United Airlines (NYSE:UAL) has canceled flights to New Delhi until further notice citing the toxic smog surrounding the city. Travel to New Delhi is suspended through at least Tuesday (Nov. 14), according to an advisory posted on United’s website.

The company’s statement said, “United has temporarily suspended our Newark-Delhi flights due to poor air quality concerns in Delhi and currently has waiver policies in place for customers who are traveling to, from or through Delhi. We are monitoring advisories as the region remains under a public health emergency, and are coordinating with respective government agencies.”

The company is letting passengers switch flights without charge and helping them find seats on other carriers. Customers with tickets for November 9 through November 13 will be able to re-book a comparable ticket leaving on or before November 18 at no additional cost.

The airline is advising the public to watch for updates, saying, “We encourage customers who are travelling over the next several days to visit United.com or download our mobile app for the latest updates and developments,”

It was unclear whether any of the other airlines were planning to follow suit. Virgin Atlantic, KLM, and Etihad Airlines all have flights that go to New Delhi. American Airlines and Delta do not fly to New Delhi.

New Delhi’s air quality is consistently ranked among the world’s worst. A recent study linked 2.5 million deaths in India in 2015 to pollution. Every winter, pollution levels rise to dangerous levels. Over the last week, New Delhi’s air quality has dropped to the worst levels of the entire year.

Some parts of New Delhi have pollution 40 times the World Health Organization-recommended safe level. One monitor showed that air pollution levels have reached 969, far in excess of the World Health Organization’s maximum safe level of 25. Those levels are based on the concentration of fine particulate matter, or PM 2.5, per cubic meter.

Photos show the city enveloped in a gray haze. People are huddled indoors with closed windows and expensive air purifiers. The illegal burning of recently harvested farm fields is sending smoke into the air. Pollution from construction projects and vehicles is making things worse.

More than 6,000 schools have been ordered closed, and construction projects have been halted. Only trucks carrying essential supplies are allowed into the city. Officials plan to put a partial ban on private cars into effect next week.