Correspondence Comment on “Vehicle Self-Pollution Intake Fraction: Children’s Exposure to School Bus Emissions” Marshall and Behrentz (1) studied infiltration of a tracer gas (The tracer gas, sulfur hexafluoride (SF6), was injected at a constant rate near the end of the school-bus’ tailpipe.) into 6 buses driven along a highly urbanized route in Los Angeles in 90-min experiments and concluded: “The school bus microenvironment contributes significantly to children’s estimated total inhalation intake of DPM [diesel particulate matter].” However, this conclusion does not follow from the article’s estimates of “self-pollution” and “self-pollution intake fraction,” which do not depend on vehicle engine technology, i.e., diesel vs. nondiesel. The authors do not explain that their tracer gas data would be identical to what was reported, even if the exhaust gas on the tested vehicle were pure air. The tracer-gas experiments reveal only inherent bus-cabin “leakiness” (i.e., predictable air exchange through windows, doors, and gaps). Moreover, the authors’ conclusion that the “intake fraction” for an individual nearby an emissions source is much larger than the intake fraction for that same source for someone far away is obvious and does not comment on the comparative contribution of all sources to a child’s total DPM exposure. The authors do not provide data on whether DPM concentrations inside the tested buses were greater or less than what children would experience while walking to school along the same route or traveling the route in another vehicle. Thus, the article’s Introduction, referring to “DPM ... cancer risk,” will raise undue alarm regarding exposures seemingly unique to the interior of a diesel-engine school bus. Yet, the tracergas data have nothing to do with “diesel engines,” and the article does not address the question of what proportion of a student’s total inhaled DPM derives from the exhaust of the school bus in which he/she rides versus other sources. “Intake fraction” can be defined as the proportion of a vehicle’s emissions inhaled by an individual (2). Intake fraction is thus dramatically dependent on the proximity of the receptor to the emission source. It is no surprise then, as Figure 2 shows, that for a child on a school bus, the intake fraction for tracer gas released under the school bus on which he/she is riding far exceeds the intake fraction for emissions from an “average vehicle” somewhere in the South Coast Air Basin, on which he/she is NOT riding. Figure 2 does not inform us about the relative contribution to a child’s total personal exposure to DPM from his/her school bus emissions vs DPM sources in the South Coast area. Notwithstanding the actual level of self-pollution on school buses, children spend only a small fraction of their time on school buses relative to time spent outdoors and in other microenvironments where exposures to DPM also occur. These caveats are not clearly acknowledged by Marshall and Behrentz. Cumulative intake fractions also ignore temporal and spatial components of exposure conditions that may be essential elements of human health risk assessment. That is, intake fractions can be misleading indicators of potential risk if toxicity depends on the intake rate, or if the exposureresponse curve has a threshold (2). On the other hand, estimation of cancer risk requires assessment of integrated (70-year) lifetime exposure. Thus, intake fractions are not equivalent to quantitative measures of an individual’s potential exposure and health risk due to air pollution. The authors’ intake fractions are based on a single measurement study, are uncertain due to methodological issues 10.1021/es050908v CCC: $33.50 Published on Web 03/30/2006
2006 American Chemical Society
associated with the SF6 tracer gas experiments [see ref 3, Ireson et al., Comments], and have limited sample size (6 buses, 16 runs). Marshall and Behrentz neither cite other studies of bus self-pollution nor do they assess the reliability of their tracer gas results relative to other techniques. In particular, experiments on a 1995 model year school bus operated using diesel fuel tagged with an iridium tracer (3) demonstrated self-pollution approximately 1-3 orders of magnitude lower than the self-pollution estimates of Marshall and Behrentz. In another iridium tracer study, a portion of the Baltimore municipal diesel-fuel supply was tagged with an iridium tracer (4). Personal aerosol samples were collected for commuting students, and the estimated student bus exposures to DPM were in the range of
