A thick grey pall hangs over Beijing's Tiananmen Square on Monday, blocking out much of the sunlight.
Two weeks after air pollution in the Chinese capital reached record-breaking levels, smog shot back up to conditions considered "hazardous".
The headquarters of China's state CCTV all but disappeared from view at a distance of just 300 metres.
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.
This fed up resident said she wanted the government to "sort out the environment".
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.
Beijing's air quality plummets again (1:19)
Jan. 28 - Beijing's air quality returns to ''hazardous levels'' two weeks after record-breaking pollution stirred public outcry in the capital. Sarah Sheffer reports. ( Transcript )