Air quality across major Indian cities deteriorated sharply on October 1, with the Air Quality Index ranging from 100 to 300, in the unhealthy to very unhealthy bands, according to an IQAir alert.

Agra, Asansol, Delhi, Hyderabad, Kolkata, Mumbai and Siliguri were among the worst affected. At 10 am IST on October 1, Delhi, Kolkata and Mumbai ranked among the most polluted major cities in the world.

India was ranked the sixth most polluted country in IQAir’s 2025 World Air Quality Report. The country’s average PM2.5 concentration in 2025 was 48.9 micrograms per cubic metre, translating to an AQI of 134, nearly 9.8 times the World Health Organization’s guideline.

The spike comes just as the winter pollution season begins in north India, when stubble burning, low wind speeds and festival fireworks combine to trap pollutants. Health experts advise limiting outdoor activity during peak hours and using masks on high-pollution days.