In New Delhi and neighboring states, slowing winds have made air pollution worse, India’s meteorological department said. In the early hours of New Year’s Day, pollution levels in the capital soared to levels more than 20 times what the World Health Organization considers safe.

The fog is often so blinding that drivers cannot see cars slowing down in front of them, causing accidents and highway pileups. In December, hundreds of flights and trains were delayed or canceled because of low visibility.

Last week, six people were killed in the city of Noida, near New Delhi, after their car skidded off the road. Investigators blamed heavy fog.

In neighboring Bangladesh, a prolonged cold spell that started in November has left at least 50 people dead, including 17 children, according to Ayesha Akhter, a medical officer in Dhaka, the capital.