The microscopic particles, which are smaller than 2.5 micrometers in diameter, are considered particularly harmful as they are small enough to enter deep into the lungs and cardiovascular system.

PM 2.5 includes pollutants such as sulfate, nitrates and black carbon. Exposure to such particles has been linked to lung and heart disorders and can impair cognitive and immune functions.

It is estimated that more than 80% living in urban areas which monitor air pollution are exposed to air quality levels that exceed WHO guideline limits, with low- and middle-income countries most at risk.

"Air pollution constitutes the most pressing environmental health risk facing our global population," the AirVisual report said.

South Asia continues to be of particular concern, with 27 of the 30 most polluted cities in India, Pakistan or Bangladesh. Pakistan's Gujranwala, Faisalabad and Raiwind, are among the ten most polluted cities, and the major population centers of New Delhi, Lahore, and Dhaka rank 5th, 12th and 21st respectively, according to AirVisual data.

Regionally, South Asia, Southeast Asia, and the Middle East are worst affected overall, with only six of 355 cities included meeting WHO annual targets, the report said.

However, South Asia has seen improvements from the previous year. National air pollution in India decreased by 20% from 2018 to 2019, with 98% of cities experiencing of varying levels of improvement. The report points to economic slowdown, favorable weather conditions, and efforts towards cleaning the air as reasons behind the decrease.

For example, the most polluted city, Ghaziabad, had an average AQI of 110.2 this year. But in 2018 it was 135.2 and 144.6. in 2017.

The report also points India's launch of the country's first National Clean Air Programme (NCAP) which aims to reduce PM 2.5 and the bigger particulate PM 10 air pollution in 102 cities by 20-30% by 2024 compared to 2017 levels.

Climate crisis and urbanization

The air quality data shows, "clear indications that climate change can directly increase the risk of exposure to air pollution" the report said, noting it impacts air quality in many cities through desertification and increased frequency of forest fires and sandstorms.

Exacerbating the problem is rapid urbanization in industrializing Southeast Asian cities, which is also a major cause of air pollution and poses severe challenges to managing PM 2.5 levels, the report found.

Indonesia's Jakarta and Vietnam's Hanoi overtook Beijing for the first time among the world's most polluted capital cities, "in a historic shift reflecting the region's rapid industrialization," the report said. The two capitals have annual PM 2.5 levels which are about 20% higher than those of Beijing, according to the report.

"Fast growing cities need to make a choice if they want to grow in a sustainable manner," said Yann Boquillod, director of air quality monitoring at IQAir.

However, it's not all bad news. People power is bringing about change, he said.

"Up to recently, growth was more important than the environment, but we're seeing a very clear trend that people are demanding more from their local governments," Boquillod said.

"During the year 2019, citizens of Hanoi have massively become aware of the air quality in their city, thanks to the deployment of air monitors. This is an example how air quality data has helped to push government to improve the environment."

"The city's rapid growth has coincided with heightened PM2.5 levels, as the growing population adds to its notorious traffic congestion, and coal-based energy demand," the AirVisual report said.

Bright spots

Chinese cities have overall seen marked improvements in recent years, with average concentrations of pollutants falling 9% from 2018 to 2019, according to the report.

China's capital Beijing has more than halved its annual PM 2.5 levels over the past decade and has dropped out of the world's 200 most polluted cities following concerted efforts to get air pollution under control.

Dust and sandstorms in the desert city of Hotan, in western China's Xinjiang region, make it the world's second most polluted city in 2019, with an average AQI of 110.1.

Another positive is that last year saw a significant increase in countries expanding their air quality monitoring, "with the number of monitoring stations increasing by more than 200% since the year prior."

More monitoring data is important to inform communities about the quality of the air they are breathing and helps tackle air pollution globally, the report said.

Continuous public air quality data is now available for the first time for Angola, the Bahamas, Cambodia, DR Congo, Egypt, Ghana, Latvia, Nigeria and Syria, the report found.

Yet there is still a large gap in air quality data for many parts of the world, the report said, meaning the total number of cities exceeding the WHO PM 2.5 threshold could be higher.

For example, Africa, a continent of 1.3 billion people, has less than 100 monitoring stations that make air quality data available to the public in real time.

"Often locations bearing the highest particulate pollution levels tend to also notably have the least monitoring data," said Frank Hammes, CEO of IQAir. "Leaving the most vulnerable communities without access to timely and relevant air quality data, necessary to guide actions to safeguard their health."