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Passenger flights serving the Indian capital were diverted as smog made visibility too poor for airlines to operate. The dire conditions in Delhi prompted politicians to trade blame for not tackling the crisis.
Flights to and from Delhi's Indira Gandhi International Airport were delayed or rerouted on Sunday as heavy air pollution enveloped the city.
Major carriers such as Air India and Vistara were forced to divert flights to airports as far away as Mumbai and Amritsar.
Concentrations of microparticles — measuring less than 2.5 microns — hit their highest level of the season, according to India's state-run System of Air Quality Weather Forecasting and Research (SAFAR).
Particulates are considered the most harmful type of air pollution, causing heart attacks and respiratory disease.
Readings hit 810 micrograms per cubic meter on Sunday morning, according to the US Embassy in Delhi, which measures levels independently. Such levels are regarded as hazardous, with the World Health Organization putting the safe daily maximum at 25 micrograms.
Politicians blamed each other for the worsening conditions, with Delhi's chief minister, Arvind Kejriwal, saying the situation had become "unbearable."
Air pollution sensors in some parts of Delhi were reported to have maxed out at their upper limit of 999 micrograms, with people advised to stay indoors and wear masks.
Kejriwal — who has announced a plan to restrict vehicle movement in the city for the next two weeks — has blamed the neighboring states of Punjab and Haryana for the pollution, citing crop stubble burning as the cause.
Federal Environment Minister Prakash Javadekar, however, accused Kejriwal of politicizing the issue and seeking to show the two states "in a bad light and presenting them as villains." He instead called for states to work together to solve the problem.
The cancellations came after German Chancellor Angela Merkel wrapped up a two-day trip to the capital, New Delhi.
During the visit, Merkel and Indian Prime Minister Narendra Modi declined to use the face masks recommended by doctors while inspecting troops at the presidential palace.