After a marked improvement on Wednesday, Dhaka's air quality slipped back to the 'unhealthy' zone on Thursday morning.

With an air quality index (AQI) score of 180 at 8.50am, the metropolis ranked second in the list of world cities with the worst air quality.

Pakistan’s Lahore and China’s Hangzhou occupied the first and third spots in the list, with AQI scores of 184 and 170, respectively.

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

Similarly, 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.

AQI, an index for reporting daily air quality, is used by government agencies to inform people how clean or polluted the air of a certain city is, and what associated health effects might be a concern for them.

Also read: Dhaka's air quality remains 'unhealthy'

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 during winter and improves during monsoon.

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.

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.