 Many of the city's most impressive buildings were built with red or blond sandstone, but during the industrial era those colours disappeared under a pervasive black layer of soot and pollutants from the furnaces, until the Clean Air Act was introduced in 1956.In recent years many of these buildings have been cleaned and restored to their original appearance.There are over 1,800 listed buildings in the city, of architectural and historical importance, and 23 Conservation Areas extending over 1,471 hectares.