Alternative transportation is defined as a mode of transportation that does not require the use of conventional petroleum-based fuel sources, such as gasoline or diesel. Mass transit vehicles that do not use petroleum-based fuel, such as some trolley cars and buses, are considered alternative forms of transportation. No-emission forms of transportation, such as walking or bicycling, are also classified as forms of alternative transportation.
Other forms of alternative transportation include railroads, bus lines, subways, trolleys, and group commuter travel vans that are powered by alternative, nonpolluting fuels.
Alternative motor vehicles, agricultural equipment, airplanes, trains, and ships have modified engines that use alternative fuels, which are specifically manufactured to reduce the emissions typically found with the burning of traditional diesel or gasoline. These lowered emission fuels include electricity, biodiesel, compressed fossil natural gas (a combustible mixture of hydrocarbon gases, consisting of mostly methane), liquefied fossil natural gas, alcohol-based fuels, and hydrogen.
The purpose of using alternative transportation is to decrease the numbers of vehicles that use polluting forms of fuel, such as petroleum-based fuel sources, and their resulting emissions. The use of alternative forms of transportation decreases the dependence on crude oil as a fuel source. Gasoline is refined from crude oil. Domestic oil production has been declining since the 1970s, while imported oil to the United States has increased 67%.
Emissions generated from traditionally fueled vehicles are considered to be the major cause of air pollution throughout the world. The 2008 World Health Organization (WHO) report on air quality and health states that even relatively low concentrations of air
pollution may have negative health effects on humans, animals, and plants. More than 113 million Americans live in cities where air quality is poor. As air quality declines, the incidence of human disease increases. Air pollution has been linked to about 800,000 annual fatalities from respiratory and cardiac-related diseases.
Environmental experts believe that a shift to alternative fuels and vehicles could lower emissions, provide cost savings over time at an individual and governmental level, and provide protection from political and geographical conflicts over dwindling traditional oil resources.
Recent increases in oil prices are believed to be indications of a dwindling global oil supply. Projections of increasing future costs as supplies decrease are universally accepted. Renewed research by private and governmental agencies into the use of alternative vehicles and alternative fuel sources is seen as a potential way to lower air pollution levels and decrease the United States' reliance on foreign oil imports.
Once considered a problem only within developed countries, the need for low-emission transportation is rapidly expanding in developing countries. Despite the lack of uniform global data on traffic-generated air pollution in developing countries, many areas of the world have moved increasingly to the use of cars, buses, and taxicabs powered by alternative or multiple fuel sources.
In many urban areas of the world and in most suburban and rural areas, mass transit systems are not available. Without access to railways, commuter bus lines, trolley cars, or subways, residents often rely on individual vehicles for transportation.
In 1787, John Fitch of the United States launched a 45-foot steamboat in the Delaware River.
In 1832, Robert Anderson of Scotland built the first electric car.
In 1880-1905, electric trolleys ran in Washington and Boston.
The German inventor of the diesel engine, Rudolph Diesel, unveiled the engine at the 1898 World Exhibition in Paris and used peanut oil for fuel. All diesel engines ran on vegetable oil until the 1920s, when petroleum prices became cheaper than that oil.
In 1908, Henry Ford's Model T ran on ethanol, gasoline, or a combination of the two.
In the 1970s, when air pollution started to become a concern, the first major fuel shortage since World War II occurred. Electric cars were made available in limited numbers to the consumer by private manufacturers.
In 1992, the Energy Policy Act (EPAct; Public Law 102-486) was passed to reduce the United States' dependence on foreign oil and to improve air quality. The act also encouraged the use of alternative fuel vehicles in metropolitan areas by requiring annual purchases of government vehicles capable of running on alternative fuel sources. The U.S. Department of Energy (DOE) still maintains this directive.
In 1998, the National Highway Traffic Safety Administration officially recognized electric cars as legal for transportation use on American roads. Major auto manufacturers have produced alternative fuel cars for the mass market.
In response to the government's support of alternative options, in November 2004, the Jobs Creation Bill HR 4520 initiated a federal excise tax credit for the production of biodiesel, an alternative low-emissions fuel. By the end of 2005, biodiesel production had increased 300% in one year to 75 million gallons.
In 2006, the American-owned company Vectix launched the first electric scooter, called the Maxi-Scooter.
In 2007, the U.S. Environmental Protection Agency (EPA), under the Energy Independence and Security Act, began implementing regulations to require a minimum volume of renewable fuel to be contained in all gasoline sold in the United States. Renewable fuel blends will increase from 9 billion gallons in 2008 to 36 billion gallons by 2022. Although some states, such as Texas, requested a waiver from a portion of the required standards, the 2009 requirement has been set by the EPA to be a minimum of 10.21% of the total gasoline sold by American refiners, importers, and blenders.
In 2009, President Barack Obama encouraged Congress to pass the American Clean Energy and Security Act, which he believed would limit greenhouse gas emissions, create clean energy jobs, and help transition the country to clean energy, such as wind and solar power. The bill would place the first national limits on emissions of greenhouse gases from power plants, factories, and oil producers. The bill would require the United States to lower its emissions by 17% by 2020 and by 83% by 2050. Opponents argue that this bill creates a tax on industry and will drive American companies to cheaper regions of the world, where air-quality controls are less stringent. Narrowly passed by the House of Representatives by a 219-212 vote, the bill requires consideration by the Senate.
General: Self-powered transportation includes walking, skateboarding, in-line skating, and bicycling. The U.S. Public Health Service is a strong advocate of self-propelled commuting.
Walking: According to researchers at the U.S. Centers for Disease Control and Prevention (CDC), walking as a form of transportation for adults has increased in the United States from 16.7% in 1995 to 21% in 2001. Walking to school (one mile or less) for youths has increased from 31.3% in 1995 to 35.9% in 2001.
Walking as a transportation method is hampered by logistical issues, such as safety, distance, and weather. In some areas, lack of safe crosswalks or sidewalks limits use, as do long-distance commutes. In some areas of the world, extreme weather conditions, such as subzero temperatures during winter months, also restrict the potential use of walking as a consistent form of transportation.
Skating: In some urban areas, restrictions have been placed on the use of skateboards and in-line skates. Local ordinances frequently ban the use of these items in sidewalk areas, commercial building zones, and school areas due to the potential of pedestrian injury.
Bicycling: Bicycles are commonly used as a form of commuter transportation in some areas of the world, such as Southeast Asia. In 1950, there were 11 million bicycles and eight million cars worldwide. By 2007, there were 130 million bicycles and 52 million cars.
In the United States,
Chicago has set a goal that five percent of all trips less than five miles should be accomplished by bicycle by 2015. New York has built 420 miles of bike paths.
Cities such as Portland, Oregon, have addressed the transit issue by upgrading a regional transportation system to its downtown area through light commuter train railways, a bus network, and a streetcar system. Two hundred twenty-one miles of bikeways allow walkers and cyclists to commute in a designated lane, away from dangerous roadways and intersections.
According to consumer advocates, not enough state or federal funding is invested into providing walkers and cyclist access to safe and appropriate commuter pathways.
Types of alternative fuels
General: Alternative fuel vehicles are available worldwide and include alternative fuel-run vehicles, modified diesel engines, modified electric vehicles, and scooters.
According to consumer information supplied by the Oregon Department of Energy, less-polluting alternative fuels are increasingly available to the consumer. Biodiesel is the most well known in the United States.
can be made from barley, soy, peanut, hemp, rapeseed, palm, sorghum, canola, sunflower, poultry fat, fish, or cooking oil.
The use of biodiesel reduces carbon dioxide and carbon monoxide emissions. It is biodegradable, meaning that it can be broken down by living organisms without harm to the environment. It can be used in traditional standard diesel engines of automobiles and ships. Biodiesel is made from two grades of diesel: B20, which is 20% biodiesel blended with 80% petroleum diesel, or B100, which is 100% biodiesel. The most frequently used blend is B20.
Currently, the U.S. Postal Service has fleets of vehicles that run on B20 biodiesel. San Francisco's fleet of fire trucks, ambulances, and buses run on biodiesel. The U.S. Army, Navy, Air Force, and Marines offer biodiesel at their bases.
In 2006, the city of Oak Ridge North was the first Texan city to run its electric generator on biodiesel. The electricity produced powered 3,000 homes in that area.
Biodiesel is the European Union's main biofuel source, comprising almost 80% of alternative fuel options (ethanol makes up the remaining 20% of the EU's alternative fuel.)
Alcohol-based fuels: Alcohol-based fuels, such as ethanol and methanol, are also available. Ethanol is usually mixed with petroleum gasoline at a ratio of 85% ethanol to 15% gasoline, called E-85.
A corn derivative, ethanol is frequently used in flexible fuel vehicles. According to fuel experts, ethanol delivers a 20-30% efficiency gain over traditional gasoline. Nitrous oxide and particulate matter are reduced, and horsepower is increased compared to biodiesel. In addition, ethanol lowers engine weight compared to an electric hybrid automobile, although testing at the Massachusetts Institute of Technology indicated that ethanol may not be as efficient as the "best full hybrid systems." At high temperatures, ethanol fuel can ignite the fuel mixture too early and create engine misfire. Efforts are being directed to counteract this potentially damaging mechanical event. Ethanol has also shown a tendency to corrode rubber components, such as the fuel line, in some vehicles. Despite these hurdles, ethanol makes up the European Union's second-most frequently used alternative fuel.
Methanol is an alcohol-based fuel that is usually produced from coal. It is expensive and caustic. Even when blended with gasoline in a mixture called M-85, its corrosive characteristics have not made it the best choice as a mass-produced fuel source.
Gaseous-based fuels are common in many parts of the world. They include compressed natural fossil gas, liquefied natural fossil gas, propane, and hydrogen.
Compressed natural fossil gas is a naturally found gas that is kept under pressure. Because it is lighter than air CNG requires a vehicle's engine to be altered to be able to use this fuel source. CNG has shown itself to be efficient. This vehicle type is popular in parts of Italy, New Zealand, Argentina, Brazil, and China. Currently, limited numbers of refueling sites in the United States have restricted consumer interest in this fuel type.
Liquefied natural fossil gas is natural gas that is kept in a cold liquid form. This fuel must be stored in an insulated storage tank and a special distribution system used within the vehicle. Liquefied natural gas systems have been used historically in heavy vehicles, such as those found in construction.
Liquefied petroleum gas, or propane, is a widely used alternative fuel. Oregon has more than 250 vehicle refueling sites within the state. Conversion of a standard vehicle is required; however, the recent increase in propane costs, in addition to the conversion expense, may make this fuel source expensive.
Hydrogen is one of the basic gaseous chemical elements found on Earth. It has been considered as an alternative fuel source as it is a clean source of energy. Its use has been hampered by technical challenges. Difficulties with transport of the gas to the engine, storage of the hydrogen gas, and the lack of hydrogen "filling stations" have moved researchers to look into the possibility of generating hydrogen within the vehicle itself.
According to the U.S. Department of Energy (DOE), current use of electricity, natural gas, steam, methanol, or coal to produce hydrogen may not fit well within the logistics of supplying it to refueling sites. Extensive manufacturing or distribution systems are not available in the United States. Fuel-celled, hydrogen-powered cars are not currently available on the American mass market.
While other forms of fuel, such as steam engines or wood gas, have been used over the years, many have been rejected as logistically or environmentally inappropriate for the world's current needs.
Scooters: In 2006, the American-owned company Vectix launched the first electric scooter, called the Maxi-Scooter. It combines a battery pack with an electric motor. The battery can run for up to 1,700 charging cycles and be fully recharged in two hours. The scooter is reported to be able to reach a top speed of 62 miles per hour, with a 55-mile distance range.
Different states view electric scooters in different ways: some states require scooter owners to have a motorcycle license, while others regard them as bicycles with a "helper" motor.
Electric bicycles: President Bush signed into law Senate bill SR 1156, which places electric bicycles under the management of the Consumer Product Safety Commission and defines a bicycle as any conveyance "with pedals, that can propel the bicycle, even if it has an electric motor, with a top speed of no more than 20 miles per hour." Electric bicycles are available in either the traditional bicycle style or the "scooter" styling with an enclosed engine housing. Electric bicycles were developed initially in Asia for single-personal use. Designed for trips shorter than 10 miles, the average speed of electric bicycles is about 18 miles per hour, with a typical 10-25-mile distance range per electric charge. Electric motors are available in the heavier power-on-demand type (without any pedaling help from the driver) or as an assist-motor that requires pedaling up hills and in high headwinds. The motor is powered by either rechargeable standard lead-acid or Ni-Mh batteries. Manufacturers claim an average of $5 per year in electricity costs. Newer folding electric bicycles are available for the urban commuter.
Electric vehicles: Electric vehicles draw their power from multiple battery storage units that power an electric motor. It is advertised to be a zero-tailpipe-emissions form of travel, with 99% lower emissions than gasoline or diesel vehicles. With no oil, engine, transmission, spark plugs, valves, fuel tank, distributor, starter clutch, muffler, or catalytic converter, many believe the electric car to be reliable and cost effective, as there are fewer parts to repair.
All road-use electric motor vehicles must be equipped with headlights, taillights, turn signals, parking brake lights, rear-view mirror, side mirrors, windshield and wipers, speedometer, odometer, brakes, seat belts, and federally required identification numbers. Some states limit the use of low-speed electric cars, such as enclosed golf carts, to roadways where the posted maximum speed limit is 35 miles per hour.
According to the California Air Resources Board, the electric car has lower emissions, up to 90% cleaner than a traditional vehicle, even if the electricity used to charge the car is produced by coal. While originally hampered by limited batteries and resulting low distance range, newer electric cars can travel 80-120 miles per charge at highway speeds, although the average range is 30-60 miles. Charging the 16-24 six-volt batteries can be done overnight at home on a standard household circuit. Some regions of the country now offer electrical "charging stations" for the public.
Electric car batteries last about three years and may be recycled, as are traditional fuel auto batteries, according to the Battery Council International.
The electric car is efficient, with the Electric Car Association citing the Smart Car, and the RAV4 EVA in particular, which can maintain a 112 mile per gallon equivalent of a traditionally fueled car. Manufacturers estimate the electric drive motor may last for as many as one million miles.
While some consumer groups have raised concerns about the increased cost connected with rising electricity prices, advocates argue that electricity costs are rising more slowly than gasoline prices. For some owners, after the additional expense of adding a solar-powered battery charging system, refueling costs of an electric car are negligible.
Flexfuel vehicles: Composite forms of alternative transportation include flexfuel and hybrid vehicles. Flexfuel or multiple-fuel vehicles can use more than one type of fuel source. The most commonly found are combinations of electricity or alcohol-based fuel with traditional gasoline as a backup source of power. Hybrid and multiple-fuel vehicles are not considered zero-emission automobiles, but they generate less air pollution than traditional gasoline fueled engines.
Hybrid vehicles: Hybrid gas/electric vehicles use a battery-powered electric motor in the car, coupled with a gasoline engine. Toyota and Honda have seen recent success in consumer sales with this power arrangement in the Honda Insight and Toyota Prius hybrid models.
The recent consumer interest in alternative vehicles also stems from an increased mile-per-gallon efficiency, even with the larger sport utility vehicle (SUV) or truck hybrids. As of the 2009 models, the EPA's mile-per-gallon estimates for the popular hybrid SUVs, such as the Cadillac Escalade Hybrid, the Chevrolet Silverado Hybrid, the Dodge Durango HEV, the GMC Sierra Hybrid, and the Chrysler Aspen HEV, were in the range of 20 city miles per gallon and 22 highway miles per gallon, as compared to earlier traditional-fuel models, which delivered an average of 14 miles per gallon.
General: Railways have also undergone evaluation for their part in air emissions. The Transportation Technology Center in Colorado, which manages the Federal Railroad Administration's specialized testing facilities, has implemented efforts to reduce the amount of train engine idling time, as well as reduce the amount of friction at the wheel-rail contact. Locomotives burn six gallons of diesel per hour and burn fuel even when in idle mode. The new reduction in idling time is estimated to save the burning of 1,560 gallons of diesel per week, or 39,000 gallons per year. By engineering new designs to decrease the amount of friction between the wheel and the rail, researchers have been able to lower the energy needed to pull the heavy trains, as well as decrease the need to produce more replacement iron rails. These efforts are anticipated to decrease the amount of per-train emissions into the atmosphere.
Maglev trains: Maglev trains are a system of rail transport that uses magnetic levitation to raise the cars above the rail and move it forward. The first magnetic levitation train was inaugurated in 1984 in England. It moved on a monorail between the Birmingham International Airport and the railway station, but it was closed in 1995 due to maintenance problems. Maglev trains patents were sought in the United States in the first half of the 1900s. Maglev trains can be found in current use in Japan, China, and Korea. Technological problems with cost, weight issues of the magnets, noise, low-speed electrical efficiency, stability, and negative impacts on magnetic storage devices have interfered with more widespread applications.
General: Scientific and political attention has been focused on decreasing transportation-generated pollution.
Air quality: Concern regarding the increasing level of air pollution and rising traditional fuel prices has created a renewed interest in alternative transportation. As global human population numbers increase, the need for efficient and nonpolluting modes of transportation also increases. It is estimated that by 2040, the world's population may increase to nine billion. Most of that population will be located within the confines of crowded urban environments. The need for efficient and nonpolluting forms of transport for people and supplies will correspondingly increase.
The exact mixture of pollutants differs from region to region, due to differences in fuel sources, geography, numbers of vehicles, and weather patterns.
While transport vehicles are considered the major source of air pollution, the role of international transport, including shipping and aviation, is still being evaluated.
As the number of petroleum-fueled vehicles increases, so does the level of air pollution. Traditional fuel sources for transportation vehicles of all sizes have been gasoline and diesel, and they are the leading cause of air pollution throughout the world.
According to researchers at the Center for International Climate and Environmental Research in Oslo, Norway, road transport has the largest effect on global mean temperature, even greater than that caused by aviation. Recent data suggest that since the pre-industrial era, road transport has generated 15% of the man-made carbon dioxide and 31% of the ground level ozone. It also contributes about 37% of man-made nitrous oxides.
Shipping is the largest contributor or airborne sulfur dioxide, at 56% of transport emission levels.
Air transport of people and supplies was once considered a modern and efficient form of shipment; however, airplanes are included on the list of vehicles adding to air pollution because of their gasoline-based jet fuel emissions.
Many railways, once a major source of pollution, have been undergoing refitting to use electricity, which many experts consider to be a lesser source of air pollution. Coal and petroleum diesel-burning railways are still found throughout developing countries.
Bicycles: Some nations have actively encouraged the use of bicycles as transportation. In Victoria, Australia, provisions to encourage the use of bicycles as a transportation method require bicycle parking with showers and lockers for all new large commercial, retail, and high-rise residential buildings. Montr©al, Canada, has plans to double existing bike paths. Bogot©, Colombia, has the most extensive bike path network in the world. In Paris, bike rentals are available at 1,451 commuter stations. Delhi, India, has a new master plan to provide fully separated bike paths on all major roads. Israel plans to narrow existing dirt roads to be used only for bike or pedestrian use. In Japan, new bike path construction began in 2008 near schools and transit stations in 98 cities. Mexico City has made Sundays car-fee from 7 a.m. to 2 p.m. in an area near the city's center. The Netherlands averages more than two bikes per person. In Amsterdam, bicycles make up 55% of commuter transportation and 33% of travel to schools. In London, one million dollars will be invested over 10 years to expand bike parking at railroad stops and create more biking lanes. Tanzania and Zambia plan to distribute 1,000 bicycles in each country to the poor and healthcare volunteers, as well as train 400 bicycle mechanics.
Alternative fuel sources: The abrupt transition of production of U.S. agricultural barley and soy into the fuel production industry produced shortages in these products and a resulting increase in consumer and animal food prices. This impact on the food supply has hindered acceptance of the biofuel industry. Climbing soybean prices, as a result of fewer agricultural incentives, has slowed the production of biodiesel and encouraged shipment of supplies to Europe.
Until 2008, federal Renewable Fuels Standards supported the use of 500 million gallons of biodiesel. According to the National Biodiesel Board, the production of biodiesel is currently falling to a projected half of the 700 million gallons produced in 2008. Without government incentive payments, many farmers are moving back into food-grade soy production.
Debate over the viability of biodiesel continues. A statement from the National Academy of Sciences indicates that if the United States proportioned all of its annual corn and soybean production to biodiesel production, that product would meet only 12% of U.S. gasoline demand and only six percent of diesel demand.
Biodiesel exhaust: Controversy has developed over the possible toxicity of biodiesel exhaust. While considered a positive fuel alternative, biodiesel has been found to produce more nitrogen oxide than traditional diesel. A 2002 evaluation by the U.S. Environmental Protection Agency (EPA) found biodiesel to produce two percent more nitrogen oxide than petroleum diesel. However, a 2006 study by the National Renewable Energy Laboratory (NREL) found no increase in nitrogen oxide levels. While health risks have been acknowledged for decades with petroleum diesel, the exact long-term impact of exposure to biodiesel combustion products is unknown.
Battery safety: The safety of the electric car's multiple batteries in a collision has been questioned; however, crash tests on the road and race track have not shown any additional risk when compared to traditional vehicle's battery and safety. Proponents argue that traditionally fueled vehicles contain similarly vulnerable engine-compartment batteries without posing a serious health threat.
Disposal of batteries from electric cars and the long-term impact on the environment is currently unknown.
In response to these issues, even considering the emissions produced during their manufacture, the EPA recommends choosing alternative transportation methods as the best way to improve emission levels. These choices could include purchasing an alternative vehicle and using renewable fuel sources. Decreasing the amount of driving and employing collective commuting options are also part of the EPA's recommendations.
Maglev trains: Potential harm to passengers with pacemakers from the large magnets have raised concern in some medical circles. Technological difficulties in shielding passengers have raised installation costs beyond acceptable limits.
FUTURE RESEARCH OR APPLICATIONS
General: According to the American Journal of Public Health, there is a public health movement that supports governmental installation of bicycle and pedestrian lanes, designated "safe routes" for children walking to school, and consumer-friendly structures, such as bicycle racks at the workplace and business locations. It is unclear if recent government stimulus money will be directed toward these types of community projects.
Biodiesel: Due to concerns over the loss of food crops to the fuel industry, researchers are looking into nonfood sources for biodiesel. Specific kinds of algae are being reviewed as renewable resources. Algae can be grown in most parts of the world. It also uses less water than current crop sources and can be grown in saltwater year round. In 2007, the Chevron Company's Technology Ventures program joined forces with the National Renewable Energy Laboratory (NREL) to research potential liquid transportation fuels using algae.
According to the state of Texas, PetroSun's research facility is experimenting with the development of algae-derived jet fuel for aviation needs. Unfortunately, as of 1996 data, biodiesel fuel made from algae is not yet cost efficient in comparison with traditional diesel fuel. Similar cost constraints have been found in the use of food oils as sources of biodiesel fuel.
Making biodiesel from only sunlight and water has been under research since the 1970s, but the process is far from commercially viable, due to technical constraints.
In India, biodiesel is being made from a nonedible tree called the jatropha. The seeds are used in South American oil lamps. The tree can grow on waste land and live as long as 40 years.
Hydrogen: Hydrogen possesses the ability to produce a clean fuel; however, the process of producing and storing hydrogen remains problematic. New research is underway looking into producing hydrogen from solar systems. According to researchers at the School of Chemical Engineering and Energy Center at Purdue University, if this is successful, the entire fuel needs of the United States could be produced without the addition of any carbon dioxide emissions.
Railroad: The Federal Railroad Administration's Transportation Technology Center is working on a new train control system that will allow an onboard electronic cruise control to manage the train. The cruise control will alter the throttle in response to weather, rail conditions, and geographic terrain. The system is expected to help lower the amount of fuel needed to move a train from the starting point to the endpoint and therefore reduce emissions.
Carpooling: The Finnish National Public Health Institute has created a futuristic model for the Helsinki area, where a central location collects commuters' requests for transportation to specific designations, assigns the commuters to 4-8-seat public vehicles, and transmits the assignment instructions to the commuters' mobile phones. The commuters then assemble in a common area for group transportation. Researchers propose that such techniques could reduce the negative impact of car traffic of Helsinki's one million residents by 50-70%.
Maglev trains: Research into using airless vacuum tunnels or tubes for magnetic trains (called "vactrains") is currently being conducted. Eliminating normal air drag within the tube would increase overall speed and efficiency of the propulsion system. Concerns with this approach involve potential passenger cabin depressurization and asphyxia, should the train malfunction or be involved in an accident within the tube.
This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).
- Agrawal R, Singh NR, Ribeiro FH, et al. Sustainable fuel for the transportation sector. Proc Natl Acad Sci USA. 2007 Mar 20;104(12): 4828-33.
- Betts, KS. Innovative technologies: Transportation fuels on the table. Environ Health Perspect. 2008 Feb; 116(2): A67.
- Ham SA, Macera CA, Lindley C. Trends in walking for transportation in the United States, 1995 and 2001. Prev Chronic Dis. 2005 Oct;2(4):A14.
- Health Effects Institute (EHI). www.healtheffects.org
- National Biodiesel Board. www.biodiesel.org
- Natural Standard: The Authority on Integrative Medicine. www.naturalstandard.com
- Pervin T, Gerdtham U, Lyttkens CH. Societal costs of air pollution-related health hazards: a review of methods and results. Cost Eff Resour Alloc. 2008 Sep 11; 6:19.
- Schmidt, CW. Biodiesel: cultivating alternative fuels. Environ Health Perspect. 2007 Feb115(2):A86-A91.
- Swanson KJ, Madden MC, Ghio AJ. Biodiesel exhaust: The need for health effects research. Environ Health Perspect. 2007 Apr;115(4):496-9.
- Tuomisto JT, Tainio M. An economic way of reducing health, environmental, and other pressures of urban traffic: a decision analysis on trip aggregation. BMC Public Health. 2005 Nov 25 5:123.
- United States Department of Commerce. www.commerce.gov
- United States Department of Energy (DOE). www.eia.doe.gov
- United States Environmental Protection Agency (EPA). www.epa.gov
- United States National Renewable Energy Laboratory (NREL). www.nrel.gov
- World Health Organization (WHO). www.who.int
Copyright © 2011 Natural Standard (www.naturalstandard.com)