Air pollution has grown by 8 percent worldwide over the past five years. Billions of people are now exposed to dangerous air, which negatively affects human health and causes millions of premature deaths per year.

While most global regions are affected, populations residing in large cities in poor countries appear to be the worst hit, data released by the World Health Organization (WHO) on Thursday revealed.

WHO's third Global Urban Ambient Air Pollution Database, which looked at the outdoor air in 3,000 cities, villages and towns across 103 countries shows that fast growing cities in Southeast Asia, Middle East and the Western Pacific felt the worst impact of air pollution with many of these cities having pollution levels between five to 10 times above the recommended levels.

The data showed fast deterioration of air quality as cities in low-income countries grow unchecked and smog and soot from construction sites, transport, industry, farming and wood-burning in home exacerbates air conditions.

Air pollutants such as black carbon, nitrates and sulphates penetrate into the body's cardiovascular and respiratory systems posing risks to human health.

WHO Assistant Director General Flavia Bustreo said that as the quality of urban air declines, risks for stroke, lung cancer, heart disease, asthma and other respiratory diseases increase for people who live in the cities.

A study published in the journal Nature in September last year showed that outdoor air pollution is linked with 3.3 million premature deaths per year, which is more than those caused by AIDS and malaria making it the world's biggest single killer.

Once dirty air covers cities, the most vulnerable urban populations, the poorest, youngest and oldest, are the most impacted. The number of deaths associated with dangerous air is likewise anticipated to increase as the number of urban dwellers rise and car numbers would approach 2 billion by the year 2050.

Health experts said that renewable power sources, cutting industrial emissions as well as rapid transit, cycling and walking networks in cities can help curb rising air pollution.

"It is crucial for city and national governments to make urban air quality a health and development priority," said Carlos Dora, from WHO.

"When air quality improves, health costs from air pollution-related diseases shrink, worker productivity expands and life expectancy grows. Reducing air pollution also brings an added climate bonus, which can become a part of countries' commitments to the climate treaty."

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