A marathon run in thick smog highlights in ironic relief the plague of air pollution that is choking Chinese cities.

I've always admired sports enthusiasts for their perseverance, but my envy stopped on Oct 19 when tens of thousands of them took to the streets for the annualBeijingmarathon.

It was a smoggy day. The level of PM2.5, particulate matter with a diameter of 2.5 micrometer or less, reached an extremely hazardous level of 344 at 8 am when the race started. That was not the worst in recent weeks, with some days so choked with dirty air you could hardly see the buildings across the street, but the Sunday on which the marathon was run was obviously unsuitable for strenuous outdoor activity.

The organizers issued warnings that anyone who felt ill at ease should stop immediately, and senior citizens were discouraged from taking part. There were 26,000 registered for the complete 42 kilometers and an additional 6,000 for the 21 km course.

But their tenacity became the fodder for online ridicule. There was a joke that the smog was largely dispersed - with PM2.5 dropping to about 100 - by early afternoon because so many people had sucked it in. This droll piece ends with a calculation that human breathing, while detrimental to your health, would not make a dent to the reduction of smog because 30,000 people acting as air purifiers would absorb only the amount of gas emissions from 600 cars covering the same distance.

The organizers were in an awkward spot. They did all they could to reduce the effects of the smog - advising runners to breathe with their nose rather than their mouth, spraying surrounding areas with water, etc. But they did not call it off. With about 40 percent of the participants from out of town, a last-minute cancellation would have affected them more adversely than it would have affected Beijingers.