Vehicular Exhaust and Air Pollution


Individual tailpipe emissions

Transport and fate in the environment

Measuring exposures

Prevention and control of exposure

Exposure Pathway

Risk assessment

Adverse effects

Harmful Effects

Dose Response

Absorption, Metabolism and Molecular Mechanisms of Action

Organ Sites of Toxicity


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Prevention or Control of Exposures

The United States relies heavily on the use of fossil fuels in transportation. The best option for preventing air pollution from mobile sources is prevent the combustion of fossil fuels. While much of the economy in the United States depends on fossil fuels, the reduction of consumption can be achieved in several ways. Increased fuel efficiency in new vehicles can also contribute to reducing the consumption of these fossil fuels. Standard combustion engines can be replaced with alternative fuels engines such as hybrid gas-electric engines or engines powered by natural gas or ethanol. Finally, offering viable and effective transportation alternatives can reduce the number of vehicles on the road and thereby reduce the amount of tailpipe emissions.

Increased Fuel Efficiency

The burning of fossil fuels from mobile sources generates greenhouses gases, including carbon dioxide. The generation and emission of these gases into the atmosphere trap heat by preventing a significant portion of infrared radiation from escaping the atmosphere. Vehicles with lower fuel economy burn more fuel and therefore produce greater amounts of greenhouse gases and carbon dioxide. Every gallon of gasoline burned by a passenger vehicle emits, on average, 20 pounds of carbon dioxide into the atmosphere. For example, a vehicle that averages 25 miles per gallon compared to one that averages 20 miles per gallon will emit 15 fewer tons of greenhouse gases over the lifetime of the vehicle.

Congress has mandated that the National Highway Transportation and Safety Administration be responsible of the establishing and enforcing fuel economy standards according to the Corporate Average Fuel Economy (CAFÉ) standards. CAFÉ is the sales weighted average fuel economy, expressed in miles per gallon, of a manufacturer’s fleet of passenger cars or light trucks with a gross vehicle weight rating (GVWR) of 8,500 pounds or less, manufactured for sale in the United States, for any given model year. Current CAFÉ standards are 27.5 miles per gallon for passenger cars and 20.7 miles per gallon for light trucks. Proponents of these standards would like to see the standards raised to 40 miles per gallon for both cars and light trucks and argue that this increase in efficiency translates to less emissions from mobile sources and reduces reliance on foreign sources of oil. Opponents of the standards, including the current administration, sight safety concerns and economic costs incurred by manufacturers as reasons not to increase the CAFÉ standards.

On May 17, 2001, the Energy Policy Development Group, led by Vice President Cheney, issued its National Energy Policy. This report made recommendations to President Bush regarding the path that the administration’s energy policy should take and included specific recommendations regarding vehicle fuel economy and CAFE. The report recommends that the President direct the Secretary of Transportation to:

  • Review and provide recommendations on establishing Corporate Average Fuel Economy (CAFE) standards with due consideration of the National Academy of Sciences study to be released in July 2001. Responsibly crafted CAFE standards should increase efficiency without negatively impacting the U.S. automotive industry. The determination of future fuel economy standards must therefore be addressed analytically and based on sound science.
  • Consider passenger safety, economic concerns, and disparate impact on the U.S. versus foreign fleet of automobiles.
  • Look at other market-based approaches to increasing the national average fuel economy of new motor vehicles.

Hybrid and alternative fuel vehicles

Several technologies are currently available that can greatly increase fuel efficiency in passenger cars. Other alternative fuel technologies use non-gasoline sources to generate power in the vehicle.

The most successful entry of high efficiency vehicles into the consumer marketplace has taken place with so called hybrid electric vehicles. Hybrid vehicles combine a small and efficient gasoline engine with an electric engine to maximize fuel efficiency. Furthermore, hybrid vehicles have batteries onboard that allow energy generated from braking and the electric engine to be stored for use in high demand situations. Because hybrids combine technologies, they can be used identically to regular passenger cars and require no special fuel considerations. Current examples of these vehicles that are available to the consumer are the Honda Insight, Honda Civic Hybrid, and Toyota Prius. According to the EPA these vehicles average between 48 and 56 miles per gallon.

In an effort to drive consumer interest in this technology, Congress has created a temporary one-time federal income tax deduction of up to $2,000 for individuals purchasing a hybrid vehicle through 2003. The deduction is currently set to decline thereafter, until it is phased out in 2006.

Alternative fuel vehicles use sources other than gasoline to generate power. Alternative fuel technology currently includes flex-fuel vehicles, bi-fuel vehicles, compressed natural gas, liquefied petroleum gas, diesel vehicles, and electric vehicles. While these technologies, with the exception of diesel, offer an alternative to gasoline, they remain unattractive to the consumer because the infrastructure to deliver these fuels remains limited. Diesel is not generally accepted as a positive alternative to gasoline because of the high level of particulates emitted by diesel engines.

Mass transit and other forms of transportation

Choosing alternative forms of transportation will limit fuel consumption and reduce the amount of pollutants emitted into the atmosphere. The Federal Transit Administration highlights the benefits of the Commuter Choice program. This program details the benefits that employers can offer employees to commute to work by methods other than driving alone.