The densely populated metropolis continues to dominate the list of cities with the worst air quality in the world.

On Tuesday, Dhaka was ranked the world's second-most polluted city, as its air quality index (AQI) was recorded at 200 at 11am, which is considered ‘unhealthy'.

China’s Wuhan occupied the top spot with an AQI score of 218, while India’s Kolkata ranked third with an AQI of 193 in the latest list of world cities with the worst air quality.

An AQI between 151 and 200 is said to be ‘unhealthy’ particularly for sensitive groups. Active children and adults, particularly those with respiratory diseases such as asthma are usually advised to avoid all outdoor activities in this situation.

Read: Experts lament lack of steps to check air pollution in Dhaka city

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".

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.

Read: Experts seek winter plan to fight Dhaka’s air pollution

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.