Pollution levels in Beijing have risen above index limits as a dense haze shrouds large parts of northern China.
Mask-wearing residents of the capital battled through a second consecutive day of pollution at hazardous levels. Municipal authorities warned those with respiratory difficulties to stay indoors.
It is at least the fourth time a dense smog has descended on northern China this winter, reducing visibility and causing flight delays, with even state media repeatedly expressing anger over the issue.
'The current environmental problems are worrisome,' Wang Anshun, who took over as mayor of the Chinese capital this week, was quoted as saying by the official Xinhua news agency.
The US embassy's air quality index (AQI) reading for Beijing stood at 495 and 'hazardous' at 11am local time on Tuesday, after having reached 517, or 'beyond index', at 6am.
The index rates anything over 150 as 'unhealthy' and over 300 as 'hazardous'. A reading above the upper limit of 500 is regarded as 'beyond index'.
The toxic air follows an extreme bout of pollution earlier this month, peaking on January 13 when state media said readings for PM 2.5, particles small enough to deeply penetrate the lungs, reached 993 micrograms a cubic metre, almost 40 times the World Health Organisation's recommended safe limit.