While all regions of the world are affected, populations in low-income cities are the most impacted: 98 percent of cities in low- and middle-income countries with more than 100,000 inhabitants do not meet WHO air quality guidelines, Stephane Dujarric said at a daily news briefing here.

In high-income countries, that percentage drops to 56 percent, showed the latest global urban ambient air pollution database presented Thursday by the UN agency.

WHO says tens of thousands of urban dwellers in lower income countries exposed to low air quality

In the past two years, the database, which now covers 3,000 cities in 103 countries, has nearly doubled, with more cities measuring air pollution levels and recognizing the associated health impacts.

Ambient air pollution is the greatest environmental risk to health, causing more than three million premature deaths worldwide every year, the spokesman said.

"Air pollution is a major cause of disease and death. It is good news that more cities are stepping up to monitor air quality, so when they take actions to improve it they have a benchmark," said Flavia Bustreo, WHO assistant director general for family, women's and children's health.

"When dirty air blankets our cities the most vulnerable urban populations -- the youngest, oldest and poorest -- are the most impacted."

As urban air quality declines, the risk of stroke, heart disease, lung cancer, and chronic and acute respiratory diseases, including asthma, increases for the people who live in them, WHO said.

During the five-year period from 2008 to 2013, WHO compared 795 cities in 67 countries for levels of small and fine particulate matter PM10 and PM2.5 -- particles smaller than 10 or 2.5 microns.

This included pollutants such as sulfates, nitrates and black carbon, which penetrate deep into the lungs and into the cardiovascular system, posing the greatest risks to human health. Data were then analyzed to develop regional trends.

Among the key trends from the period include that global urban air pollution levels increased by 8 percent, despite improvements in some regions, according to the agency.

In general, urban air pollution levels were lowest in high-income countries, with lower levels most prevalent in Europe, the Americas and the Western Pacific region.

The highest urban air pollution levels were experienced in low- and middle-income countries in WHO's Eastern Mediterranean and South-East Asia regions, with annual mean levels often exceeding five to 10 times the limits set by the agency, followed by low-income cities in the Western Pacific region.

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.