Schools have been closed in New Delhi after “dangerous” toxic smog worsened.

The authorities in IndiaThe capital, home to more than 33 million people, has also halted construction work as part of efforts to control air pollution.

Airlines have warned of delays due to visibility falling to around 100 meters due to the worst pollution of the season so far.

Air quality is much worse than the safe limit recommended by the World Health Organization (WHO).

Live rankings from Swiss technology company IQAir warn that New Delhi is the world’s most polluted city, according to Reuters. IQAir also says the air quality is “hazardous”, with pollution levels that can cause illness and heart problems.

India’s top environmental agency, SAFAR, said the pollution was serious.

Classes for most grade levels are being held online, while no trucks will be allowed into the city except those carrying essential items.

Some older, diesel-guzzling vehicles have been banned from the city. Authorities also urged children, the elderly and people with respiratory problems or chronic illnesses to avoid going out as much as possible.

Pollution levels have continued to increase in recent weeks, and forecasts predict that poor air quality will continue in the coming days.

New Delhi residents complained of headaches and coughs on social media.

Air pollution in northern India is increasing every year, particularly in winter, as farmers burn crop residues in agricultural areas.

Combustion coincides with colder temperatures, which trap smoke in the air.

The smoke is then released into cities, where vehicle emissions add to the pollution.

Farmers in neighboring Uttar Pradesh state set fire to their fields over the weekend, releasing plumes of gray smoke that winds likely carried to New Delhi and other nearby areas.

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Emissions from industries and the burning of coal to produce electricity are also linked to pollution.

Studies estimate that more than a million Indians die each year from pollution-related illnesses.

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Indian authorities often resort to short-term measures, including water sprinklers and anti-smog guns, to try to control the haze.

But others urged authorities to introduce a long-term solution that would significantly reduce the pollution itself, instead of measures to mitigate the effects once it reaches cities.