Abroad and at home, there is a rising chorus questioning the effectiveness of air pollution control in China, most recently from the thousands of runners who finished the Beijing Marathon in smog on Sunday. Still, officials and experts alike voiced their confidence about reining in air pollution, although they concede it will take time.

When about 30,000 runners from around the world came to Beijing for the event, some of them got cold feet due to the lingering smog. Runners with a variety of masks were caught in the media spotlight, raising questions about how long it will be before blue skies are back in our lives.

"It will take China about 20 to 30 years to tackle the problem of air pollution fundamentally, based on the experiences of developed countries like Japan, the United States and the UK in reining in air pollution," said Song Guangsheng, head of the National Indoor Environment Monitoring Center, at a forum on ecological cities on Monday, China National Radio reported on Tuesday.

If the country uses the toughest restrictions and advanced technology to reduce emissions of air pollutants, it will take at least 15 to 20 years to see a thorough improvement in air quality, he said, citing statistics from an annual report in May on a low-carbon economy.

Considering the strong determination of governments to improve air quality in three to five years, many people are hopeful and feel positive about the environment in the near future, said Zhao Zhongxiu, an economics professor from the University of International Business and Economics and co-author of the low carbon report. But a fundamental improvement cannot be realized in the short term, Zhao said.

Many government officials doubted that it would take just two decades to return to good air quality.

"It will take years to cure the problems, which were accumulated during decades of development," said Lyu Wen, director of the air pollution control department of the Hebei Environmental Protection Bureau, on Tuesday, adding that there was no scientific research on how long it will take to rein in pollution using increasingly stronger measures.

Some experts said the current air pollution problem can be traced to emissions of past decades. One of them, Ma Zhong, a professor of environmental protection at Renmin University of China, said the highly polluting industries of steel and cement and other heavy industries discharged many pollutants over the years that have accumulated in the air.

Coal consumption from 2006 to 2010 was increased by around 1 billion tons more than planned, the annual report on energy consumption said in 2011.

"But we have started to take tough measures to cut the emissions, although it slows growth," Lyu said.

Hebei, which since last year has been the province with the worst air pollution, has shut down hundreds of polluting companies, including some that conform with national emission standards. In the first half of this year, its GDP growth was ranked second to last in China.

Some residents said they have witnessed improving air quality in the province.

"I noticed the improvement recently, and the slower economic growth is worth it for the better air quality," said Wang Wenyi, a vendor selling snacks in ShijiazhuangHebei province.

In September, Shijiazhuang was not among the top 10 cities with the worst air pollution, according to a report released on Tuesday by the Ministry of Environmental Protection.

Beijing Marathon kicks off in haze	Smog affects tours to China