The densely populated capital of Bangladesh continues to gasp for fresh air. In fact, it is still the world's third-most polluted city.

Dhaka'd air quality index (AQI) was recorded at 268 at 10am on Thursday, which is considered "poor" or  "very unhealthy".

India’s Delhi and Pakistan’s Lahore occupied the top two spots with AQI scores of 513 and 306, respectively, in the latest list of world cities with the worst air quality.

Read: No respite for Dhaka residents as air quality still ‘unhealthy’

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.

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.

A report by the Department of Environment (DoE) and the World Bank in March 2019 pointed out that the three main sources of air pollution in Dhaka "are brick kilns, fumes from vehicles and dust from construction sites".

Read:Dhaka breathes ‘unhealthy’ air as AQI dips

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.