Diesel exhaust is dirty -- far dirtier than exhaust from gasoline. It causes worse pollution and it has harsher effects on human health. Diesel exhaust is a major source of nitrogen dioxide, for example, which causes smog.
The tiny particles diesel engines emit are also dangerous. Virtually all of them are smaller than one micron. (There are 25,400 microns in one inch -- the period at the end of this sentence is hundreds of microns across.) Particles that small can evade the respiratory system's defenses and embed themselves in the lungs. (In general, particles less than 10 microns in diameter pose this problem.) They can cause short-term effects, like wheezing and coughing, as well as chronic respiratory problems. Breathable particles are especially harmful to elderly people and those with existing respiratory or cardiovascular problems. Recent studies show that chronic exposure to breathable particles is associated with a higher rate of death from respiratory and cardiovascular disease.
Diesel exhaust is also known to cause lung cancer, and may cause bladder cancer as well. Lifetime exposure to diesel pollution could mean premature death from cancer for more than 14,000 Californians, according to the California Air Resources Board. Studies in Southern California indicate that 70 percent of the estimated lifetime risk of cancer from air pollution is attributable to diesel exhaust. Children are particularly susceptible to diesel's health effects, because they are particularly vulnerable to air pollution. Their still-developing lungs are more sensitive, and they breathe more air (and thus more pollution) relative to their size than adults do.
Growing Reliance on Diesel Fuel
Californians are not only burning greater amounts of diesel -- we are also using more of it relative to other fuels. In the 1950s, diesel represented 4.7 percent of all fuel consumed; by the 1990s, the figure had jumped to 13.8 percent.
The California Air Resources Board estimates that 28,000 tons of diesel exhaust were emitted into California's air in 2000. On-road mobile sources (heavy-duty trucks, buses, light-duty cars and trucks) contributed 27 percent of this exhaust, or 7,560 tons. Other mobile sources (such as farm and construction equipment, ships, trains, and boats) emitted an estimated 71 percent, and stationary sources, such as diesel generators for electricity, contributed the remaining 2 percent.
More Diesel Trucking
The major reason for the steady growth in diesel emissions in California is the huge increase in the quantity of goods shipped by diesel truck around the state. But a growing number of sport-utility vehicles and light trucks also run on diesel fuel. Although these vehicles use fuel more efficiently than their gas counterparts, their emissions are far more hazardous. Behind all these trends is the minimal regulation of diesel, which so far has escaped most of the stringent government regulations imposed on gasoline. That will change in 2004, when stricter state and federal diesel regulations go into effect.
The Bay Area's public buses also use diesel fuel. They don't have to: alternative fuels exist. Many cities in California are already switching to buses that run on these cleaner fuels, but the Bay Area has lagged behind. Among Bay Area counties, only Sonoma has committed to stop buying diesel buses and to invest instead in buses that use compressed natural gas, a fuel that pollutes much less.
Most school buses still use diesel fuel as well. Many local, state, and federal funding sources can help schools offset the costs of switching to alternative-fuel buses, though still more funding is needed. Also, clean-fuel buses can cost less to run and maintain, so over time, a school district can recoup the initial cost of buying new buses.