About 30,000 runners - some donning gas masks - took part in an international marathon in Beijing yesterday, braving the choking smog that again blanketed the capital. The government issued a blue warning for air pollution - meaning the air quality was unsuitable for outdoor activities. There has been controversy over whether the organisers should have postponed the event as the haze was forecast days ago. The smog also blanketed other northern cities in Hebei , Shandong and Henan provinces, as well as Tianjin , with the National Meteorological Centre issuing a yellow warning for haze. Pollution readings in Dingzhou , in the southwest of Hebei, went beyond the highest level on the index. Watch: Beijing Marathon runners brave hazardous air pollution Late on Saturday, organisers of the marathon warned competitors that there would be "slight or moderate smog" on race day, and asked runners to take precautions based on their own health conditions, advising the elderly and people with respiratory diseases to carefully consider whether to participate. Beijing's official Air Quality Index, however, climbed above 400 early yesterday morning - showing the pollution was at "hazardous" level and that all groups of people should avoid outdoor activities - when runners started the race at Tiananmen Square. At 10am, the concentration of PM2.5, the tiny pollution particulates that can penetrate people's lungs, reached 310 micrograms per cubic metre, more than 12 times the safety recommendation set by the World Health Organisation. Some runners wore masks, but most of the athletes ran with no protection, according to participant Wang Xing. "It's impossible to run with a mask on, especially for the second half," said Wang, who dropped out of the 42km marathon after 25km due to health concerns. Hector Xu, who travelled from Shanghai for the event, dropped out after running just 100 metres wearing a mask. The amateur runner said he had to quit a marathon in Shanghai last year due to heavy smog. "Running is supposed to be for health," Xu said. "I feel bad seeing so many people running in such heavy pollution without protection." The race was won by Ethiopia's Girmay Birhanu Gebru in two hours, 10 minutes and 42 seconds. His compatriot, Fatuma Sado Dergo, was the fastest woman with a time of 2:30:03. Organisers said it was impossible to change the date of the run because nearly half of the participants had travelled from other cities or abroad. Instead, they handed thousands of sponges to runners "to clean the skin that has been exposed to smog". State broadcaster CCTV reported that so many people had applied to join the marathon this year that the organisers had to approve applications by lucky draw. Only 14 per cent of applicants were able to race. Beijing weather authorities had forecast this round of smog last Monday, days after the city endured a week of haze earlier this month. Xinhua yesterday ran a commentary blaming some local governments for losing momentum in tackling air pollution a year after the country announced an all-out campaign to clean the nation's air, backed by an estimated investment of 1.7 trillion yuan from 2013 to 2017. It said some local governments had made no substantial effort in responding to smog, and only waited for wind to blow the pollutants away.