Thousands of schools were closed in India and Pakistan and a public health emergency was declared as thick smog continues to make life miserable for hundreds of millions who live in the region.

Some of the worst air quality readings were in Delhi state, home to some 20 million people in northern India. In New Delhi, India's capital city, air quality readings earlier this week revealed the dangerous air particle PM 2.5 soared above 700 micrograms per cubic meter , the New York Times reported, citing United States Embassy data. That's the equivalent of smoking more than two packs of cigarettes per day, the report added.

"Delhi has become a gas chamber ," Arvind Kejriwal, the chief minister of Delhi state, said in a tweet.

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Schools will remain closed through at least the end of the week, and most trucks were prevented from entering New Delhi in recent days, according to the Associated Press. Officials tried to hand out pollution masks, but some residents refused them, the report added.

In Pakistan's Punjab province, at least 10 people have died and another 25 have been injured since Monday in car accidents blamed on poor visibility, a highway police official told the AP in a separate report. Air pollution has soared to four times above the World Health Organization's limits, on average, in Pakistan's major cities, the AP also said.

The smog's invasion has become a yearly occurrence during the fall months in northern India. It's caused by farmers burning crops, construction dust, pollution emitted from factories and garbage fires , according to the Los Angeles Times. Then as temperatures fall and winds die down, the problem worsens, the report added.

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It's not just a nuisance – the smog is taking lives. The L.A. Times cited a study from the Lancet medical journal that found as many as 2.5 million people died in India in 2015 alone from pollution-related illnesses. Doctors are seeing widespread lung problems, even with nonsmokers, and they say they know why.

"I don't see pink lungs, even among healthy nonsmoking young people," chest surgeon Arvind Kumar told the New York Times. "The air quality has become so bad that even if you are a nonsmoker, you are still suffering."