Elementary schools will be closed Wednesday, the government announced. Many schools suspended outdoor activities and urged parents to give their children gas masks.

Dangerous PM 2.5 particles, which are small enough to enter peoples' bloodstreams, peaked at 742 micrograms per cubic meter. The World Health Organization's safe limit for the particles is 60.

In some parts of the city, air quality readings reached a peak of 999, the worst that instruments can measure.

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The chief minister of Delhi called the city a “gas chamber.”

The dip in air quality happens regularly around this time of year. In previous years, it was linked to fireworks burned during the festival of Diwali, and crop burning in surrounding states. This year, an expert said, the cause was “local dust” meaning pollution from inside the city.

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Other parts of north India, and parts of Pakistan, too, are choking in the smog.

A sponsor of the city's upcoming half marathon threatened to withdraw funding from the event if the government doesn't intervene to curb pollution.