Bangladesh's capital Dhaka topped the list of cities with worst air in the Air Quality Index (AQI) on Thursday morning.
It had an AQI score of 370 at 10am. The air was classified as "hazardous".
An AQI score is between 301 and 500 implies health warnings of emergency conditions and the entire population is more likely to be affected.
Myanmar's Yangon, Nepal's Kathmandu and Pakistan Lahore stood second, third and fourth in the list of poor air cities with AQI values 194, 193, and 192 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.
Air quality is categorised as good when the AQI value is between 0 and 50. The air is classified as moderate when the score is 51-100. But when the number is between 101 and 150, the air is classified as unhealthy for sensitive groups.
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 has long been grappling with air pollution. The air quality usually improves during monsoon.