The annual Beijing Marathon kicked off on Sunday morning as planned despite the heavy smog, while competitors wearing masks triggered controversy.
The organizers said they are considering changing the date of the event in the future to avoid the capital's peak smog season.
The marathon is one of China's biggest athletic events. The race, from Tian'anmen Square to the Bird's Nest national stadium, was expected to have 30,000 participants who qualified to attend through a lottery.
However, after the smog hit the capital and raised health concerns, many chose to quit.
Chen Xiaohui, chief executive officer of Cheers Publishing, gave up what was supposed to be his eighth marathon since Sept 8 last year.
"A marathon represents a healthy way of living life, not the opposite," said Chen, 41. "Since it does harm to my health, I have no choice but to quit."
According to the Ministry of Environmental Protection, the air quality index in Beijing was above 300 at 8 am, suggesting the air was hazardous to people's health.
Jin Bo, a manager with Lenovo in Beijing, joined the event for the first time with dozens of his friends and even the CEO of the company. He estimated that about 30 per cent of participants didn't show up. About 1 per cent of them wore masks.
"I also had concerns about air pollution before it started. But I had prepared for this for a long time and it would be a shame if I quit," said Jin, 33. "After I started, it turned out to be not that horrible. For some time it was difficult to see what lay ahead, but that's OK."
"This was an amazing experience, like a festival for runners. I mean, running on the roads which should be packed with cars and people if not for this marathon, it was great," Jin said.
"The event was organised well, with volunteers helping keep everything in order. I plan to participate more in the future."
Li Ming, a correspondent based in New York who was a fan of marathons in China, said she totally understood why many people chose to continue.
"The Beijing Marathon is a goal for many marathon fans," Li said. "It is not about what kind of health it can bring you in a few hours."
"I think what they did can also help society and the government pay more attention to air pollution in Beijing."
Wang Dawei, secretary-general of the event's organising committee, said they didn't foresee the smog a few days ago.
"We surely don't want this to happen. It has always been our goal to organise a great event for participants," Wang said.
"To reduce the effect of heavy smog on participants, we have done many things. We informed all 30,000 people who enrolled about the situation and potential risks on Saturday. We recommended they make their own choice based on their health."
"For those who still wanted to participate, we gave them detailed guidelines and cooperated with the environmental protection authority," Wang added.
Wang said the organising committee may change the date in the coming years after research and consultation with environmental protection and meteorological authorities to find a time less prone to smog.