California Air Resources Board

SACRAMENTO – Vehicle pollution inside many California school buses may be worse than levels found in roadway air, a California Air Resources Board (ARB) study has found.

A previous ARB study found in-vehicle pollution concentrations to be much higher than those found in the ambient air. The in-bus pollution came from two sources: first, driving through polluted areas or behind high emitting vehicles, where ambient pollution entered the passenger compartment; second, from the bus exhaust itself, which was shown to leak into the cabin, particularly for older, higher emitting buses.

"These findings underscore the need for further improvement in emission controls for all school buses," said ARB Chairman, Dr. Alan C. Lloyd. "We need to assure our children that their biggest dose of air pollution doesn't happen on their commute to and from school."

School bus transportation remains the safest way for children to commute to and from school. On a per mile basis, the risk of death by vehicle crash is about five times higher for an adult-driven passenger car compared to school buses, according to the Transportation Research Board.

The study was performed by the University of California at Los Angeles and University of California at Riverside. Total ARB funding for the study was $450,000, with the South Coast Air Quality Management District contributing $59,000 and the U.S. Environmental Protection Agency funding $72,000.

During the tests, seven buses were driven on Los Angeles Unified School District bus routes, including two urban routes and a rural / suburban route. The vehicles in the test included five diesel buses built between 1975-1998, one 1998 diesel bus equipped with a particulate trap, and one 2002 compressed natural gas (CNG) bus.

The study found that the worst in-bus pollution levels occurred in older, uncontrolled buses on urban routes with heavy traffic while the windows were closed. Vehicle-related pollution levels inside each bus were greatly affected by the bus' own exhaust (self-pollution), traffic congestion and the direct influence of other vehicles being followed, as well as window position.

The study recommends minimizing commute times, using the cleanest buses for the longest commutes, accelerating the retirement of older buses, and decreasing bus caravanning and idling time to reduce children's exposure to bus-related air pollutants. Since the study's completion, health risks from idling buses at schools, bus stops and after school activity sites has been significantly reduced by the Board's adoption of a school bus idling measure in 2002, which eliminates unnecessary idling.

The trap-equipped diesel bus and the CNG bus had lower emission levels inside the cabin compared to the uncontrolled buses, but in most cases still registered self-pollution emissions. The CNG bus was the cleanest of the tested fleet; however, formaldehyde levels in the cabin were higher due to higher emissions of this pollutant from natural gas-powered engines. Available catalyst technology can control many of these emissions.

For most Californians, their highest exposure to air pollution on any given day occurs during their daily commute in a passenger car. For more information about the ARB's in-car study, click here.

For more detailed information on the school bus study, click here.