The United States consulate in Shanghai has begun issuing its own pollution data, giving a much darker assessment of air quality than government statistics.
Hourly data from a monitoring device in the consulate compound is being posted online and on Twitter.
And throughout yesterday morning, air quality was classified as "unhealthy," even as the local environment bureau called conditions "good."
The discrepancy raised concerns among local residents.
"I firmly believe the US data should be trusted," blogger Xiao She posted. " It's beyond imagination ... that our monitoring center said air quality is good when it looks full of dust."
Shanghai officials blamed different measurement standards and said the US figures only relate to a "limited area around the consulate."
Most mainland cities base air-quality information on particles of 10 micrometers or larger, known as PM10, and do not take into account finer particulates that experts say are most harmful to health. But the US consulate figures, like those already being issued by Washington's embassy in Beijing, do monitor the smaller particles known as PM2.5.
State media said Shanghai will release readings that include PM 2.5 particulates from next month.