The study, written by a US environmental consultant and reported in the International Herald Tribune, concluded that there were "irregularities" in the city government’s system of measuring air pollution.
The study said that Beijing had changed its formula for measuring pollution in 2006. Officials stopped including readings from two stations in polluted areas, the report said, and began using readings in three other stations in less polluted locales.
Without this switch, Beijing would have fallen short of its goals in 2006 and 2007 for the number of "blue sky days" that met national air quality standards, the report said.
Du Shaozhong, deputy director of the Beijing Environmental Protection Bureau, was reported as saying in response to the report: "Beijing’s air pollution has visibly improved… This is an indisputable fact."