What are the adverse health effects of diesel exhaust?
Diesel exhaust is a mixture of smog-forming pollutants, soot particles, and other toxic constituents. These small particulates can penetrate deep into the lungs, exacerbating a wide variety of respiratory problems including asthma, a growing health problem now afflicting some 9 million American children. Smog and soot also exacerbate cardiac disease with particulate matter directly linked to premature death. More than 40 compounds found in diesel exhaust are believed to cause cancer. These harmful emissions pose an especially high risk to children because their respiratory systems are still developing and they spend more time outdoors than adults, increasing their relative exposure to air pollution. Indeed, there is no known safe level of exposure to diesel exhaust for children. According to a 2002 Yale University study, children riding buses were exposed to 5 to 15 times more particulate matter than normal ambient levels. Additionally, a 2003 California Air Resources Board (CARB) study estimates that children traveling two hours per school day from kindergarten through high school experience a five percent increase in lifetime cancer risk.
Aren’t all school buses on the road required to meet today’s safety and pollution standards?
No. Although new buses must meet a stringent series of federal motor vehicle safety standards, there are about 3,000 school buses on the road that were built before these standards were required. These older buses did not have to meet safety requirements for passenger seating and crash protection, rollover protection, body joint strength, and fuel system integrity. It is true that pollution standards for buses have become progressively stronger, providing better protection for children and their maturing lungs. However, buses built before 1990 and 1991 are still on the road; they are allowed to emit at least six times more toxic soot (particulate matter) and four times more smog-forming nitrogen oxides (commonly called NOx) than newer models. Older school buses expose children, whether they are riding the bus or waiting for it in the schoolyard, to greater levels of air pollution. In addition, diesel school buses may release more pollution – particularly soot – under real-world driving conditions than estimated from government laboratory tests. Here is the current status of the U.S. School Bus Fleet
|5.8 billion miles per year
|Individual bus mileage
|11,400 miles per year
|Fuel of Choice
|Average Yearly Emissions (entire fleet)
|Particulate matter (soot)
|Average Yearly Emissions (per bus)
|Particulate matter (soot)
|Reduction in Particulate Matter as a result of:
What technologies can be used to help clean up our school buses?
Retrofit and cleaner-fuel technologies play a key role in school bus cleanup strategies, a.k.a., “the five Rs”: retrofitting, refueling, replacement, repair, and reduced idling.
- Retrofitting: More than 25,000 school buses have been retrofitted with diesel pollution controls, including particulate traps and crankcase filtration controls, reducing soot by 59 tons in 2005.
- Refueling: Nearly 13,000 school buses are currently running on biodiesel or cleaner alternative fuels such as natural gas and propane, cutting soot pollution by 23 tons in 2005.
- Replacement & Repair: Older diesel buses pollute more than newer buses because engine performance degrades. Accordingly, the highest priority for cleaning up America’s school bus fleet is to replace the oldest school buses first. Routine maintenance and periodic engine rebuilds are also crucial for keeping school bus engines cleaner over their lifetime.
- Reduced idling: Idling school buses not only waste fuel and money, but can also unnecessarily expose children to harmful pollution. Anti-idling measures are important for reducing pollution.
What is the difference between conventional diesel and natural gas school buses?
A conventional new diesel bus (Model Year 2005) releases about ten times more soot and 35 percent more smog-forming pollutants than a new natural gas bus. The trace amount of soot from natural gas buses is generally attributed to lubricating oil, not the fuel itself. In addition, the soot released from a standard diesel bus is more toxic than natural gas emissions. The State of California has listed over 40 chemicals in diesel exhaust as toxic air contaminants and attributes diesel pollution with 70 percent of the State’s cancer risk from airborne pollution. Luckily, new pollution controls for diesel buses can make them a lot cleaner.
How does a new low-emission diesel school bus compare to the conventional diesel school buses?
Diesel emission control technologies are evolving and improving, and new low-emission diesel buses are starting to enter the market. Emissions from diesel buses can be reduced through a combination of engine improvements, cleaner fuel and oil, and exhaust control equipment. If these technologies live up to their potential, they can reduce smog-forming pollutants and toxic soot by 85 percent or more when compared to today’s standard diesel school bus. But these technologies are still under development, and they must be monitored to ensure that the emissions reductions continue over the sometimes 20 to 30 year lifetime of the bus.
Do new pollution controls make diesel buses as clean as natural gas?
Just about, though natural gas remains a cleaner alternative today. It reduced particulate matter to very low levels. Unfortunately, nitrogen oxides (NOx) are more difficult to control, and NOx emissions are still higher from the average diesel than from the average natural gas engine. In order to function, new pollution controls for diesel require the use of a special low-sulfur fuel, which will not be nationally accessible until mandated by federal law in 2006. Plus, these nascent pollution control technologies must still be monitored to prove their effectiveness over the range of real-world operating conditions. If they fail, degrade, or are disengaged, diesel buses will continue to pollute the air with black toxic soot and smog-forming emissions. But we remain hopeful that new pollution controls developed for diesel will prove to be a valuable part of the clean school bus solution.
A natural gas school bus initially costs between $30,000 and $40,000 more than a diesel bus. This initial investment is often recouped by school districts that benefit from lower maintenance and operational costs.
Do natural gas buses pose increased risks from accidents?
The US Department of Energy (DOE) considers natural gas buses to be as safe as their diesel counterparts on the road and possibly even safer during maintenance and refueling. Although both natural gas and diesel fuels are flammable and require specially-designed precautions and fire protection equipment, natural gas lacks some of the risks of diesel. In fact, DOE reports that natural gas fuel tanks are much stronger and safer than either diesel or gasoline fuel tanks. Diesel tanks can leak and contaminate groundwater, which is not a risk with natural gas. Diesel bus facilities often store much larger quantities of fuel on site than natural gas facilities. And, while natural gas vapors are odorless and nontoxic to breathe, diesel vapors contain toxics that are dangerous to ingest or breathe.
Are our nation’s school buses getting cleaner?
Clean school bus programs have made significant strides during the past several years. Nationally, soot pollution from school buses has been reduced more than two percent through local, state, and federal actions. California and Washington lead the country in cleanup programs, reducing school bus soot by more than seven percent through retrofits and cleaner fuels. But we are still a long way from ensuring that our children are traveling on cleaner school buses. Many states have not prioritized the problem of school bus pollution and most states lack the resources to develop effective clean school bus programs.
Should federal dollars be aimed at replacing the oldest, dirtiest school buses or at cleaning up newer buses?
Both, federal dollars should be used to fund a two-part clean-up strategy:
- Get Rid of the Oldest, Dirtiest Buses First: More than one-third of the school buses on the road today are over ten years old, and in need of replacement. Luckily, there are new school buses available today powered by alternative fuels and to a lesser extent low sulfur diesel that can provide both safe and clean transport for children. The highest priority is to replace the school buses on the road that were built before 1991, some of which do not even have to meet minimum safety requirements
- Retrofit buses built 1991 to today: For buses that have years of service remaining, the best alternative is to retrofit them with clean-up technologies. Though not as clean as a new bus, emission controls can reduce toxic soot 25 to 85 percent.