Dhaka's air quality showed no improvement Friday morning as it continued to be in the 'unhealthy' zone.

With an air quality index (AQI) score of 166 at 9 am, the metropolis ranked sixth in the list of world cities with the worst air quality.

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

Read more: Dhaka’s air ‘unhealthy’ this morning

With the advent of winter, the city’s air quality starts deteriorating sharply due to the massive discharge of pollutant particles from construction works, rundown roads, brick kilns and other sources.

Pakistan’s Lahore, India’s Delhi, and China’s Wuhan occupied the first three spots in the list, with AQI scores of382, 373 and 178, 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.

Read more: Dhaka’s air ‘unhealthy’ this morning as winter approaches

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 a 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.