Dhaka has once again topped the list of world cities, for the wrong reason, with the worst air quality on Friday morning.

With an air quality index (AQI) score of 208 at 9am today, Dhaka ranked first in the list of cities with the worst air pollution.

Read more: Dhaka's air quality still ‘very unhealthy' as it ranks 2nd worst polluted city

An AQI between 201 and 300 is said to be "very unhealthy," while a reading of 301 to 400 is considered "hazardous," posing serious health risks to residents.

India's Mumbai and Ghana's Accra occupied the second and third spots in the list, with AQI scores of 189 and 188, respectively.

In Bangladesh, the AQI is based on five criteria pollutants – particulate matter (PM10 and PM2.5), NO2, CO, SO2 and Ozone.

Dhaka has long been grappling with air pollution issues. The city's air quality usually turns unhealthy in winter and improves during monsoon.

Read more: No improvement in Dhaka’s air quality, still ‘ very unhealthy’

Air pollution consistently ranks among the top risk factors for death and disability worldwide.

Breathing polluted air has long been recognised as increasing a person's chances of developing heart disease, chronic respiratory diseases, lung infections and cancer, according to several studies.

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