NEW DELHI (Reuters) - Tens of thousands of runners have signed up for the Indian capital’s half marathon and other races on Sunday, officials said, despite the air quality hitting dangerous levels in one of the most heavily polluted cities in the world.

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New Delhi’s air quality index was around 300 on Thursday, classified as very poor and meaning prolonged exposure can cause respiratory illness.

Delhi Chief Minister Arvind Kejriwal, who has described the city as a “gas chamber” in winter, has ordered emergency measures, including restricting the number of private vehicles on the roads under an “odd-even” scheme based on number plates.

Race organisers said pollution was a worry but they would take steps to reduce the impact on runners. Hours ahead of and throughout the race, the course will be sprayed with water.

“The air quality is a concern and will remain a concern, there is no question about it,” said Vivek Singh, joint managing director of Procam International that conducts the race sponsored by telecom operator Bharti Airtel.

“The measures that we take for those few hours to give our runners a good experience have worked in the past.”

The race has been moved this year to avoid a sharp rise in pollutants during Diwali, the Hindu festival of lights, when hundreds of thousands of firecrackers are lit.

But farmers burning crop stubble in the states north of Delhi have turned the air over Delhi toxic. The forecast for the next few days and into Sunday is “very poor”.

A record 40,633 people have signed up for the 21-km, 10-km and a 5-km races. Last year there were 34,916 runners, many of whom wore masks.

A former Olympic gold medallist, Carmelita Jeter of the United States, is the international event ambassador.

Doctors have advised citizens to restrict their outdoor activities and said runners must be made aware of the risks they are taking.

“Just two weeks before the odd-even scheme comes into play, how have the civic authorities allowed more than 30,000 people to expose themselves to toxic air?” asked said Desh Deepak, senior chest physician at the city’s Ram Manohar Lohia Hospital.