Dhaka, April 10 -- Dhaka ranked first among the world's most polluted cities on Friday morning, with an Air Quality Index (AQI) of 195 at 9:30am.

The city's air was classified as 'unhealthy', indicating a serious risk to public health, according to the AQI report.

India's Delhi and Vietnam's Hanoi ranked second and third, with AQI scores of 192 and 171 respectively.

In contrast, San Francisco in the United States and Oslo in Norway recorded some of the cleanest air, with AQI scores of 6 and 11 respectively.

According to the AQI scale, a reading between 50 and 100 is considered 'moderate', meaning air quality is generally acceptable, although sensitive people should limit prolonged outdoor activity.

Levels between 101 and 150 are desc...