Dhaka, the overcrowded capital of Bangladesh, has topped the list of cities worldwide with the worst air quality yet again.

With an AQI index of 278 at 9:12am today, the air was classified as 'very unhealthy'.

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.

China's Shanghai and India's Kolkata and Delhi occupied the second, third and fourth spots on the list, with AQI scores of 199, 198 and 189, respectively.

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.

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.