Hundreds of people take part in the weekend race on Nov 9 at Gucun Park in Shanghai, organized by Shanghai Running. Fortunately, the races happened under blue skies. Photo by Gai Erqiang / China Daily

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While the situation is improving — no Chinese city features on the US-based Blacksmith Institute's annual report of the top 10 toxic threats for 2013, for example — concerns about the impact of smog from factories, cars and coal burning remain, especially in the country's northern band.

Harbin Airport in Heilongjiang province's capital was forced to close in November, along with local schools, as the level of particulate matter in the air was equivalent to 50 times the World Health Organization's recommended limit. In January, Beijing saw its PM readings —including that for PM 2.5 particles so small they penetrate deep into the lungs — average 300.

"For me, air pollution is a problem, but you don't know exactly how it affects your body, and not running is also bad for your health," says Xia Bing, a businessman from Xi'an in his early 40s. He recently moved from Shanghai to Shenzhen.

Shanghai residents coughed through one of the city's most polluted days in recent memory on Nov 7, two days before weekend races organized by Shanghai Running. Fortune smiled on the day, and the races were staged under clear blue skies.

"We were praying for a break. And luckily there was a lot of wind last night," says Michael Lee, a 43-year-old ethnically Korean Chinese from Jilin province.

"Otherwise, we probably wouldn't have come."

Meanwhile, the Shanghai International Marathon saw a record number of runners compete on Dec. 1 despite the air quality index hovering around 300, or "hazardous."

The hazy skies and murky rumors — often involving a friend of a friend who received unpleasant X-ray results — have left many questioning whether big-city runners do more harm than good by eschewing the treadmill for the gray outdoors.

"Every man and his dog has weighed on this argument about whether it's worse to run amid all the pollution or not," Mark Woodward says.

The 41-year-old Briton founded the local hash group DOGS (Distinguished Overseas Gentleman of Shanghai).

"But I smoke and run ultra-marathons, so maybe I'm a bit more blase about it."

Bad air quality doesn't deter Yang Qiming, a 36-year-old IT technician in Beijing, from running at night.