guide to green

Automobiles and the Environment

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. Over the dozen or so years of a vehicle's life, nearly 90 percent of lifecycle ("cradle to grave") greenhouse gas production for a typical automobile is due to fuel consumption.

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 after "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).

Petroleum products now provide 96 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. The 1991 war with Iraq was directly related to securing our oil supply. The tragic situation in which the United States finds itself since September 11, 2001, presents many grave challenges for national defense and security. Choosing more fuel-efficient vehicles to reduce our reliance on a world oil market in which Middle Eastern countries play a dominant role is one way we, as individuals, can assist in energy-related aspects of national security.

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. Over 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 gasoline price run-ups of the past two years are just the latest examples 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 economic costs and moral liabilities of oil dependence.

Major Pollutants Associated with Automobiles

Our focus in ACEEE's Green Book® 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 stricter standards are now commonly available 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 Book®.

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 tailpipes 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, and inventories show a modest decline in NOx emissions over the past five years. Transportation-related NOx emissions continue to increase, however, preventing faster progress overall.

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, the benzenes found in gasoline are carcinogens.

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 and scrappage facilities are at the highest risk. When vehicles are scrapped, bioaccumulative toxins such as lead, chromium, and mercury make their way into the soil, water, and air where they can last for a long time and build up in our bodies and those of other organisms. Vehicles also emit toxics in use, due to fuel evaporation while the tank is being filled or while the car sits in the sun, for example, as well as toxic emissions 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 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 the 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 four other countries in the world (China, Russia, Japan, and India). Our vehicles produce nearly as much CO2 as all of India, which has more than triple our population. U.S. cars and trucks emit more than 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.

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 a vehicle's fuel economy, but standards are weaker for many gas-guzzling light trucks. Moreover, 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. Upstream emissions associated with a vehicle are proportional to its fuel consumption. For an average car, upstream hydrocarbon and PM10 emissions are about twice the tailpipe emissions of those pollutants. Lesser but still-significant amounts of other pollutants are also related to the amount of fuel burned. Examples include NOx from tanker trucks delivering gasoline and a whole soup of pollutants from oil refineries. Thus, higher fuel consumption implies greater pollution.

Efficiency and Safety

Other things being equal, a smaller, lighter vehicle is more fuel efficient and 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 generally inflicts higher forces on the occupants of the lighter vehicle. Thus, while individuals may gain a measure of protection by driving heavier vehicles, they do so at others' expense. In and of itself, weight does not improve overall safety. Any type of vehicle, including a small car, can be safe when it has well-designed features that improve occupant protection (such as stable, energy-absorbing structures), and properly used occupant restraints (like seat belts, air bags, and child safety seats).

Many of these safety-enhancing features are reflected in crash-test scores and the related safety ratings published by National Highway Traffic Safety Administration (NHTSA) and several consumer publications which we list at the end of this section. These safety ratings are particularly helpful for identifying vehicles that have superior structures, since such features are engineered into a vehicle and are not obvious based on size, shape, or body style.

Better vehicle structures absorb crash energy, cushioning occupants from the severity of an impact. Well-designed structures also act as a "safety cage" that protects occupants from being crushed or otherwise injured by intruding parts of a crashing vehicle. Seat belts protect occupants from striking the inside parts of a vehicle or being thrown out of it. Using seat belts doubles the chance of surviving a serious crash in any vehicle, and air bags further enhance that protection. Improved stability lowers the risk of rollover, a form of accident associated with high fatalities and serious injuries. All of these design features improve safety without adverse trade-offs.

Large and heavy personal vehicles are a mixed bag when it comes to safety. For one thing, heavier vehicles are a greater menace to others on the road, including pedestrians, bicyclists, motorcyclists and occupants of smaller vehicles. Sport utility vehicles (SUVs) illustrate the fallacy of "larger is safer" simplistic thinking. While SUVs are heavier on average than passenger cars, they don't necessarily have lower fatality rates. Not only are SUVs more hazardous to others on the road, they threaten their own occupants with higher rollover risks. Many SUVs are less stable than passenger cars and provide poor occupant protection in rollovers. Pickup trucks have the same safety deficiencies as SUVs and many vans are only moderately better. Newer "crossover" body styles, such as all-wheel-drive wagons and sport wagons, reduce some of the safety liabilities of traditional SUVs. In general, because they are detrimental to both safety and the environment, SUVs should be avoided by consumers without an ongoing need for their larger power, capacity, or off-road ability.

The best guidance is to check the safety ratings as well as the Green Scores of models that you are considering, so that you can find the safest and greenest vehicle that meets your needs. The government safety ratings measure how well a car or truck protects its occupants in various crash tests; recently added rollover ratings indicate how stable a vehicle is. The ratings do not tell the whole story about vehicle safety, since they fail to account for how harmful a vehicle is to others on the road and how well it protects its own occupants in a rollover crash. Nevertheless, crash-test scores are a good comparison of the relative safety of vehicles within a given size class. Look for the "Buying a Safer Car" feature on NHTSA's website ( Additional advice, information on crashworthiness, and descriptions of safety features by make and model are provided by The Ultimate Car Book and Consumer Reports.

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How to buy green

Sorting out Standards

EPA Fuel Economy Estimation (Cars and Light Trucks)

Why Buy Green?

Automobiles and the environment

Major Pollutants Associated with Automobiles

Cars, Trucks, and Global Warming


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