Days after record air pollution caused an outcry in Beijing, smog in the capital on Wednesday (January 23) shot back up to levels considered 'hazardous,' highlighting the long-term environmental challenge facing authorities.<br/> <br />Beijing's official air quality monitoring system showed pollution hovering between 300 and 400 on an index that measures particulate matter in the air with a diameter of 2.5 micrometers, known as PM2.5.<br/> <br />The U.S. embassy in the Chinese capital measured an index of 435 at 11am (0300gmt) on Wednesday.<br/> <br />A level of 300 is considered dangerous while the World Health Organisation recommends a daily level of no more than 20.<br/> <br />On January 12, levels exceeded 700, increasing the number of patients visiting the city's hospitals with respiratory complaints.<br/> <br />Emissions from factories and heating plants, fumes from millions of vehicles and the burning of coal bricks to heat homes often conspire to blanket the city in a pungent haze that can become trapped when weather conditions are right.<br/> <br />A thick, grey mist hung over the metropolis of 20 million people on Wednesday, obscuring the red-tiled rooftops of the Forbidden City at its centre.<br/> <br />Smog also covered the Beijing's suburbs on Wednesday, and visibility was low at Beijing's Capital International airport.<br/> <br />Around ten percent of flights were delayed in the early morning, the airport's hotline told Reuters, but most flights were leaving on time by the early afternoon.
