Dhaka has once again topped the list of cities worldwide with the worst air quality, with an AQI of 219 at 8:54am today.

India's Delhi and Pakistan's Lahore occupied the next two spots, with AQI scores of 197, and 179, respectively.

An AQI between 151 and 200 is said to be "unhealthy" while 201 and 300 is considered "very unhealthy", and 301 to 400 is ranked "hazardous", posing serious health risks to residents.

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.

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.