California has more personal vehicles than many countries have people. Nearly all of the state’s vehicles run on gasoline. According to the California Air Resources Board, gasoline and diesel-powered vehicles produce about 50% of the criteria pollutants in California. Vehicle emissions include:

• Volatile organic compounds—any organic compound that evaporates readily to the atmosphere. VOCs significantly contribute to smog.
• Carbon monoxide—a colorless, odorless, highly poisonous gas.
• Oxides of nitrogen (NOx)—smog-formers and major components of acid rain.
• Particulate matter—microscopic solid and liquid particles that remain suspended in the air for some time. Particles affect visibility and have adverse affects on health and the environment.

The California Air Resources Board has required increasingly cleaner vehicles, assigning them designations of LEV, SLEV, SULEV, PZEV and ZEV. Fuel cell vehicles are ZEVs—zero emissions vehicles—meaning no smog-forming emissions come from the vehicle.