The Indian capital -- which has a population of 30 million -- once again ranked as the world's most polluted city Sunday, according to monitoring firm IQAir.

New Delhi topped a real-time list of the world's most polluted cities compiled by Swiss group IQAir, which put the city's air quality index (AQI) at 640 in the "hazardous" category on Friday. 

The air pollution in Delhi, India’s most polluted city, continue to be above the "severe" level for the third day as authorities caution people against staying outdoors for more than three to four hours.