Children on school buses collectively inhale as much or more exhaust emitted from those buses as does the rest of the city's population, according to a new analysis by researchers at the University of California.
The results highlight the problem of "self-pollution," or exhaust from the vehicle leaking into the cabin, particularly among older buses. This also is the first study to specifically look at how much exhaust is breathed in on school buses.
"Although environmental regulators focus on controlling the amount of exhaust emitted by vehicles and other sources, knowing how much of a pollutant is inhaled is a better indicator for related health impacts," said Julian Marshall, a Ph.D. student at the University of California, Berkeley's Energy and Resources Group and lead author of the study, which is scheduled to appear in the April 15 issue of the journal Environmental Science and Technology, but is available now online.
"Diesel is the last big source of air pollution that has yet to be reigned in," said Marshall. "As a policy matter, it seems clear from this analysis that reducing emissions from school buses should be a very high priority."
The researchers noted that children are especially vulnerable to air pollution because, compared with adults, their immune systems are less mature and, per body weight, they inhale more air per day.
"For every metric ton of pollution emitted by a school bus, the cumulative mass of pollution inhaled by the 40 or so kids on that bus is comparable to, or in many cases larger than, the cumulative mass inhaled by all the other people in an urban area," said Marshall. "That the values were even close was shocking."
The researchers analyzed results from tracer-gas experiments conducted by scientists at UCLA and UC Riverside. They measured the air in six empty school buses traveling through established routes in south-central and suburban Los Angeles, all areas within the Los Angeles Unified School District. Nine runs were made with windows open and seven runs with windows closed in April, May and June 2002.
Five of the buses were powered by diesel fuel and built between 1975 and 1998. One of the diesel buses, a 1998 model, was equipped with a particle trap to reduce emissions. The sixth bus, built in 2002, ran on compressed natural gas.
Sensitive detectors on the buses tested for sulfur hexafluoride, a gas used as a tracer because it can be picked up at levels as low as 10 parts per trillion and is not present in background air. Sulfur hexafluoride was released into the engine exhaust manifold of the buses so researchers knew that any traces of the gas must have come through the exhaust system of the bus.
Eduardo Behrentz, a post-doctoral researcher at the Environmental Science and Engineering Program and the School of Public Health at UCLA, participated in the experiments and co-authored the new analysis.
"We determined that concentrations of key air pollutants were higher inside the bus cabins than outside the cabins," said Behrentz. "While the conditions inside the cabin were affected by the emissions of other vehicles on the road, our tracer gas measurements revealed that a significant amount of the pollutants found inside the buses originated from the buses' own exhaust systems, especially when the windows were closed."
The experiments included measures of exhaust from neighboring vehicles, but this new analysis focuses on the amount of emissions inhaled from the school bus exhaust system.
The analysis assumed a typical ridership of 40 children per school bus with an average breathing rate of 15 liters of air per minute. The researchers calculated the collective amount of bus emissions inhaled for all riders, called the self-pollution intake fraction, as well as the average intake fraction for individuals.
The researchers found that for every million grams of pollutants emitted by the bus, 27 grams would be inhaled by all 40 riders, or 0.67 grams per child.
"In comparison, a city of 1 million people will inhale about 12 grams per million grams of exhaust emitted," said Marshall. "In a single day, a child riding a school bus will breathe in anywhere from 7 to 70 times more exhaust from that bus than a typical L.A. resident will inhale from all school bus emissions in the area."
Not surprisingly, the highest levels of self-pollution were found with the two older buses, particularly when the windows were closed. The intake fraction for a 1975 model diesel bus measured 94 grams of pollution inhaled per million grams emitted, a level 3.4 times greater than average.
The newer model diesel buses are more representative of those found in current school bus fleets. But notably, a survey by School Bus Fleet magazine, a trade publication that tracks statistics in school transportation, finds that California has the highest percentage of pre-1977 school buses in the country. Ten percent of California's fleet consists of pre-1977 buses, while second-place Missouri reports six percent of its fleet made up of pre-1977 buses. In comparison, 35 other states have no buses built before 1977 in use, and nine states have 1 percent or fewer pre-1977 buses in use.
Interestingly, the differences among the newer buses, including the one with the particle trap on the exhaust pipe and the one running on compressed natural gas, were inconsistent. A 1993 bus with windows closed had an intake fraction level of 10 per million, equivalent to the value for a trap-equipped bus with windows open.
The researchers said this may be because the exhaust is leaking into the cabin of the bus further up the system than the tailpipe, which is where the particle trap is located. Exactly how that exhaust is entering into the cabin is a subject of further study.
"The broader message from this study is that there are many exposures to air pollution that are flying below the radar because they are not being picked up by our current air monitoring system," said William Nazaroff, a UC Berkeley professor of civil and environmental engineering who was not part of the study.
Diesel exhaust particulates are considered by public health officials to be a toxic air contaminant and a major source of cancer risk from outdoor air pollution.
"Because so many children ride school buses, reducing the emissions of a school bus would give policymakers more bang for their buck than the same reduction of emissions from other diesel vehicles, such as an 18-wheeler or a construction truck," said Marshall.
Yet despite the findings, the researchers said that riding school buses is still safer than being driven to school in passenger vehicles and that parents shouldn't yank their kids from bus ridership.
"School buses are built like a tank, and the chances of children getting killed or seriously injured from a traffic accident in a private passenger vehicle are significantly greater than if they are on a bus," said Marshall.
This work was supported in part by the University of California Toxic Substances Research and Teaching Program and the University of California Transportation Center.