There are environmental benefits and drawbacks associated with using diesel fuel. The benefits of diesel fuel include better fuel economy than regular gas and less emissions from a cold start.

However, vehicles that use diesel fuel also produce more fine particles (particulate matter). These particles pose a threat to human health being linked to aggravating and sometimes causing respiratory illnesses such as asthma.

Diesel engines power most of the nation's trucks, buses, trains, ships, and off-road machinery. But each diesel engine can produce tons of air pollutants over its lifetime. With mounting evidence that diesel exhaust poses major health hazards, reducing diesel pollution has become a public priority.

Most diesel engines used today power heavy vehicles such as freight trucks, buses, construction and agricultural equipment, trains, and barges. Diesel passenger vehicles make up only a small share of the current US market, but automakers are working to reintroduce diesel engines into sport utility vehicles, pickups, and passenger cars. While diesel cars are more efficient than their gasoline counterparts, regulations permit them to emit far more pollutants. Such a tradeoff between efficiency and clean air is both unwise and unnecessary.
Diesel engines emit large quantities of particulate matter (called PM) and nitrogen oxides (NOx), the latter a precursor to particulates and smog. Collectively, diesel-powered vehicles account for nearly half of all nitrogen oxides and more than two-thirds of all particulates from US transportation.

Health Impacts

Particulates irritate the eyes and nose and aggravate respiratory problems, including asthma, which afflicts 13 million Americans. Very small particles, called fine particulates, have also been directly associated with an increased risk of premature death. One recent landmark study found that the risk of premature death in areas with high levels of fine particulates was 26 percent greater than in areas with lower levels. Researchers estimate that, nationwide, tens of thousands of people die prematurely each year as a result of particulate pollution. Diesel engines contribute to the problem by releasing particulates directly into the air and by emitting nitrogen oxides and sulfur oxides, which transform into "secondary" particulates in the atmosphere.
Diesel-related emissions of nitrogen oxides also contribute to ozone, the major ingredient in the smog engulfing major cities. High up in the stratosphere, ozone shields the earth from harmful ultraviolet rays. But at ground level, ozone--formed when nitrogen oxides and hydrocarbon emissions combine in the presence of sunlight--irritates the respiratory system, causing coughing, choking, and reduced lung capacity. Urban ozone pollution has been linked to increased hospital admissions for respiratory problems such as asthma, even at levels below the current standard. Ozone presents hazards for healthy adults as well: one study of nonsmoking adults in the ozone-heavy Los Angeles area found that their breathing capacity was reduced as much as that of pack-a-day smokers.

In addition to contributing to mainstream air pollution problems, public health agencies consider diesel exhaust a potential human carcinogen. Exposure to high levels of diesel exhaust causes lung tumors in rats, and studies of humans routinely exposed to diesel fumes indicate a greater risk of lung cancer. For example, occupational health studies of railroad, dock, trucking, and bus garage workers exposed to high levels of diesel exhaust over many years consistently demonstrate a 20 to 50 percent increase in the risk of lung cancer or mortality. The International Agency for Research on Cancer classifies diesel exhaust as a probable human carcinogen, and the US Environmental Protection Agency has proposed the same classification. The California EPA estimates that 450 out of every million Californians are at risk of developing cancer due to diesel exhaust exposure.

Solutions

The public-health problems associated with diesel emissions have intensified efforts to develop viable solutions. But while improvements to existing diesel engines and fuels are necessary, they are not a long-term solution. Alternative fuels and advanced engines can provide larger gains. Fortunately, these new low- or no-pollution technologies are winning acceptance as alternatives to major new investments in diesel-based solutions.
Heavy vehicles. In the heavy-vehicle market, where most diesel is used today, transit buses have led the way toward nondiesel solutions. One in five buses currently on order will run on an alternative, less-polluting fuel such as natural gas. And advanced technologies, like fuel cells, are now entering transit fleets and eliminating polluting emissions altogether. In addition to providing a quieter, cleaner ride for transit passengers, advanced buses provide a springboard to using these improved technologies and fuels to clean up all heavy vehicles. Development of advanced heavy-vehicle options is progressing rapidly, but requires more public funding for research, development, and demonstration. Policymakers must also help develop stronger regulatory and market incentive programs to develop the necessary fueling infrastructure and move these technologies onto the road.

Light vehicles. While diesel powers relatively few automobiles or light trucks today, industry and government are currently working to reintroduce this technology into passenger vehicles to meet fuel economy and climate change goals. But advanced technologies such as battery, hybrid, and fuel cell electric vehicles powered by alternative fuels provide better solutions to air quality, climate change, and energy security problems. Research into advanced vehicles should therefore focus on these inherently cleaner choices. At a minimum, regulators should close historic loopholes that permit diesel cars to pollute more than those powered by gasoline. Government policy should also target the largely untapped potential for improving gasoline vehicles while working to help bring truly clean and efficient vehicles to market.