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Heavy smog covers New Delhi, India on November 7, when AQI readings hit as high as 451. Photo: VCG

Schools closed across large swaths of north India on Thursday as a hazardous fog of toxic pollution cloaked the region for a third day, with growing calls for urgent government action to tackle what doctors are calling a public health emergency.

Punjab's government said it was closing all 25,000 schools in the state for the rest of the week due to the acrid air blanketing north India and parts of neighboring Pakistan.

The decision came a day after New Delhi authorities ordered all 6,000 schools in the capital to shut until Sunday.

Low winds and the annual post-harvest burning of crop stubble in Punjab and neighboring areas have caused the levels of dangerous pollutants in the air to spike to many times the levels considered safe.

Air quality typically worsens before the onset of winter as cooler air traps pollutants near the ground and prevents them from dispersing into the atmosphere, a phenomenon known as inversion.

Figures on the US embassy website showed levels of PM2.5 - the smallest particulates that cause most damage to health - spiked at over 1,000 on Wednesday afternoon in Delhi, though by Thursday they had fallen to 590.

The World Health Organization's guidelines say 25 is the maximum level of PM2.5 anyone can safely be exposed to over a 24-hour period.

"Delhi once again has become a veritable gas chamber with denizens finding it difficult to breathe," The Times of India said Thursday, joining growing calls for government action to curb the chronic pollution, which the Indian Medical Association this week termed a public health emergency.

"Air pollution during winter months has become a catastrophe for large parts of north India," the country's most read English-language newspaper said in an editorial blaming "political apathy."

"It's high time the question is asked: why can't authorities enjoy jurisdiction over the national capital of an aspiring great power ... come up with concrete measures to tackle the world's worst air pollution."

As pressure mounted on the government, authorities in Delhi ordered a ban on all construction work and barred trucks from entering the city.