Delhi was ranked the world’s most polluted major city on IQAir’s live ranking on Saturday morning, as residents prepared for Diwali, according to Reuters. At 10:35 a.m. local time, IQAir listed the city with a US air-quality index reading of 581. India’s Central Pollution Control Board reported an index of 363 at 10 a.m., classed as “very poor”.

Those numbers use different scales and methods and should not be treated as direct equivalents. The readings nevertheless point to a difficult day for residents already facing hazardous conditions. Reuters reported that authorities had asked schools in and around Delhi to offer online classes and encouraged people to work from home before Sunday’s festival.

Firecrackers are commonly used during Diwali and can worsen air quality, but they are not the only source of pollution. The immediate concern is limiting exposure while the city’s air remains poor. Public-health guidance emphasises reducing strenuous outdoor activity and following local advisories.