Dhaka occupied the 4th position on Monday morning in the list of world cities with the worst air quality.

The city's Air Quality Index (AQI) score was 178 around 09.44 am, i.e. well in the range of the 'unhealthy' category and close to being classified as "very unhealthy" (200 and above).

Pakistan's Lahore, India's Delhi and Croatia's Zagreb occupied the first three slots with AQI scores of 398, 220 and 197 respectively.

An AQI between 101 and 200 is considered 'unhealthy', particularly for sensitive groups of people like children and elderly population.

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), Nitrogen Dioxide (NO2), Carbon Monoxide (CO), SO2 (Sulphur Dioxide) and ground level ozone.

Dhaka has long been grappling with air pollution. Its air quality usually turns unhealthy during winter and improves during monsoon.

Last July, the capital even earned a spot among the world's top ten cities having clean air from July 22-31 last. Besides, the city's AQI score remained below 100 for most days during the month.

An AQI score 0-50 is considered as good air quality, while 51-100 as moderate, 101-150 as unhealthy for sensitive groups, 151-200 as unhealthy, 201-300 as very unhealthy and 301 and above as hazardous.

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".

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.

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, they said.