Millions of cars have been banned from the streets of Delhi and dozens of flights diverted, with giant air purifiers installed at the Taj Mahal as India struggles with a pollution crisis engulfing the north of the country.

A pall of toxic smog sweeping the capital was so thick that more than 30 planes were turned away on Sunday. In parts of the city the air quality index soared to 1,200 — far higher than the internationally recommended safe limit of 50 — making it by far the world’s most polluted capital. About 150 miles south of Delhi, the authorities in Agra have deployed industrial air purifiers in a desperate attempt to protect the Taj Mahal, the country’s most popular tourist attraction.

Delhi hospitals have