The United Nations warns that unchecked urban air pollution is devastating human health around the world with city dwellers in low- and middle-income countries suffering the most from it.

The newest World Health Organization (WHO) air pollution database shows urban air pollution continuing to rise at an alarming rate globally. It found that 98 percent of urban residents in low- and middle-income countries with over 100,000 inhabitants are breathing in air that fails to meet WHO safety standards. That percentage drops to 56 percent in cities in wealthier countries, however.

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Overall, some 80 percent of the city dwellers worldwide breathe poor quality air, thereby increasing their risk of contracting life-threatening diseases, according to WHO. And as urban air quality declines, the risk of stroke, heart disease, lung cancer, and chronic and acute respiratory diseases, including asthma, rises for the people who live in them.

"Urban air pollution continues to rise at an alarming rate, wreaking havoc on human health," said Maria Neira, head of WHO's department of public health and environment.

The database reveals an overall deterioration of air in the cities, and highlights the growing risk of serious health conditions. It focused on outdoor rather than household air and compared data collected from 795 cities in 67 countries between 2008 and 2013.

WHO found air quality was worsening in developing regions -- notably the Middle East and Southeast Asia. Air quality, however, was generally improving in richer regions like Europe and North America.

The report said contaminants in outdoor air cause more than three million premature deaths a year.

A look at air quality in selected mega-cities with a population above 14 million reveals New Delhi as the most polluted, followed by Cairo and Dhaka, capital of Bangladesh.

Among richer countries, the air quality in Rome was slightly worse than in Berlin, followed by London and Madrid.

WHO noted several factors determining urban air quality. The first of these is transportation. WHO said cities that succeed in reducing vehicle traffic while promoting walking, cycling and mass public transport inevitably see their air quality improve.

The second factor is energy inefficiency. This is especially true with respect to heating and cooling buildings. WHO said energy inefficiency is a major cause of dirty air, which in turn is caused by the widespread use of diesel generators.

The third is waste management with smoke generated by burning garbage ranking among the top pollutants. This problem is prevalent in developing countries.

Tagsurban air pollution, United Nations, World Health Organization, WHO, New Delhi