More than half of the world’s population is exposed to increasing air pollution despite global efforts to tackle toxic fumes, a study has found.

In some regions pollution has soared five times above safe levels set by the World Health Organisation (WHO), according to researchers at the University of Exeter.

They warned poor air quality posed a “major, and in many areas growing, threat to public health".

Professor Gavin Shaddick, the university’s chair of data sciences and statistics, said: "While long-term policies to reduce air pollution have been shown to be effective in many regions, notably in Europe and the United States, there are still regions that have dangerously high levels of air pollution, some as much as five times greater than World Health Organisation guidelines, and in some countries air pollution is still increasing."

The study, carried out with WHO, found the highest levels of air pollution in middle-income nations, particularly in central and southeastern Asia and sub-Saharan Africa.

Inefficient energy use in households, industry, agriculture and transport, as well as coal-fired power plants, are major sources of fine particulate matter air pollution. In some regions, sand and desert dust, waste burning and deforestation also contribute.

For the study, the research team examined trends in global air quality between 2010 and 2016 against a backdrop of global efforts to reduce pollution through short- and long-term policies.

They used ground monitoring together with satellite data to provide yearly, air-quality profiles for individual countries and wider regions.

Professor Shaddick added: "Although precise quantification of the outcomes of specific policies is difficult, coupling the evidence for effective interventions with global, regional and local trends in air pollution can provide essential information for the evidence base that is key in informing and monitoring future policies."

WHO estimates 4.2 million deaths a year across the world can be attributed to outdoor air pollution.

It advises that annual mean concentration of fine particulate air pollution should not exceed 10 micrograms per cubic metre.

The proportion of the global population exposed to levels above that guideline fell from 94 to 90 per cent between 2010 and 2016, driven largely by decreases in North America and Europe.

But the Exeter study, published in the journal Climate and Atmospheric Science, found levels “remained virtually constant and extremely high” in other regions.

Fifty-five per cent of the global population is exposed to increasing levels of pollution, the research found.

More than 99 per cent of the population in central and southeastern Asian nations is breathing unsafe air.

This pollution is not confined to cities, with the vast majority of people living in rural areas also exposed to dangerous levels.

“Addressing air pollution in both rural and urban settings should therefore be a key priority in effectively reducing the burden of disease associated with air pollution,” the study said.

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