Bangladesh's capital Dhaka ranked worst in the Air Quality Index (AQI) on Sunday morning.
It had an AQI score of 258 at 08:44am. The air was classified as 'very unhealthy'.
Everyone may experience more serious health effects when the AQI score is between 201 and 300.
India's Delhi and Pakistan's Lahore occupied the second and third positions in the list of cities with worst air with AQI scores of 257 and 255 respectively.
The AQI, an index for reporting daily air quality, informs 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 (O3).
The Department of Environment has also set national ambient air quality standards for these pollutants. These standards aim to protect against adverse human health impacts.
Dhaka, an overpopulated megacity surrounded by brick kilns, has long been battling air pollution.
Brick kilns have been identified as the leading cause of air pollution in the capital in a report by the World Bank and the Department of Environment.