Frequently Asked Questions About Diesel Technologies and Fuels

May, 2004

Does diesel exhaust pose a serious health risk?

Yes. Diesel exhaust is a serious public health risk and has been listed as a known human carcinogen since 1990. In 1998, the California Air Resources Board (CARB) formally listed diesel particulate as a toxic air contaminant. In fact, diesel particulates are the most significant source of air toxics in California and account for 70 percent of the cancer risk from toxic air contaminants statewide, according to CARB. Diesel exhaust also exacerbates lung diseases such as asthma and emphysema. Public health risks are greatest from older model diesel engines that emit the highest levels of diesel particulates.

Are there new technological advances that make diesel burn cleaner?

Yes. Advanced technology diesel buses equipped with particulate filters can substantially reduce particulate emissions, but only when they are used with low sulfur diesel fuel. Results from a recent CARB study comparing “low-emitting diesel” and natural gas transit buses indicate that particulate matter (PM) levels are extremely low for both advanced diesel buses equipped with traps and low-sulfur fuel and CNG buses equipped with oxidation catalysts. For the latest information on this research, go to www.arb.ca.gov/research/cng-diesel/cng-diesel.htm. In addition, later model diesel vehicles (post-1994) can be upgraded with particulate traps that achieve 85 percent reduction in diesel exhaust particulates.

However, all diesel technologies still emit more smog forming nitrogen oxides than alternative fuels such as natural gas . In addition, it is important to remember all advanced diesel technologies with particulate traps must use low-sulfur fuel to be effective. Low sulfur diesel fuel is available in some areas, but will not be widely available in California until mid-2006. More data also needs to be collected on the long-term durability of pollution control systems for diesel buses and the actual on-road emissions of advanced technology diesel buses during regular use.

What does the term “Green Diesel Technology” mean?”

One diesel engine manufacturer has patented the name Green Diesel Technology™ for a lower-emitting diesel school bus produced by the company. As with other advanced diesel technology, this school bus can achieve low particulate levels when used in conjunction with a particulate trap. However, this bus still emits more nitrogen oxides than a comparable CNG bus with an oxidation catalyst.

In addition, the term, “green diesel” is sometimes used to describe bio-diesel fuels that can be used in any diesel vehicle. Bio-diesel fuels are fuels made from vegetable oils, fats or greases that are usually blended with 80 percent standard diesel fuel and only 20 percent “bio” fuel. Due to the high proportion of diesel fuel in the mixture, these 20 percent “bio” fuel blends still have high pollution emissions. For in-use diesel vehicles, use of a diesel particulate filter with low-sulfur fuel is far more effective in reducing emissions.

Are alternative fuels available today for heavy-duty buses and trucks?

Yes. Compressed natural gas (CNG), liquified natural gas (LNG), electric, and some fuel cell powered buses and trucks are already in use today. In the future, more advanced technologies such as fuel cells will be available for heavy-duty applications. While transition to natural gas technology is now a viable option for key segments of the heavy-duty market, there are some markets, such as long-haul trucking, where transition to alternative fuels is not practical on a large scale. In the future, as technology advances and more fueling locations become available, alternative fuels should become viable for these additional markets.

What are the advantages of natural gas over diesel?

Natural gas is an inherently cleaner alternative fuel that produces very low particulate and nitrogen dioxide emissions. Utilizing natural gas fuel not only provides greater overall emission reductions, but also supports the goals of fuel diversity and reducing petroleum dependence.

Do CNG Buses Need Pollution Controls?

Yes. Although natural gas is an inherently cleaner fuel, there are some particulate emissions from CNG buses that are probably due to the use of lubricating oils in the engine. All new CNG buses should be equipped with oxidation catalysts to achieve the lowest possible emissions.

Is there funding available to support purchases of cleaner and alternative fuel transit and school buses?

Programs are available to assist owners and operators of diesel equipment, including transit and school buses, to upgrade older equipment and convert to cleaner fueled vehicles. These programs include the Carl Moyer Air Quality Attainment Program and the Lower Emission School Bus Program administered by CARB and local air districts. These programs have been funded in past years through state budget appropriations and citizen initiative. This year, the State Legislature is considering several proposals to raise additional funds to continue these important air quality programs.

Funding for cleaner school buses is especially important. Children are among those most vulnerable to the health risks of diesel exhaust exposure, yet they ride on some of the oldest and most polluting diesel buses on the road today, sometimes for hours at a time. In fact, a report by the Union of Concerned Scientists revealed that nearly half the buses are older than 10 years. Constant, significant exposure to diesel exhaust, coupled with a child’s heightened vulnerability to pollution, is widely recognized as a potential cause of severe health problems in children. It is well known, for example, that children raised in heavily polluted areas face the prospect of reduced lung capacity and prematurely aged lungs. A report by the National Environmental Trust shows that children exposed to toxic air pollution, such as diesel particulates, are also at greater risk of developing cancer.

What is the American Lung Association of California doing to reduce diesel pollution?

The American Lung Association of California is supporting legislation to identify a stable, long term and adequate source of funding for key diesel pollution reduction programs such as the Carl Moyer program and the lower emission school bus program. These programs provide funds to replace and upgrade older diesel trucks, buses, farm equipment, marine engines and other on-road and off-road diesel applications to reduce diesel exhaust. The American Lung Association of California is also closely following the “Diesel Risk Reduction Program” at the California Air Resources Board and advocating for the strongest possible state regulations to reduce diesel pollution in new and on-road vehicles as part of that program.