After a marked improvement in the past week, Dhaka's air quality slipped back to the 'unhealthy' zone yesterday morning.

With an air quality index (AQI) score of 137 at 8.44am, the metropolis ranked fourth in the list of world cities with the worst air quality.

Saudi Arabia's Riyadh, the United Arab Emirates' Dubai and India's Kolkata occupied the first three spots, with AQI scores of 164, 158 and 139, respectively.

An AQI between 101 and 200 is considered "unhealthy". Similarly, an AQI between 201 and 300 is said to be "poor", while a reading of 301 to 400 is considered "hazardous".

AQI, an index for reporting daily air quality, is used 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.