When Grand Central was being redeveloped, the ceiling mural restoration was pretty much just a cleaning. Not that cleaning a very large ceiling 125 feet off the ground is easy, especially using materials that are environmentally friendly and wouldn't damage the mural.
A little patch was left uncleaned. People had assumed it was most diesel smoke, but spectrographic analysis showed that it was primarily the tar from the cigarettes that the passengers used to smoke. Imagine what that stuff was doing to the lungs of smokers - and others who were breathing it.
