The transportation sector is a predominant source of air pollution in the U.S. Particularly in urban areas, mobile sources – including passenger cars, trucks, buses, ships, locomotives and aircraft – and the fuels on which they run, collectively can produce 90 percent or more of local air pollution, including levels of ozone (smog) and particle pollution (soot). Vehicles can also produce a significant amount of toxic  pollutants, as well as greenhouse gases and emissions that contribute to the formation of acid rain. While tremendous progress has been made in cleaning up mobile sources and fuels over the past four decades, additional emission reductions can and must still be achieved from this source category.

The U.S. passenger car fleet consists of more than 200 million cars and light-duty trucks (referred to collectively as light-duty vehicles). Forty years ago almost all personal vehicles were cars. Today, at least half of all vehicle sales for personal use are sport utility vehicles (SUVs), vans and pickup trucks. Title II of the Clean Air Act required EPA to promulgate a series of rules to reduce passenger car tailpipe emissions, as well as emissions from refueling and the evaporation of gasoline in the tank.

Diesel-fueled trucks and buses have served the nation’s freight and transportation needs for over half a century because of their reliability, durability and relative efficiency. This source category includes a wide variety of vehicles from 18-wheel tractor-trailer combinations, to school and transit buses to dump trucks and refuse haulers. These vehicles can be found in large numbers on major highway, urban streets and rural roads.

Controlling the content and properties of transportation fuels can offer great benefits in terms of protecting public health and the environment, particularly when cleaner fuels are combined with vehicle and engine emission standards. In 1974, EPA put in place a program to gradually reduce lead in gasoline. Lead in gasoline has been banned entirely since 1996. More recently, federal programs have required "reformulation" of gasoline and significant reductions in sulfur levels of gasoline, allowing the pollution control equipment to work more efficiently.

Large ships, locomotives and aircraft are all considered “nonroad” vehicles or engines. Ocean-going ships – such as diesel-fueled container ships, cruise ships, tankers and bulk carriers – travel around the world and contribute significantly to air pollution in many of our nation’s cities and ports, as well as inland areas. Likewise, locomotive engines, which power trains that transport vast quantities of goods and also serve as a mode of transportation, run on diesel fuel and play a role in air pollution levels across the country. Meanwhile, the rapid growth in worldwide air travel has heightened concerns over the influence of aviation on air quality while piston-engine aircraft, used for general aviation, are fueled with leaded aviation gasoline and are a significant source of lead emissions.