Air quality still severe but United Airlines resumes flights to Delhi

United Airlines said it had resumed flights from Newark, New Jersey to New Delhi, India on Sunday, after suspending the service temporarily over concerns about poor air quality in the Indian capital.

UA Flight 82 had been cancelled on Friday and Saturday, data from flight tracking website FlightRadar24 showed, while the airline's website said it had waiver policies in place for passengers travelling to, from or through Delhi until Monday.

UA Flight 82 has resumed operations, but "we will continue to monitor conditions over the next few days", a spokesman said.

The third-largest US carrier is monitoring advisories as the New Delhi region remains under a public health emergency, and is coordinating with respective government agencies, the United Airlines spokesman had earlier told Reuters.

Last week, New Delhi declared a pollution emergency as toxic smog hung over the city for days, with tourism operators reporting cancellation of bookings for the Christmas holidays.

US rivals Delta Air Lines Inc and American Airlines Group Inc said they do not operate flights to New Delhi, while several Asian airlines contacted by Reuters said they had not cancelled flights.

Verisk Maplecroft, a risk consultancy, says India has the worst air quality out of a list of 198 countries it measures, and that New Delhi ranks among the world's top 10 most polluted cities with many urban areas around the capital also among those with the worst air quality.

"At the national level, India tops the index rankings (i.e. the country with the worst air quality), followed by Bangladesh and Thailand," said Richard Hewston, Verisk Maplecroft's global head of environment and climate change.

The company's Air Quality Index assesses the atmospheric concentrations of fine particulate matter smaller than 2.5 micrometres, known as PM 2.5.

A US embassy measure of PM 2.5 showed a reading of 481 in New Delhi on Monday morning, local time. The outer limit of "good" air is 50.