Dhaka, the densely populated capital of Bangladesh, ranked 4th among the world’s most polluted cities on Wednesday morning, recording an Air Quality Index of 144 at about 9:00am.

On Wednesday air was classified as ‘unhealthy for sensitive groups’, referring air pollution levels are high enough to cause health issues for specific vulnerable populations, though the general public is likely unaffected, according to the AQI index.

Chiang Mai of Thailand, Shanghai in China and Kolkata in India occupied the first three spots on the list, with AQI scores of 235, 160 and 156, respectively.

AQI levels of 101–150 are ‘unhealthy for sensitive groups’, 151–200 is ‘unhealthy’, 201–300 is ‘very unhealthy’, and readings above 301 are considered ‘hazardous’, posing severe health risks.

The AQI, which reports daily air quality, informs residents how clean or polluted the air is and highlights potential health effects.

In Bangladesh, the AQI is calculated based on five key pollutants — particulate matter (PM10 and PM2.5), nitrogen dioxide (NO2), carbon monoxide (CO), sulfur dioxide (SO2), and ozone.

Dhaka has long struggled with air pollution. Air quality usually deteriorates in winter and improves during the monsoon season.

The World Health Organisation estimates that air pollution causes about seven million deaths worldwide each year, primarily from stroke, heart disease, chronic obstructive pulmonary disease, lung cancer, and acute respiratory infections.