Pollution levels in Beijing soared above index limits, the US embassy said, as a dense cloud of haze shrouded large swathes of northern China.
People in the capital, some wearing masks, Tuesday battled through a second consecutive day of pollution at hazardous levels. Beijing municipal authorities warned those with respiratory difficulties to stay indoors.
It is at least the fourth time a dense cloud of haze 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, after reaching 517, or "beyond index", at 6am.
The index rates anything over 150 as "unhealthy", over 300 as "hazardous", while a reading above the upper limit of 500 is regarded as "beyond index".
Meanwhile, the Beijing Municipal Environmental Monitoring Centre gave the figure at 10am as 393, indicating the air in the capital was "severely polluted".
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 per cubic metre, almost 40 times the World Health Organisation's recommended safe limit.
The pollution sparked online uproar from residents and outrage from state media who urged officials to confront Beijing's poor air quality.
At the height of the smog, many residents rushed to buy face masks and air purifiers, and doctors at two of Beijing's major hospitals said the number of patients with respiratory problems had increased sharply during the period.
China's pollution problems are blamed on the country's rapid urbanisation and dramatic economic development.
A total of 61 flights were delayed at Beijing Airport just before 9.30am on Monday, according to its website.
[Image via Agence France-Presse]