Dhaka was ranked fourth worst in the Air Quality Index (AQI) yesterday morning. Dhaka had a score of 165 at 10:44am, indicating that the air quality was unhealthy. A numerical value between 151 and 200 indicates that everyone may begin to experience health effects. Members of sensitive groups may experience more serious health effects.

India's Delhi, Pakistan's Lahore and Nepal's Kathmundu occupied the top three spots respectively. The AQI, an index for reporting daily air quality, tells people how clean or polluted the air of a certain city is, and what associated health effects might be a concern for them.

Dhaka, one of the most densely populated cities in the world, has been grappling with air pollution for a long time. The air quality generally worsens during summer and shows signs of improvement in monsoon when the dust settles.

The United States Environmental Protection Agency developed the Air Quality Index (AQI) to measure air quality. The AQI is divided into six categories, indicating increasing levels of harm to one’s health.

An AQI value over 300 represents hazardous air quality, and below 50 the air quality is good.

The index is based on the five criteria of pollutants regulated under the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide.

The AQI debuted in 1968, when the National Air Pollution Control Administration undertook an initiative to develop an air quality index, and to apply the methodology to Metropolitan Statistical Areas.