Thick smog blanketing Beijing went "beyond" measurable pollution levels on February 21, the US Embassy said, as a Chinese official warned people to stay indoors and avoid outdoor activities.
The independent assessment by the embassy said pollution was either hazardous or "beyond index," meaning that air quality had plunged below the worst level on the scale.
The Beijing Environmental Bureau said air quality in most of the city was at level five - the worst rating.
"Obviously elderly people and children should not go outside," an official at the bureau who refused to identify herself told AFP.
Particulate pollution, rising temperatures and a lack of wind caused the stifling smog, which reduced visibility in parts of the city to just 200 metres, the Beijing weather bureau said in a report.
Air pollution in Beijing has been consistently listed as among the worst in the world by international organisations such as the United Nations.
Strong winds kept Beijing's skies clear and blue for most of January despite the city's numerous coal-fired power stations and more than 4.8 million cars on the roads - the capital's main sources of air pollution.