Few sights capture New York's beauty like the grand old apartment buildings that ring Central Park.
But for decades, they've also been a source of urban grit.
Like other buildings throughout the city, many of the graceful towers around the park have long burned the dirtiest heating fuel around: a nasty sludge called No. 6 oil that puts more soot into the air than all the city's automobiles.
But that has begun to change as a result of regulations put in place in 2011.
Since then, at least 1,500 city buildings have switched to cleaner fuels.
Changes are under way at many more, including Central Park palaces like The Dakota and The San Remo, which are switching to natural gas.
Environmentalists say the conversions will save lives.