Shanghai warns children to stay indoors on air pollution A participant wears a face mask as he competes in the Shanghai International Marathon. The US consulate stated that the air quality readings on the day were "very unhealthy." Shanghai warned children and the elderly to stay indoors as the level of the most harmful pollutants exceeded more than 10 times the level deemed safe by the World Health Organization. The air pollution index in the nation's commercial hub exceeded 300 as of 1.41pm, placing it in the "severe" range and the highest of six levels, the Shanghai Environmental Monitoring Center said on its website Monday. PM2.5 pollutants - particles smaller than 2.5 microns in diameter that pose the biggest health risk - reached 288.9, more than 10 times the WHO threshold, before falling to 211.5. Heavy pollution may undermine Shanghai's plans to attract foreign investment and multinational firms, as the city implements a free-trade zone as part of a broader goal to become a global financial and logistics center by 2020. Monday's smog warning comes a day after about 35,000 runners from 84 countries turned out for the 2013 Shanghai International Marathon, according to the People's Daily newspaper. Pollution levels began rising on the day of the Shanghai marathon, with the air quality index surpassing 200 at 1 pm, according to the center. In October, the Shanghai government announced a plan to cut 2012 PM2.5 readings by 20 per cent by 2017. "The sky was pretty bad," said Bridget O'Donnell, a U.S. citizen living in Shanghai who ran in the race. It didn't really affect me during the race but toward the end of the race I started to feel a little sick. After the race and today my lungs are really hurting. O'Donnell, who also ran in the race last year, said that she noticed other runners wearing face masks for the first time. The marathon started at the city's historic riverside Bund and wound through the city center. Air pollution isn't the only environmental concern in Shanghai, home to the larger of China's two stock exchanges and country headquarters for multinational companies including General Motors. Authorities found more than 10,000 hog carcasses floating in the Huangpu River in March, likely dumped by farmers wanting to avoid disposal fees. A Shanghai government report in August found more than half of the city's rivers and lakes are "severely polluted," boosting concerns about water quality. Chengdu, the capital of southwestern Sichuan province, also registered air quality index readings exceeding 300 Monday, while Beijing had readings above 150, in the "unhealthy" category, according to aqicn.org, a website that compiles air pollution data. Chinese Premier Li Keqiang pledged in March to clean up pollution including cutting coal consumption, shutting steel plants and controlling the number of cars. Pollution has become the top cause of social unrest in China, Chen Jiping, a former leading member of the Communist Party's Committee of Political and Legislative Affairs, said that same month.