California ARB publishes school bus pollution study

16 October 2003

The California Air Resources Board (ARB) published the results of a new study on pollution levels inside shool buses. Increased on-board pollutant levels were found in all tested buses, including conventional diesel, advanced emission-controlled diesel, and compressed natural gas (CNG), with the highest exposures in uncontrolled diesel buses. The results of the shool bus study were consistent with the previous ARB study which found in-vehicle pollution concentrations in passenger cars to be higher than those found in the ambient air.

The in-bus pollution came from two sources: (1) driving through polluted areas or behind high emitting vehicles, where ambient pollution entered the passenger compartment; and (2) from “self-pollution” with the bus exhaust itself, which was shown to leak into the cabin, particularly for older, higher emitting buses.

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 diesel particulate filter (DPF), and one 2002 compressed natural gas (CNG) bus. The CNG bus used in the study was not equipped with an emission control catalyst. The researchers measured black carbon (as a surrogate of diesel particulates), particle-bound PAHs, formaldehyde and many other vehicle-related pollutants. The level of self-pollution was determined by introducing a tracer gas to each bus’ own exhaust, and by performing tests with windows open and closed. The study measured exposures inside the buses and did not include tailpipe emission tests. The study did not evaluate exposures from shool bus idling, which is not allowed in California.

It was found that buses could have high on-board pollutant concentrations due to self-pollution. This effect was worse when windows were closed, and worse for older buses. Pollutant concentrations were up to 2.5 times higher—and the tracer gas levels up to 2.6 times higher—when windows were closed compared to tests with open windows for a given diesel bus on a given route. The study also confirmed that influence of other traffic was a key determinant of exposure. Pollutants were 2-3 times higher on the congested primary urban route compared to the suburban/rural route for a given diesel bus at the same time of day with windows open.

The CNG-powered bus and the DPF-equipped diesel bus, while also showing measurable self-pollution, showed significantly reduced on-board concentrations of pollutants compared to uncontrolled diesel buses. Pollutant exposures were 2-5 times higher on uncontrolled diesel buses as compared to the CNG or DPF-equipped bus during closed window conditions. When windows were open, these differences were markedly reduced due to much higher ventilation rates. The CNG bus was found to have the lowest on-board levels of diesel particulate matter but the highest level of formaldehyde exposure among all seven tested buses. Both diesel particulates and formaldehyde are classified in California as human carcinogens.

Health risk analysis using ARB’s own health risk factors showed that children commuting on uncontrolled diesel buses were subject to small risks of increased cancer (~4%) from diesel PM exposure, as well as of lower respiratory symptoms (~6%) and daily hospitalizations for asthma (~1%) from PM2.5 exposure.

Authors of the study noted that both the number of buses studied and the number of runs conducted for each bus were small, and the variability in measured concentrations from bus to bus and from run to run was large. To be able to generalize the performance of buses by fuel type or control technology, direct measurements of the bus emission rates would be needed, along with a large enough number of buses to be representative of a particular type of bus.

The school bus study was performed by the University of California at Los Angeles and University of California at Riverside from late April to early June 2002. It was funded mainly by the ARB ($450,000), with contributions from the US Environmental Protection Agency ($72,000) and the South Coast Air Quality Management District ($59,000).

California and US school transportation industry responded to the study by urging parents not to overreact by taking children off school buses. While emphasizing that school buses remain the safest method of school transportation—safer than walking, bicycling or riding in a car with parents—the industry also spoke in favor of clean bus technologies and called on state and federal legislators to provide funds to accelerate the replacement of old school buses.

California ARB: ARB Shool Bus Exposure Study | NASDPTS | CASTO