China has warned that foreign embassies issuing their own air-quality readings in Chinese cities are acting illegally. Data from sources other than the Chinese government are “not in accordance with the Vienna Convention on Diplomatic relations,” according to Wu Xiaoqing, a vice environment minister.

China has some of the most polluted cities in the world, despite this, until recently official air-quality measurements by the Chinese government rated their air quality as good. However, since 2008 the United States embassy has been monitoring pollution and recent data releases by the U.S. embassy in Beijing have shown extremely high pollution levels.

The U.S. embassy in Beijing has been posting hourly readings of pollution levels on Twitter and has gained more than 19,000 followers since 2008. The U.S. expanded the readings into the Guangzhou consulate last year and the Shanghai consulate last month.

Wu Xiaoqing went on to say that "The monitoring and publishing of China's air quality are related to the public interests and as such are powers reserved for the government."

Xiaoqing did not explicitly name the U.S. but called on all embassies to abide by Chinese laws. Despite this, the warning was clearly aimed at the U.S. embassies hourly releases.

China’s air quality is one of the worst in the world, with international organisations citing huge coal consumption and car-filled city streets as the main problems. The Environmental Performance Index compiled by Yale University ranked China 128th out of 132 countries for air quality.

The Chinese air-quality monitoring system base their information on 10 micrometers or larger, ignoring smaller particulates that experts say are the most harmful to human health, being linked to cancer and respiratory problems. In response to criticism of this method, the Chinese government ordered 74 cities including Beijing and Shanghai to begin monitoring small particles and publish the results this year.

Xiaoqing had said in March that the new methods of monitoring pollution would be implemented nationwide by 2016 as China looks to control the sources of particulates, such as coal burning and auto emissions.