Runners in the Beijing Marathon concentrated intently on the sound of the starting pistol on Sunday, as heavy smog blurred their vision, casting a pall over the athletic event.

The smog that shrouded the capital, as well as surrounding areas in Hebei province, is expected to disperse beginning Monday night as a north wind blows away the pollutants, the National Meteorological Center said on Sunday.

The air pollution worsened from Friday night and reached its highest level on Sunday, when the annual Beijing Marathon was run. The smog was expected to thicken at night, reducing visibility to 2 kilometers in Beijing, Tianjin and Hebei province, based on an alert from the regional meteorological forecast and warning center serving the three areas.

In Beijing, at 8 am, the poor air quality reached a level 13 times greater than the safety level set the World Health Organization, causing many foreign runners to choose not to take part.

Having witnessed such smog frequently covering cities since last year, Chinese residents listed air pollution and problems involving the environment as the biggest threats for the world, according to a Pew survey released on Thursday.

The survey was conducted by the US-based Pew Research Center, with more than 48,000 adult respondents from 44 countries.

About one-third of Chinese respondents said pollution and environmental issues were the worst problems for the world, worse than other options including religious and ethnic hatred, inequality, nuclear weapons, AIDS and other diseases.

Residents in neighboring countries considered nuclear threats the greatest danger to the world.

Many people have voiced agreement and complained about becoming human vacuum cleaners in the polluted metropolis, both on online and at the marathon.

"I think air pollution should be given more attention from governments," said Liu Haoyuan, who attended the marathon, "because the lingering smog could destroy the image of China in the world".

"That's bad for China," said the 35-year-old engineer, who has lived in the United States for years, but traveled back to his hometown of Beijing for the event.

In spite of the bad air quality, Liu finished the race. "I thought the air pollution didn't have much influence on my running, because my result was almost the same as I did in the US," he said.

However, a doctor and expert on air pollution recommended that people avoid exercising in such conditions.

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Tourists wearing masks visit the Temple of Heaven during a severe hazy day in Beijing on Sunday. Li Wen / Xinhua