Environment authorities in Beijing will no longer use the method of counting "blue sky days" as a gauge of air quality, which had been practiced for more than a decade. The city launched the "Defending the Blue Sky" project in 1998, when it had only 100 days of "blue skies."
Beijing has used a five-grade classification of air quality on the basis of pollution indices, with grade I being the best and grade V the worst. Days with grade I or II air quality are considered "blue sky days."