There are 1.25 million diesel-fueled engines and vehicles operating in California. These diesel vehicles account for only 2 percent of on-road motor vehicles in the state, but they produce 30 percent of the nitrogen oxides and 60 percent of the particulate matter directly emitted from California motor vehicles. The particulate matter in diesel exhaust has been identified as a toxic air contaminant by ARB, and it has been linked to lung cancer. The harmful substances in fuels can enter the environment in a number of ways throughout the entire cycle of fuel production, manufacturing, transportation, storage, distribution and usage. Most commonly, they come out the tailpipes of vehicles as exhaust or unburned fuel.
Fuel vapors escape directly from automobile engines and gas tanks, especially on hot days.
They can also escape into the air during refueling, or when liquid fuel evaporates from a spill. Fuels can enter lakes and reservoirs through accidental spills or from motorized boats and personal watercraft. Fuels spilled on the ground or leaking from fuel storage tanks can contaminate groundwater. Substances in airborne engine exhaust settle directly onto water, soil and vegetation, or they can be washed down onto these surfaces when it rains. Even groundwater can be contaminated in these ways.