India’s pollution watchdog classifies readings between 301 and 400 as very poor and anything above 400 as severe. The index covers eight major pollutants, including PM2.5, particles about 30 times smaller than a human hair that can lodge deep in the lungs.
The festival is marked with lights, lamps and fireworks that release fine particles and harmful gases. Last year Delhi woke to a toxic haze the morning after, despite a court order to use less-polluting crackers.
Winter’s onset typically brings a familiar deterioration in Delhi’s air. With Diwali days away, officials and residents fear fireworks will drive levels higher still.
Breathing such polluted air for prolonged periods can cause respiratory problems and worsen existing illness. Children and older people are particularly vulnerable.
Green firecrackers, which use modified chemical formulations meant to produce fewer emissions than conventional ones, have been promoted as an alternative.
Authorities have for years restricted firecrackers, including bans on manufacture, sale and use, but enforcement has proved difficult.
During severe pollution episodes, Delhi authorities have previously introduced hybrid schooling, work-from-home arrangements and limits on certain vehicles.
Crackers may be set off only between 06:00 and 07:00 and between 20:00 and 22:00 local time on 7 and 8 November, the Diwali weekend. Fireworks are also banned within 300 metres of hospitals, care homes, schools and other designated silence zones.