Carlos Barria / Reuters
A view of the Oriental Pearl TV Tower, right, and downtown Shanghai seen through the haze on May 15, 2012.
Aly Song / Reuters
A young man wearing a mask walks along the Bund in Shanghai on May 15, 2012.
The U.S. Consulate in Shanghai began posting hourly air quality readings for the city this week, with data showing "very unhealthy" conditions at times on Tuesday afternoon.
The consulate's classification reflects U.S. pollution standards but operates on a different scale than the Shanghai Environmental Protection Bureau, which called conditions "slightly polluted".
A similar monitor on the roof of the U.S. Embassy in Beijing has long been seen as the most reliable source of information on air quality in the Chinese capital.
Reuters contributed to this report