Automobiles affect the environment in many ways. Impacts begin when a vehicle is manufactured (including the production of all the parts and materials that go into the car) and end with its scrappage in a junkyard (which can recycle many parts but also involves the disposal of many wastes). Over the life of an average motor vehicle, however, much of the environmental damage occurs during driving and is greatly associated with fuel consumption. The figure on the following page illustrates the breakdown of lifecycle ("cradle to grave") energy use for a typical automobile. Nearly 90 percent is due to fuel consumption over the dozen or so years of a vehicle's life.

Environmental impacts start with mineral extraction and the production of the raw materials that go into the parts of a car. For example, iron ore gets turned into steel, which now accounts for most of the mass in vehicles. Steel can be recycled, of course. On average, today's automobiles are about 75 percent recyclable, and using recycled steel helps reduce energy use and pollution. Other metal components, such as aluminum (used in some engine parts and wheels, for example) and copper (used for wiring) are also largely recycled. The lead and acid in batteries are poisonous and dangerous. But batteries can be recycled, if they are returned to a service station, a parts store, or brought to a municipal hazardous waste facility. Plastics, which are mostly made from petroleum, are more difficult to recycle. In any case, some degree of pollution is associated with all of these components, much of it due to the energy consumption, air pollution, and releases of toxic substances that occur when automobiles are manufactured and distributed.

Most of the environmental impact associated with motor vehicles occurs when they are used, due to pollution in their exhaust and pollution associated with supplying the fuel. In the United States, nearly all of today's automobiles use gasoline; a lesser number use diesel fuel. In some areas, various alternative fuels are being introduced, but these are not widely available for most drivers. When gasoline, diesel, or other fuels are burned in car engines, combustion is never perfect, and so a mix of hazardous pollutants comes out the tailpipe.

If combustion were perfect and didn't create noxious by-products, the exhaust would contain only water vapor and carbon dioxide. Carbon dioxide (CO2) isn't directly harmful to health, at least not in low concentrations. After all, CO2 is also what we exhale as a result of our "burning" the calories in the food we eat. However, CO2 from fossil fuels like gasoline and diesel is very harmful to the environment because it causes global warming--more on this pollutant shortly.

Motor fuel is itself a product and so, like a car, environmental damage occurs throughout its lifecycle as well. For gasoline and diesel, the product lifecycle begins at the oil well and ends when the fuel is burned in the engine. Fuel cycle impacts are the forms of pollution and other environmental damage that occur between the oil well and the fuel tank. Gasoline and diesel fuel are poisonous to humans, plants, and animals, and their vapors are toxic. Other energy sources have their own fuel cycles. With battery-powered electric vehicles, for example, no fuel is burned onboard the vehicle, and so nearly all of the fuel-cycle pollution and energy use occurs at electric power plants and in producing the fuels that run the power plants. Many of the same air pollutants that spew from vehicle tailpipes are also spewed from power plants and oil refineries (as well as the tanker trucks that deliver gasoline to your local filling station).

Gasoline and diesel fuel now provide 97 percent of America's transportation energy needs. Air pollution isn't the only problem associated with these petroleum-based fuels. Oil extraction lays waste to many fragile ecosystems, harming tropical forests in South America and Southeast Asia, deserts and wetlands in the Middle East, our own coastal areas, and the fragile tundra and arctic coastal plains of Alaska. Millions of gallons of oil are spilled every year. Sometimes the disasters are well known, such as the 1989 Exxon Valdez spill in Prince William Sound. More often there are rarely reported but still tragic smaller spills that occur in the oceans and in coastal waters, bays, and rivers throughout the world. In our own communities, groundwater is sometimes tainted by leaks from underground fuel storage tanks and miscellaneous spills that occur during shipping and handling of the 120 billion gallons of fuel we use each year.

In addition to these environmental harms, gasoline and diesel consumption bring economic and security risks. The Middle East contains the largest concentration of the world's oil. The United States maintains a global military presence partly to maintain access to foreign oil. Most recently in the 1991 war with Iraq, but several times throughout our history, U.S. troops were committed and blood was spilled to secure our oil supply.

Major recessions were triggered by oil crises in the 1970s and early 1980s, causing unemployment and inflation. Oil imports drain over $50 billion per year from American pockets, representing lost job opportunities even when our economy seems to be doing fine. Half of U.S. oil is now imported and our dependence on foreign sources is steadily rising, perpetuating the risk of future oil crises. The past year's run-up in gasoline prices is just the latest example of how petroleum dependence can squeeze family budgets only to enrich oil producers.

Our addiction to gasoline and diesel fuel also involves moral compromises. It entails deals and economic arrangements with some oil-rich countries whose standards of human rights and environmental protection may not be the same as what we expect at home. Of course, these issues go beyond strictly environmental concerns. Nevertheless, choosing greener vehicles that consume less fuel not only protects the environment, but also helps protect U.S. jobs while reducing the security risks and moral liabilities of oil dependence.

Major Pollutants Associated with Automobiles

Our focus in ACEEE's Green BookTM is on air pollutants related to car and truck fuel consumption, because they are such a large part of a vehicle's environmental damage and because they are the main impacts that can be reduced through your choice of make and model. The adjoining figure shows the amounts of major air pollutants caused by the average new passenger car and light truck in a year. The pollution coming from vehicles can differ depending on the standards they meet (and how well their emissions controls work), how they are driven and maintained, and the quantity and quality of the fuel they burn. Many vans, pickups, sport utilities, and other light trucks meet less stringent emissions and fuel economy standards than vehicles classified as passenger cars. As a result, and as the ratings in this guide indicate, the average light truck pollutes more than the average car.

All new vehicles must meet either the emissions standards set by the U.S. Environmental Protection Agency (EPA) or those set by the California Air Resources Board (CARB). Generally, California standards are more stringent than the Federal standards. A number of Northeastern states have adopted the California standards, and vehicles meeting these standards are becoming more and more common nationwide.

Vehicles are also tested for fuel economy, as measured by miles per gallon-MPG. Fuel economy standards apply to manufacturers, rather than individual vehicles, and are set so that an automaker can sell a variety of vehicles as long as the average MPG of its sales meets the applicable standard. Manufacturers calculate the fuel economy of each model they sell using laboratory tests similar to those conducted to determine tailpipe emissions. Because these tests give fuel economy estimates higher than what most people experience in real-world driving, the MPG measurements are adjusted downward by EPA. These adjusted MPG numbers are printed on new vehicle stickers and listed here in ACEEE's Green BookTM.

Although a wide variety of pollutants are formed in the various stages of an automobile's lifecycle, our ratings are mainly based on the serious air pollutants that are regulated to control vehicle emissions. All of these pollutants are more damaging to health when emitted from vehicle tailpipes than when a similar quantity is emitted from a power plant, since tailpipe pollution is literally "in your face," subjecting people to more direct exposures during daily activities.

Particulate Matter (PM)

Fine airborne particles are an established cause of lung problems, from shortness of breath to worsening of respiratory and cardiovascular disease, damage to lung tissues, and cancer. Certain people are particularly vulnerable to breathing air polluted by fine particles, among them asthmatics, individuals with the flu and with chronic heart or lung diseases, as well as children and the elderly. PM also soils and damages buildings and materials. It forms haze that obscures visibility in many regions. Soot and smoke coming from exhaust pipes are obvious sources of PM, but among the most deadly forms of airborne particulate matter are the invisible fine particles that lodge deeply in the lungs. PM has been regulated for some time, but the regulations were based on counting all particles up to 10 microns in size (PM10). However, PM10 standards fail to adequately control the most dangerous, very fine particles. The U.S. EPA has recently started to regulate fine particles up to 2.5 microns in size (PM2.5), which better focuses on the most damaging category.

Properly functioning new, fuel-injected gasoline vehicles directly emit very little PM2.5. But they indirectly cause significant PM pollution as a result of their NOx, SO2, and HC emissions, not only from tailpipe but also from vehicle manufacturing and fuel refining (see below). These emissions result in "secondary" particle formation. This phenomenon refers to the way that the gaseous pollutants agglomerate ("glom up") at microscopic scales to form fine particles that are largely invisible but cause the health problems mentioned. Transportation sources account for about 20 percent of directly emitted PM2.5. Diesel engines are the major source of direct PM emissions from motor vehicles. Although most such emissions come from heavy trucks and diesel buses, even the smaller diesel engines in some cars and light trucks emit significant amounts of fine PM.

Nitrogen Oxides (NOx)

NOx refers mainly to two chemicals, nitrogen oxide (NO) and nitrogen dioxide (NO2) that are formed when nitrogen gas, which comprises 78 percent of air, reacts with oxygen during the high temperatures that occur during fuel combustion. NOx is truly a noxious pollutant in many ways. It is directly hazardous, an irritant to the lungs that can aggravate respiratory problems. It reacts with organic compounds in the air to cause ozone, which is the main reason for "smog alerts" that still happen too often in many cities and regions. NOx is a precursor of fine particles, which cause respiratory problems and lead to thousands of premature deaths each year. It is also a precursor of acid rain, which harms lakes, waterways, forests, and other ecosystems, as well as damaging buildings and crops. Airborne NOx also contributes to nitrification-essentially an over-fertilization-of wetlands and bays, leading to algae blooms and fish kills.

As an air pollutant, NOx is one of the most difficult to control since it is such a pervasive product of combustion. Nationwide, most NOx comes from electric power plants and industrial sources. Natural gas and oil-fired home furnaces and water heaters also produce NOx in their flue gases. Motor vehicles account for about one-third of nationwide NOx emissions. Many of these emissions come from heavy-duty diesel trucks, but cars and light trucks are also a major source. NOx has also been one of the most difficult pollutants to get out of our air. EPA air quality regulations have helped keep emissions from growing as fast as they might have, but nationwide, overall NOx emissions are higher than they were a decade ago. A good portion of this growth in NOx pollution has been from cars and light trucks.

Sulfur Dioxide (SO2)

Gasoline and diesel fuels also contain varying amounts of sulfur, which burns in the engine to produce sulfur dioxide (SO2). This gaseous chemical is another source of secondary particulate formation, and is itself a lung irritant as well as a cause of acid rain. SO2 also interferes with the operation of catalytic converters. Some of the cleaner, reformulated versions of gasoline have very low sulfur levels. Most gasoline sold nationwide still has too much sulfur, but levels are being reduced under recently established EPA regulations.

Cars and light trucks are not the largest source of SO2 emissions, which come mainly from power plants and industrial facilities. However, because cars and light trucks are so numerous and gasoline has a high average sulfur content, cars and light trucks cause twice as much fine PM pollution as heavy freight trucks. Making all gasoline as clean as the cleaner, low-sulfur fuels already available in California would greatly reduce this PM pollution from all cars and trucks on the road, both new and used.

Hydrocarbons (HC)

Hydrocarbons are a broad class of chemicals containing carbon and hydrogen. Those hydrocarbons that cause various forms of air pollution are also known as volatile organic compounds since they are forms of HC that are either gases or readily evaporate into the air. Many forms of HC are directly hazardous, contributing to what are collectively called "air toxics." These compounds can be directly irritating to the lungs and other tissues and they can also cause cancer, contribute to birth defects, and cause other illnesses. During daylight hours, and particularly during hot summer weather, HC reacts with NOx to form ozone smog (see box below). Controlling ozone is one of the major environmental challenges in the United States. Although progress has been made over the past several decades, many cities and regions still have smog alerts when ozone levels get too high.

Gasoline vapor contains a mix of hydrocarbons. Thus, HC pollution is produced whenever we fill our tanks. Some regions have special nozzles on fuel pumps to help trap such vapors. Other HC vapors are released at various stages along the way from the refinery to the filling station. Vapors seep out, even when a car is parked and turned off, due to the imperfect sealing of the fuel tank, pipes and hoses, and other components leading to the engine. HC also comes out of the tailpipe, as a result of incomplete combustion and the less-than-perfect cleanup of exhaust gases by catalytic converters and other vehicle emissions controls. Diesel fuel is less volatile than gasoline, so evaporation is less of a problem. Nevertheless, diesel exhaust still contains many toxic hydrocarbons and other compounds. Overall, transportation is responsible for about 36 percent of man-made HC emissions in the United States.

Ozone: Helpful in the Stratosphere, But Harmful in the Air We Breathe

Ozone (O3) is a highly reactive form of oxygen that occurs naturally in various parts of the atmosphere but gets artificially produced in dangerously high concentrations due to emissions from cars, trucks, and other combustion sources.

Up in the stratosphere, ozone helps protect us from ultraviolet radiation. Loss of this protective ozone layer at high altitudes can lead to increased skin cancer. Such concerns have led to restrictions on ozone-depleting chemicals such as those once found in some spray cans and others that have been phased out of use in refrigerators and air conditioners (including automotive air conditioners).

Down in the lower atmosphere, in the air the we breathe, ozone is a health hazard. It is the main ingredient of the smog that causes pollution alerts in many cities around the country. Ozone produced by pollution at low altitudes is of no help in restoring the protective ozone layer at high altitudes. Inhaling air polluted by ozone damages the lungs, reduces breathing ability, and makes us more susceptible to other respiratory problems. Ozone can be deadly to individuals with asthma and other lung conditions, as well as to people with heart conditions. It is also harmful to both adults and children who are otherwise healthy. The risks of shortness of breath, chest pain, lung congestion, and other symptoms caused by ozone are the reasons why public health officials warn us to stay inside and avoid strenuous exercise on severe air pollution days.

Although cars and trucks do not directly emit ozone, they are a major cause of ozone smog. They add to the amount of HC in the air, and tailpipe NOx reacts with HC to form ozone. Cities without major industries and power plants still have serious smog problems, mostly caused by pollution from cars, trucks, and vans. Although many U.S. cities are seeing better air quality, we'll have to do better at cutting motor vehicle pollution to ensure progress.

Toxic Chemicals

Toxic releases are just that -- any number of a wide range of chemicals that can cause cancer, birth defects, cardiovascular, respiratory, and neurological damage, or other forms of health harm. Many smog-forming hydrocarbons are directly toxic; for example, benzene is a known human carcinogen. Other toxics include solvents and metallic compounds such as lead and chromium. Toxics are released during many industrial activities and car and truck manufacturing is a significant source. Workers and communities near factories are at the highest risk. Vehicles also emit toxics, due to fuel evaporation while pumping gas and while a car sits in the sun, for example, as well as from the tailpipe. Diesel exhaust, in particular, has been implicated as a harmful toxic release.

Toxic emissions from cars and trucks, as well as toxic releases during the production and assembly of vehicles and their components, are controlled by various regulations. Factories and other manufacturing facilities are required to report toxic emissions from each site. But controls are far from perfect, and there are many ways in which industry could do a better job of preventing toxic pollution. You can find out the source and amount of toxics that are emitted in your community from the Environmental Defense's toxic pollution Scorecard.

Carbon Monoxide (CO)

Carbon monoxide is an odorless, colorless, but potentially deadly gas that is created by the incomplete combustion of any carbon-containing fuel, including gasoline and diesel. When inhaled, CO combines with the hemoglobin in our blood, impairing the flow of oxygen to our brain and other parts of the body. We've all heard stories of people being killed by carbon monoxide poisoning, from vehicles in closed garages, during fires, or in homes when indoor CO concentrations are raised by malfunctioning stoves or furnaces. Even if it doesn't cause death, CO exposure can cause permanent damage to the nervous system. At lower concentrations, CO is still harmful, particularly for people with heart disease. In some areas, cars and trucks can create enough CO to cause health risks outdoors.

Large amounts of CO are produced when a vehicle first starts up and its engine is cold. Poorly designed and malfunctioning engines and emission controls systems are also responsible for excess CO pollution. Motor vehicles are responsible for about 60 percent of CO emissions nationwide.

Cars, Trucks, and Global Warming

The gasoline-powered automobile was invented just over 100 years ago, when the industrial revolution was still young. Streams had long been dammed to turn mills, and coal was on its way to widespread use-it was already powering steamships and locomotives. But most energy used by humans still came from traditional fuels such as wood. In 1890, the world population was about 1.5 billion but growing rapidly. The amount of carbon dioxide (CO2) in the atmosphere was just over 290 parts per million, not yet noticeably over its level throughout pre-industrial civilization.

The world population has now topped six billion and is still growing rapidly. During the past century, the amount of fossil fuel we consume has risen nearly five times faster than population. As a result, the amount of CO2 in the atmosphere is now over 360 parts per million and climbing. This rapid increase in CO2 concentration represents an enormous impact of our energy-consumptive lifestyle on the planet, and it is causing dangerous changes to the earth's climate. The past decade has already seen many years with above-normal temperatures. The changes in weather patterns and increases in severe events are consistent with climate disruption. Recent years have been among the warmest ever recorded.

Carbon dioxide is the most important of what are known as greenhouse gases, compounds that enable the earth's atmosphere to trap heat, like a greenhouse, but on a global scale. Too much greenhouse gas in the atmosphere causes global warming, an increase in global average temperatures above what they normally would be.

The risks of global warming are many. Human health is threatened by more frequent and severe heat waves and the spread of tropical diseases. Lives can be lost because of rising sea levels and more severe storms, which can also damage regional and national economies. The disruptions to climate are unpredictable but certainly risky. While some areas may see greater coastal flooding and inundating rains, other regions may experience droughts. Both agriculture and natural habitats can be harmed. Future generations will bear the brunt of these risks, but the effects of global warming have already been detected. Although we cannot attribute any given event to climate change, the increased risks have created a call for action to curtail CO2 emissions around the world.

Oil is now the world's dominant fuel. There are over 600 million cars and trucks in the world. Both here and abroad, transportation accounts for most oil use. In the United States, we now have more motor vehicles than licensed drivers, and we travel over 2 trillion miles per year, burning 120 billion gallons of gasoline. Not counting the "upstream" emissions from producing the fuel, the result is over a billion tons of CO2 pollution each year.

U.S. cars and light trucks alone account for more energy-related CO2 than the nationwide emissions of all but three other countries in the world (China, Russia, and Japan). Our vehicles produce more CO2 than all of India, which has more than triple our population. U.S. cars and trucks emit twice as much fossil-fuel CO2 as the economies of either South Korea or Mexico and over three times as much as the whole of Brazil. Although some of these countries are growing and industrializing rapidly, it will be decades before their level of CO2 pollution per person approaches ours.

The Kyoto Protocol

In December 1997 the nations of the world gathered in Kyoto, Japan, to negotiate a treaty to control emissions of greenhouse gases that are causing global warming. The resulting agreement is known as the Kyoto Protocol. In it, the United States and other wealthier, developed nations-who have produced most of the world's global warming pollution-agreed to significantly reduce their emissions over the next 10 to 15 years. Greener vehicles will be essential for accomplishing that goal. For further information on government actions needed to address global warming, contact the Climate Action Network.

Fuel Economy and Air Pollution

The amount of CO2 emitted by a vehicle is essentially proportional to the amount of fuel burned. Thus, fuel-efficient vehicles are the best choice for helping to stop global warming. And gas guzzlers are global polluters.

For other forms of air pollution, the relation between fuel economy and emissions is more complex. Automobile emissions are regulated to a given number of grams per mile, independently of how much fuel they burn (although standards are weaker for many gas-guzzling light trucks). But several factors cause NOx, HC, CO, and PM pollution to be higher when a vehicle's fuel economy is lower.

In real-world use, most vehicles' emissions are much higher than the standards levels. The reasons include the fact that automakers' and EPA's emissions tests fail to fully represent real-world driving, malfunction of emissions control systems, deterioration of components, inadequate or incorrect maintenance, and sometimes tampering. A portion of this excess pollution is proportional to a vehicle's rate of fuel consumption. Automobiles that meet a more stringent emissions standard are generally cleaner than those that meet a less stringent standard. However, among vehicles that meet the same standard, those with higher fuel economy generally produce less air pollution.

A significant amount of pollution also occurs in supplying vehicles with fuel. These so-called upstream emissions occur everywhere from the oil well and refinery to the filling station and gas tank, before the fuel gets to the engine. The relationship between fuel consumption and upstream emissions is strongest for hydrocarbons (HC). For an average car, about 11 grams of excess HC pollution (beyond what comes out of the tailpipe) occurs for every gallon of gasoline burned. Lesser but still-significant amounts of other pollutants are also related to the amount of fuel burned. Examples include NOx and PM from tanker trucks delivering gasoline and a whole soup of pollutants from oil refineries. Thus, higher fuel consumption implies higher upstream pollution.

Efficiency and Safety

Other things being equal, a smaller, lighter vehicle is more fuel efficient and therefore less polluting than a larger, heavier vehicle. But are smaller, lighter vehicles less safe? The answer is more complex than one might think. In a two-car collision, occupants of the heavier vehicle are typically subjected to lower crash forces. However, the heavier vehicle will inflict higher crash forces on the occupants of the lighter vehicle. Thus, any protective benefits of mass itself, if any, come at the expense of harm to others.

While individuals may protect themselves by driving heavier vehicles, they then become a greater menace to others on the road-including pedestrians, bicyclists, and motorcyclists as well as occupants of smaller vehicles. Thus, public safety is not likely to be enhanced by a shift to heavier vehicles. Safety is enhanced by design features that improve occupant protection, such as energy-absorbing structures and passenger restraints such as seat belts. Using seat belts doubles the chance of surviving a serious crash in any vehicle. Air bags, better vehicle structures, and improved stability (designs less prone to rollover) all improve safety far more than added mass.

The fallacy of simplistic "larger is safer" thinking is well illustrated by sport utility vehicles. While large SUVs have low fatality rates (though not quite as low as some cars, in spite of the fact that they are heavier), large SUVs are deadly to other people on the road. Small SUVs tend to handle poorly, are prone to rollover, and have among the highest accident rates. Thus, SUVs are not as safe as some people believe, and neither are large pickup trucks and vans.

Evidence from accident statistics now indicates that the rising popularity of large SUVs and pickups is contributing to increased traffic fatalities and serious injuries. Because of their higher bumpers, more rigid structures, and heavier mass, these light-truck class vehicles now pose a serious threat to the occupants of passenger cars. Even though cars still outnumber light trucks on the road by two to one, many more people are killed when light trucks collide with cars than in two-car collisions. In short, SUVs and other large light trucks harm both public safety and the environment, and are more costly to own and operate. Avoid these vehicles unless you truly have an ongoing need for their larger power and capacity.

Government crash-test scores measure how well a given vehicle protects its occupants. However, the scores do not tell the whole story about vehicle safety, since they fail to account for vehicle aggressivity-how harmful a vehicle is to others on the road. Nevertheless, crash-test scores are a good comparison of the relative safety of vehicles within a given size class. Well-designed vehicles that provide better crashworthiness without high aggressivity can protect their occupants without imposing greater risks on others. Crash test scores are published by the National Highway Traffic Safety Administration (NHTSA) and, along with discussions of other auto safety features, are covered by a number of publications. Advice, information on crashworthiness, and listings of safety features by make and model are provided by The Ultimate Car Book and Consumer Reports.

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