Clouds of smoke from Lunar New Year fireworks sent air pollution in Beijing soaring under a new more sensitive measurement system.
Readings of fine particulate matter called PM2.5 were about 100 times worse on the January 22 eve of the holiday than the amount considered good for 24-hour exposure, the city's environmental bureau said.
The reading drew wide publicity in the local media.
Concern has grown over air pollution from cars and other sources, prompting the city this month to begin measuring PM2.5 - particles less than 2.5 micrometres in size.
That is about 1/30th the width of an average human hair.
Because of their small size, the particles can lodge deeply in the lungs and are believed to pose the greatest risk to health.