In the Eastern Mediterranean and South-East Asia regions and in low-income countries in the Western Pacific region, levels of urban air pollution increased by more than 5 percent in more than two thirds of the cities.
The WHO noted that urban air pollution data remain sparse in the African region, although the available data revealed particulate matter levels above the median. The database now contains particulate matter measurements for more than twice as many cities than previous versions.
Ambient air pollution, made of high concentrations of small and fine particulate matter, is the greatest environmental risk to health -- causing more than three million premature deaths worldwide every year, the WHO said.
"Urban air pollution continues to rise at an alarming rate, wreaking havoc on human health," said Maria Neira, director of the WHO's Department of Public Health, Environmental and Social Determinants of Health.
"At the same time, awareness is rising and more cities are monitoring their air quality. When air quality improves, global respiratory and cardiovascular-related illnesses decrease."
Most sources of urban outdoor air pollution are well beyond the control of individuals and demand action by cities, as well as national and international policymakers to promote cleaner transport, more efficient energy production and waste management, the WHO said.
More than half of the monitored cities in high-income countries and more than one third in low- and middle-income countries reduced their air pollution levels by more than 5 per cent in five years.
Reducing industrial smokestack emissions, increasing use of renewable power sources such as solar and wind, and prioritizing rapid transit, walking and cycling networks in cities are among the available and affordable strategies, the agency noted.
"It is crucial for city and national governments to make urban air quality a health and development priority," said WHO official Carlos Dora.
"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," he added, referring to the Paris Agreement on climate change, which was reached in December 2015.
WHO's air quality guidelines provide global guidance on thresholds and limits for key air pollutants that pose health risks.
The guidelines indicate that by reducing particulate matter pollution from 70 to 20 micrograms per cubic metre, air pollution-related deaths could be reduced by about 15 percent.
