.
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 crisis would cost governments "enormous" amounts globally.
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.
She called on governments to take responsibility for tackling the deadly threat by ensuring they become more eco-friendly.
"This is one of the biggest public health issues we have ever confronted," she told the Press Association.
"It is an enormous cost not only in terms of mortality, but in terms of treating diseases and the costs of hospitalisation - as most of these diseases are chronic.
"It will also lead to less working days and a lower quality of life."
Her conclusions are based on data which has been collected on 2,000 world cities, showing many populations are exposed to levels of air pollution exceeding the WHO standards.
Figures recently revealed that air pollution limits for the whole of 2016 had already been breached in London.
Last week, the viability of the Heathrow Airport expansion was scrutinised as Prime Minister David Cameron said "the question about air quality" had to be answered before the matter could progress.
Dr Neira added that improved public transport systems, a greater number of energy-efficient houses and a commitment to renewable energy could mitigate the crippling effects of poor air quality.
"There is also a role to be played on an individual level, like choosing not to take the car," she added.
"I think it is a societal decision, but it is important that, as well as the Government stepping in, citizens are also informed."
An Environment Department spokesman said: "Tackling air pollution is a priority for this Government. Last month we published plans that clearly set out how we will improve the UK's air quality through a new programme of Clean Air Zones, which alongside national action and continued investment in clean technologies will create cleaner, healthier air for all."