Dhaka once again topped the list of world cities with the worst air quality yesterday morning.

With an air quality index (AQI) score of 317 at 8:00am yesterday, Dhaka ranked first in the list of cities with the worst air.

An AQI between 151 and 200 is said to be 'unhealthy', while a score between 201 and 300 is considered 'very unhealthy'.

On the other hand, an AQI score between 301 and 400 is considered 'hazardous' and is supposed to be posing serious health risks to residents.

India's Kolkata and Mumbai occupied the second and third spots in the list, with AQI scores of 196 and 189, 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. Its air quality usually turns unhealthy in winter and improves during the monsoon.

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.