Dhaka, the overcrowded capital city of Bangladesh, has topped the list of cities with the worst air quality with an AQI index of 255 at 09:01am on Friday.

The air was classified as 'very unhealthy', according to the air quality and pollution city ranking.

An AQI between 151 and 200 is considered 'unhealthy' while 201-300 is 'very unhealthy' and 301-400 is considered 'hazardous', posing severe health risks to residents.

Vietnam’s Hanoi, Uzbekistan’s Tashkent and India’s Delhi occupied the second, third and fourth spots in the list, with AQI scores of 234, 201 and 189, respectively.

Dhaka’s air quality ranks world's 2nd worst on Thursday

The AQI, an index for reporting daily air quality, informs people how clean or polluted the air of a certain city is and what associated health effects might be a concern for them.

The AQI in Bangladesh is based on five pollutants: particulate matter (PM10 and PM2.5), NO2, CO, SO2, and ozone.

Dhaka has long been grappling with air pollution issues. Its air quality usually turns unhealthy in winter and improves during the monsoon.

Dhaka's air quality is worst in the world this morning

As per World Health Organization (WHO), air pollution kills an estimated seven million people worldwide every year, mainly due to increased mortality from stroke, heart disease, chronic obstructive pulmonary disease, lung cancer, and acute respiratory infections.