Two weeks after air pollution in the Chinese capital reached record-breaking levels, smog shot back up to conditions considered “hazardous”, Reuters reported on Monday.
It said Beijing’s official air quality monitoring system showed noon readings of around 350 on an index that measures particulate mass in the air.
A level of more than 300 is considered dangerous, while the World Health Organization recommends a daily level of no more than 20.
By 1pm a reading from the U.S. embassy in Beijing peaked at 423.
Emissions from factories and heating plants, fumes from millions of vehicles and the burning of coal often conspire to blanket the city in a pungent haze that can become trapped in certain weather conditions.
Extreme air pollution is known to cause cardiopulmonary disease, lung cancer and acute respiratory infections.