Dhaka has again topped the list of cities around the world with the worst air quality this morning.

With an air quality index (AQI) score of 241 at 9:00am, Dhaka ranked first in the list of cities with the most polluted air.

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

Pakistan's Lahore, Vietnam's Hanoi, and India's Delhi occupied the second, third and fourth places with AQI of 219, 194 and 191 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.

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.