Beijing - China's capital was shrouded in thick smog on Thursday, cutting visibility down to a few hundred metres as a count of small particulate pollution reached more than 20 times recommended levels.
A grey haze filled the sky, leaving an industrial, burning smell hanging in the air as PM2.5 reached over 500 micrograms per cubic metre in parts of Beijing, according to official statistics.
The WHO recommends an exposure level of no more than 25 over a 24-hour period.
The air quality index reached the top of the scale at 500.
Figures from the US embassy, which also monitors air quality, were in a similar range.
China's cities are often hit by heavy pollution, blamed on coal-burning by power stations and industry, as well as vehicle use, and it has become a major source of discontent with the ruling Communist Party.
Authorities have become more open about pollution levels, in part as a response to public pressure, but officials have implied that it will take years before the situation improves.
The pollution has been linked to hundreds of thousands of premature deaths, and has tarnished the image of Chinese cities including Beijing, which saw a 10 percent drop in tourist visits during the first 11 months of 2013.
China's State Council, or cabinet, said last year that “concentrations of fine particles” in the capital's air would fall by approximately 25 percent from 2012 levels by 2017.
Other major Chinese cities on China's affluent east coast, including Shanghai and Guangzhou, will see reductions of between 10 and 20 percent from 2012 levels over the same period, it said.