Nakul Butta, an ultra-runner and coach, always checks the Air Quality Index before every run. If PM 2.5 is over 400, he recommends that runners should stay indoors.
"Around this time of the year we do a lot of indoor running for our shorter runs, and for the longer runs we come out and wear pollution masks," Mr Butta says.
According to doctors, exposure to pollutants can cause chronic obstructive pulmonary diseases like asthma, chronic bronchitis, cardiac problems and even strokes. Dr Ashok Ahuja, who specialises in sports medicine, does not think masks are the solution.
"The fine particles of 2.5 microns which are 10 times less than your hair size will filter through, even these masks can't help," Dr Ahuja opines. "They can also hamper cardio-respiratory endurance," he warns.
But some runners in Delhi are not willing to put their health at risk. Aman Yadav, 32, has run the Airtel Delhi Half Marathon seven times, but he dropped out last year and is planning to drop out again this year.
"You wake up in the morning, you're gung ho I'm going to get a good workout, and then you realise that going out and running may actually be harmful for your health," explains Mr Yadav. He pointed out that the best weather for running coincides with the worst pollution, making training impossible. "I don't think anyone who has some sanity in themselves will go out and train in this weather," he adds.