Dhaka's air quality was in the unhealthy zone on Sunday morning.

With an air quality index (AQI) score of 153 at 9am, the capital of Bangladesh ranked fourth in the list of cities around the world with the worst air quality.

An AQI between 101 and 200 is considered unhealthy, particularly for sensitive groups.

Thailand's Chiang Mai, Nepal's Kathmandu and China's Wuhan occupied the first three spots on the list, with AQI scores of 162, 159 and 157, respectively.

An AQI between 201 and 300 is said to be poor, while a reading of 301 to 400 is considered 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.

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.