Authorities with Shanghai's environmental protection bureau said on Friday afternoon that it's illegal for the US consulate in Shanghai to publish assessments of air quality to the general public.

Zhang Quan, director of the Shanghai Environmental Protection Bureau, said at a media conference that the consulate cannot legally reveal data it collects in PM2.5 tests, which measure the concentration of particulate matter of 2.5 microns or less in diameter.

"There are certain criteria that must be met if these data are to be published officially," Zhang said. "A city needs at least eight to ten monitoring stations in various spots, depending on its size, to measure PM2.5. An analysis of a city's air quality cannot be done using readings from a single machine."

The US consulate in Shanghai began in mid-May to publish PM2.5 readings it had taken, saying they are "an indicator of overall air quality in the area surrounding its Huaihai Middle Road offices", according to a notice on the consulate's micro blog on Monday.

The consulate said it was publishing the data to inform US citizens living in Shanghai about conditions in the city. It said the monitor is an unofficial source of information and is meant to protect the health of the consulate community.

The discrepancy seen in PM2.5 readings provided by the local government and the US consulate in Shanghai have raised concerns among Shanghainese. On May 15, for instance, the former deemed the readings as being "favorable" while the latter said air conditions were "very unhealthy".

Zhang said the difference was the result of the two countries' using different evaluation criteria.

"As a matter of fact, the readings we've got are similar," he said. "But the US follows the standards for developed countries while we follow the relatively low standards for countries that have just begun taking PM2.5 readings."