The Environmental Protection Agency (EPA) recently implemented new laws mandating the use of ultra-low sulphur diesel (ULSD). ULSD is 97% cleaner and more refined than the previous diesel fuel standard.

ULSD contains less sulphur, which is a natural part of the crude oil from which diesel fuel comes from. Sulphur creates particulates or soot in the exhaust (which is why diesel fumes typically are black) which are then expelled in the diesel exhaust fumes and is one of the main contributors to air pollution.

Previous diesel fuel standard exhaust is a known carcinogen and is especially dangerous to children.

To learn more, go to:
http://auto.howstuffworks.com/fuel-efficiency/alternative-fuels/how-clean-diesel-fuel-works1.htm
http://www.hybridcars.com/related-technologies/clean-diesel.html