Our focus on greenercars.org 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 US 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 on Greenercars.org.

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 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 a 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 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 US 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 where 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.