The scheme was simple: Stuff cotton into air-monitoring equipment so that the air being read would be filtered and seem cleaner.

The people doing this to produce better — but false — air-quality readings in Xi’an, the provincial capital of Shaanxi, got away with it for months, until inspectors noticed irregularities in the data.

An investigation resulted in the head of the air-monitoring station in the city’s Chang’an district and several members of his staff being caught, according to a report by Chinese Business View, a newspaper in Shaanxi. Five people have been detained, including He Limin, chief of the Chang’an branch of the Xi’an Environmental Protection Bureau.

The report was widely reprinted online and discussed by internet users this week in China, and it raised questions about the accuracy of air-quality data gathered in scores of cities across the country. In January 2013, China began releasing real-time air-quality data from 74 cities and 496 air-monitoring stations. Since then, many more cities have been added to the system, overseen by the Ministry of Environmental Protection.