The authorities in New Delhi closed schools and urged people to stay indoors as toxic smog, which has plagued neighboring Pakistan for weeks, choked India’s capital in what officials called a medical emergency.

       New Delhi and the surrounding metropolitan area, home to about 55 million people, had the world’s worst air pollution on Monday, according to IQAir, a Swiss company that measures air quality. The reading on its index rose to over 1,600.

       Anything above 301 on that index is considered hazardous, potentially leading to severe eye and throat irritation and serious heart and lung conditions. The U.S. Environmental Protection Agency essentially considers anything beyond 500 to be off the charts.

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