Philadelphia has a long history of concentrated industry in and around its center. Its extraordinary productivity stemmed in large part from the close proximity of workers' residences to the factoriess and plants in which they worked. Up to World War II, commuting was thus mostly by street car (originally horse-drawn, but fully electric from the 1920s on) or on foot. However, in addition to coal-fired foundries and steam plants, Philadelphia was also the terminus of two major railroads (the Pennsylvania and the Reading) that used steam traction until the 1950s, with trains running into the center at all times of day and night.The result was very high levels of particulate pollution, especially coal ash and sulphides.
By the 1960s much of the center-city industry had either shut down or moved away, but most of the Academy's buildings had been there for more than 60 years with their windows open all summer.
