Because hydrogen can clean the exhaust right from a cold engine start, the plasma reformer method is 20% to 50% more efficient at reducing NOx than other clean-diesel technologies, allowing less unburned fuel to pass through in the process.

FORBES: Clearing the Air

There may also be substances such as unburned jet fuel that may have come from the two hijacked jetliners terrorists piloted into the buildings.

CNN: CDC, EPA: Little risk to public from sites