Dangerous Exhaust Levels Found Inside School Buses


Air inside the big, yellow buses used to shuttle children to and from school can contain up to 8.5 times more diesel exhaust than people typically breathe in smoggy California--enough to expose children to dangerous levels of cancer-causing soot, according to new research.

The concentrations of diesel greatly exceed limits the federal government has established to protect communities from toxic pollution at factories, oil refineries and small businesses, according to the study. The research was prepared by scientists at UC Berkeley and two environmental advocacy groups--the Natural Resources Defense Council and the Santa Monica-based Coalition for Clean Air.

Closing the windows doesn’t help--soot concentrations more than double when that happens--and air in the back of the bus is slightly worse than in front, the study found. Leaks in the floorboards and swirling exhaust backwash apparently allow exhaust to seep into bus interiors.


“The smoke leaks in, but doesn’t leak out as fast,” said Gina Solomon, a scientist for the Natural Resources Defense Council. “We found higher levels of diesel inside the buses than we did in the streets of L.A.”

The study has some notable limitations. Most importantly, the researchers studied only four buses, all in the Los Angeles school district.

But the buses studied were newer than the majority of those in use in the state, and the findings are consistent with other research done by the California Air Resources Board. In one study two years ago, researchers for the Air Board, the state agency that oversees antipollution efforts in California, found that motorists on Los Angeles freeways are exposed to up to 10 times more toxic air pollution inside their cars than outside.

A second study in Sacramento found that levels of tiny soot particles inside a diesel-powered school bus were twice as high as levels outside.

Health experts say other pollution sources, such as secondhand tobacco smoke, pose a greater hazard to children, and they note that lung cancer is rare in children. Also, a child’s exposure to diesel exhaust on a bus varies considerably depending on the length of the trip.

The state already has moved to reduce children’s exposure to diesel fumes--part of an overall effort to reduce diesel emission 75% by the end of the decade. In December, the air board approved $50 million to replace 375 school buses and reduce emissions from 1,875 others. Half the money must be used for alternative-fueled coaches, powered primarily by natural gas. Nearly $13 million is earmarked for filters that reduce soot by up to 85% on old, diesel-powered buses.


The new report is bound to add to the debate over the appropriate fuels to power California’s cars, trucks and buses. The authors of the study advocate replacing diesel engines with alternative fuels such as natural gas.

“Most of the children will not get cancer, but it may mean some of them will have increased risk,” said S. Katharine Hammond, professor of environmental health sciences at UC Berkeley and author of the study.

Environmental groups have mounted a major campaign to reduce drastically the use of diesel engines in California. But manufacturers of diesels have argued that new technologies allow them to make far cleaner engines. They also note that diesel remains considerably cheaper than alternatives.

Opponents of diesel note that the exhaust contains gases and black, carbon-based particles that contribute to hazy skies. Many of the particles are smaller than the diameter of a human hair, are inhaled deeply into the human lung and not easily dislodged. Medical studies link such microscopic particles to bronchitis, missed school days, and hospitalizations.

Research at UCLA has also shown that diesel soot can aggravate, and even cause, allergic reactions. That is significant because 5 million children suffer chronic inflammation of the airways and more are diagnosed with asthma each year, according to the American Lung Assn.

Diesel Particles Can Lead to Allergy

“Diesel particles don’t just cause someone who is allergic to have worse symptoms, they cause people who are not normally allergic to some agents to become allergic,” said David Diaz-Sanchez, an immunologist at UCLA. “Children in highly polluted areas are more likely to become allergic than children in less polluted areas.”


Three years ago, the state air board declared diesel exhaust a toxic air contaminant, citing evidence that diesel can cause respiratory disease, cancer and premature death.

Children are especially vulnerable to air pollution because they are active, spend more time outdoors and have growing lungs. About 24 million children nationwide ride school buses. The Los Angeles Unified School District runs one of the largest bus fleets in the nation.

The new study estimates that of every 1 million children who ride a school bus, from 23 to 46 will someday contract cancer as a result of exposure to diesel exhaust. That projection, based on the assumption that the average bus ride is 30 minutes to an hour each way, is still small, however, compared to the one-in-five chance of contracting cancer that everyone in the United States faces.

While the risk may not be overwhelming, “it’s important to reduce exposures to diesel because of the uncertainty over how much diesel it takes before you impact someone’s health,” said Melanie Marty, chief of the air toxicology and epidemiology section of the state Office of Environmental Health Hazard Assessment. “Kids are physiologically not just little adults, and that may make them more susceptible,” she added.

The buses used in the study are part of the fleet of 2,600 coaches used by the Los Angeles school district. Researchers drove them last year through city streets and highways--the same routes students normally travel to school--and measured pollutant levels inside. They also measured diesel exhaust inside a pilot car for comparison.

The buses, built in the mid-1980s, are cleaner than many others in use around the state. Many of the 24,000 school buses in California are so old they have no pollution controls. About 70% are diesel-powered, releasing 13 tons of soot into the sky daily near playgrounds, classrooms and bus stops frequented by children all over the state.


Measurements taken inside the buses showed that concentrations of diesel particles averaged from two to 19 micrograms per cubic meter. That is up to four times as much pollution as someone inside a car or on the street would encounter, the study shows.

When buses labored on grades, or chugged downhill, carbon concentrations increased to nearly 30 micrograms in a cubic meter of air. Californians are exposed to an average of two micrograms of soot particles daily, according to the state air board.