Monitors at the United States Embassy in the suburb of Liangmaqiao, in Beijing's inner east, said the concentration of airborne PM 2.5 particulates reached 886 micrograms per cubic metre at 8pm on Saturday night, believed to be the highest since it began measuring in 2008.
"I can tell you the machine is working properly," confirmed US Embassy spokesman Nolan Backhouse.
A concentration measure of 500 corresponds with an Air Quality Index of 500
An AQI reading above 300 is classed as "hazardous", according to US environmental standards, but the AQI does not compute concentrations above 500, which in developed countries is normally associated with bushfires.
PM 2.5 particulates, 2.5 microns in diameter or smaller, are considered the most harmful to health because they can penetrate easily into human tissue.
Last month aLancetstudy said a record 3.2m people died worldwide from air pollution in 2010, four times the number in 1990, with 1.2 million of those deaths in East Asia including China.
This ranked pollution for the first time in the world's top 10 list of killer diseases, mostly because of vehicle exhaust.
The US Embassy pollution monitor is published on a Twitter feed, which is blocked by censors in China, but which is picked up and placed on several popular Chinese websites and iPhone applications.
Beijing temporarily improved its air quality for the 2008 Olympics including by improving vehicle emissions standards, banning coal stoves and shifting heavy industry to poorer parts of the country.
Much of those gains appear to have been offset by an explosion in the numbers of cars on the roads.
Most major cities were required to report standardised PM 2.5 readings from January 1.
Previously the Chinese Government measured but refused to release the data, publishing only the larger and less dangerous PM 10.
Concentrations of PM 2.5 particulates at an official monitoring station at Xizhimen, in Beijing's inner west, hit a staggering 993 at 7pm Saturday, according to an online report by the Beijing News,.