Environ. Sci. Technol. 2003, 37, 4823
Comment on “Fuels for Urban Transit Buses: A Cost-Effectiveness Analysis” This paper by Cohen et al (1) will be useful for decisionmaking on alternative transportation technologies to improve air quality. However, several aspects of the report require amplification and clarification to maximize its usefulness. One of us (R.O.M.) served on the Advisory Panel for the Cohen et al. study. We have both served as advisors to the International Truck and Engine Company Green Diesel Technology Program. Advances in diesel technology have been occurring rapidly. Cohen et al. (1) used data for a 1995 conventional diesel (CD) transit bus using 350 ppm sulfur fuel, a 1998 emission controlled diesel (ECD) transit bus using 30 ppm sulfur fuel, and a compressed natural gas (CNG) fueled transit bus. These buses have already been superseded by buses with improved emissions control using improved fuels with attendant reduced emissions. In a recent study at Southwest Research Institute (SWRI), the emissions of a Green Diesel Technology ECD school bus using 15 ppm sulfur fuel were compared to a CNG bus (2). The ECD school bus had lower emissions of PM and NOx. Using these data in the model developed for the paper, Cohen (personal communication) calculated 26% lower health impact for the ECD bus than the CNG bus. Low emissions from the ECD school bus relate to use of an integrated system; (a) low-sulfur fuel, (b) advanced engine technology, and (c) exhaust treatment with optimized PM filters and NOx adsorbers. This integrated system, designed to meet new stringent PM and NOx standards, is more effective and consistent than the add-on approach used in retrofitting transit buses with particle filters, which were compared with CNG transit buses by Cohen et al. (1). The bottom line message from Cohen et al. (1) is that a municipality could upgrade from CD to ECD buses at onetenth the cost of upgrading to CNG. As a result, upgrading to ECD yields six times the reduction in emissions, with attendant estimated health benefits, as using the same funds to upgrade with CNG. This result occurs because of the substantially greater incremental costs for CNG buses as compared to ECD buses. The estimated reduction in emissions and estimated health benefits are even greater using the results from the contemporary Green Diesel Technology school bus described above. While it is clear that both ECD and CNG transit buses have reduced emissions, the estimates of health benefits for the alternative technologies are subject to much uncertainty as noted in the original paper. Cohen et al. (1) appropriately note that it is not known if a causal association exists between PM and ozone and the health effect end points evaluated. Obviously, if these associations are not causal the calculated Quality Adjusted Life Years (QALYs) are without validity. For a fleet of 1000 buses, the low, central, and upper estimates of QALYs saved for ECD were 0, 6, and 41, respectively, and for CNG were 0.01, 9, and 65, respectively, per year of operation. These highly uncertain estimates suggest the need for caution in adopting the Cohen et al. (1) conclusion that one technology offers a larger health benefit. It is noteworthy that estimated health benefits and associated uncertainties were primarily related to PM emissions and a 60-fold difference in exposure-response functions for all-cause mortality from PM2.5. The American Cancer Society (ACS) cohort provided central and upper estimates 10.1021/es030451u CCC: $25.00 Published on Web 09/20/2003
2003 American Chemical Society
of 500 life years lost per 1 000 000 exposed per µg/m3 PM2.5. An extrapolation from PM10 analyses in the National Morbidity, Mortality and Air Pollution Study (NMMAPS) provided a lower estimate of 8 life years lost per 1 000 000 exposed per µg/m3 PM2.5. The ACS estimate would be lower if it had been based on higher exposures experienced by the cohort early in life. The recent re-analysis of the NMMAPS time-series data using stringent convergent criteria reduced the original overall exposure-response coefficient by about one-half. The large uncertainty in the NMMAPS value is apparent from consideration of individual city data: in the re-analysis only 3 of 88 cities had statistically significant effects of PM10 (New York and Oakland had increased mortality and Little Rock had reduced mortality associated with PM10). The uncertainty in the exposure-response functions deserves additional research. Ironically, cancer risks of emissions and climatic effects of greenhouse gas emissions previously identified as contentious issues were not quantitatively important in determining benefits. There were also uncertainties in the incremental costs of the alternative technologies. The low, central, and high costs per bus per year for ECD were $1300, $1700, and $2100, respectively, and for CNG they were $4800 (low cost region)$5800 (high cost region), $15 000-$21 000 and $26 000$36 000, respectively. The lower and relatively certain incremental costs for ECD transit buses relate to lower costs for base procurement, infrastructure, and maintenance of the widely used and established diesel technology as compared to the new CNG technology. Cost-effectiveness (CE) ratios (QALYs/incremental costs) calculated by Cohen et al. (1) varied widely. For ECD they were as small as nearly $30 000 per QALYs to infinite (no health improvement per dollar spent). For CNG, they ranged from $70 000 to more than $2 billion (low cost areas) and from $90 000 to more than $3 billion (high cost areas). The uncertainties in estimated health benefits suggest that the specific CEs should be used with caution and emphasis placed on the much more certain and measurable reductions in emissions. The good news is that, as the EPA 2007 diesel emission standards are phased in, both PM and NOx emissions from ECD-fueled vehicles will become very low. With similar low emissions for ECD and CNG vehicles, the emphasis when considering purchase of alternative technologies should be on incremental costs which, at this time, greatly favor ECD over CNG to provide the greatest improvements in air quality per dollar expended.
Literature Cited (1) Cohen, J. T.; Hammitt, J. K.; Levy, J. I. Environ. Sci. Technol. 2003, 37, 1477-1484. (2) Ullman, T.; et al. Comparisons of Exhaust Emissions, Including Toxic Air Contaminants, from School Buses in Compressed Natural Gas, Low Emitting Diesel and Conventional Diesel Engine Configurations. SAE Tech. Pap. Ser. 2003, No. 200301-1381.
Roger O. McClellan* Toxicology and Human Risk Analysis 13701 Quaking Aspen Place NE Albuquerque, New Mexico 87111
Charles Lapin Lapin and Associates 1870 Calafia Street Glendale, California 91205 ES030451U VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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