delhi airpolutionUnited Airlines canceled flights to the Indian capital city of New Delhi because of poor air quality. But later resumed on Sunday.

Air Quality Index (AQI) recordings have consistently hit the maximum of 999 for Delhi. Exposure to such toxic air is akin to smoking more than two packs of cigarettes a day, say doctors.

“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,” said the company in a statement.

Passengers will be given vouchers to switch flights, or given assistance in flying with a different airline.

However, later on Sunday Flight UA82 was operated. Airlines were still flying to the national capital and the Indira Gandhi International Airport’s website had no notices about other cancellations. IndiGo, India’s biggest carrier and having a hub in the city, didn’t cancel any flights as well.

The levels of the deadliest, tiny particulate matter -- known as PM 2.5, which lodges deep in a person’s lungs -- was at 495 as of 9 a.m. local time Monday, according to a U.S. embassy monitor. The level had soared to 721 as of 1 p.m. Sunday. World Health Organization guidelines suggest levels above 300 are “hazardous.”