Urban residents in poor countries are by far the worst affected, WHO said, noting that nearly every city (98 percent) in low- and middle-income countries has air which fails to meet the UN body’s standards.
That number falls to 56 percent of cities in wealthier countries.
“Urban air pollution continues to rise at an alarming rate, wreaking havoc on human health,” Maria Neira, the head of WHO’s department of public health and environment, said in a statement.
The UN agency’s latest air pollution database reveals an overall deterioration of air in the planet’s cities, and highlights the growing risk of serious health conditions also including stroke and asthma.
The report, which focused on outdoor rather than household air, compared data collected from 795 cities in 67 countries between 2008 and 2013.
Tracking the prevalence of harmful pollutants like sulfate and black carbon, WHO found that air quality was generally improving in richer regions like Europe and North America, but worsening in developing regions, notably the Middle East and southeast Asia.
Overall, contaminants in outdoor air caused more than 3 million premature deaths a year, the UN body said.
The quality of air pollution data provided by individual countries varies considerably, and WHO does not compile a ranking of the world’s most polluted cities.
But, in a sample of selected mega-cities with a population above 14 million, New Delhi was the most polluted, followed by Cairo and Bangladesh’s capital Dhaka.
Crucially, key African centres like Nigeria’s mega-city Lagos were excluded from the list because of the sparse availability of air quality data in many parts of the continent, WHO said.
A sample of European data showed that Rome had slightly worse air than Berlin, followed by London and Madrid.
Carlos Dora, coordinator at WHO’s public health and environment department, pointed to several key factors that determine the quality of a city’s air.
First was transportation, Dora said, noting that cities which succeed in reducing vehicle traffic while promoting walking, cycling and mass public transport inevitably see their air quality improve.
Energy inefficiency — especially with respect to heating and cooling buildings — is a major cause of dirty air, along with the widespread use of diesel generators as a replacement for cleaner electricity sources, Dora added.
Another crucial factor, especially in developing countries, is waste management, with the smoke generated by burning garbage ranking among the top pollutants.