In the last five years, outdoor air pollution has risen by 8 percent around the world, based on the new World Health Organization (WHO) data. This suggests that billions of people in more than 3,000 cities are being exposed to unhealthy air.

According to the recent WHO data, several cities in southeast Asia, western Pacific and the Middle East showed increasingly high levels of air pollution. The levels in these fast-growing cities are five to 10 times higher than the recommended limits.

Based on the new report's average particulate matter (PM) 2.5 readings, the top three cities with worst air pollution levels are Zabol in Iran, followed by Gwalior and Allahabad in India.

The top 10 cities in the average PM 2.5 readings also included Riyadh and Al Jubail in Saudi Arabia and Baoding and Xingtai in China.

However, according to the data, the world's most polluted city is Onitsha in Southeast Nigeria. Onitsha is a transit and port city that has nearly 600 micrograms per cubic meter of PM 10s.

This rate is about 30 times higher than the WHO recommended levels, which is just 20 micrograms per cubic meter.

WHO said that in the last two years, their database of air pollution levels in cities and countries has almost doubled. This means that more cities and countries are monitoring not just the levels of air pollution in their areas, but also the health consequences that come with it.

"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," said WHO.

Local and national governments should prioritize improvements in air quality as it affects not just health, but also life expectancy and productivity levels, experts warn.

"Air pollution is a major cause of disease and death. It is good news that more cities are stepping up to monitor air quality," said Dr. Flavia Bustreo, assistant director general of Family, Women and Children's Health at WHO.

Bustreo added that the children, seniors and the poorest people become the most vulnerable when air pollution envelops cities.

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