Dhaka, April 3 -- Dhaka, the densely populated capital of Bangladesh, ranked 13th among the world's most polluted cities on Friday morning, with an Air Quality Index (AQI) of 123 at 8:40am.

The air was classified as 'unhealthy for sensitive groups', meaning it could pose health risks to people who are more vulnerable, according to the AQI scale.

India's Delhi, Thailand's Chiang Mai and China's Beijing were the three most polluted cities, with AQI scores of 352, 223 and 192.

An AQI between 101 and 150 is considered 'unhealthy for sensitive groups', while 151-200 is 'unhealthy', 201-300 is 'very unhealthy', and anything above 301 is 'hazardous' and can cause serious health problems.

The AQI measures daily air quality, showing how clean ...