Environ. Sci. Technol. 2003, 37, 4824
Response to Comment on “Fuels for Urban Transit Buses: A Cost-Effectiveness Analysis” In response to our cost-effectiveness analysis of alternative propulsion technologies for urban transit buses (1), McClellan and Lapin make two points (2). First, they note that emission controlled diesel (ECD) technology is improving rapidly. Any comparative evaluation of propulsion systems is of course complicated by the “moving target” nature of the technologies, a factor that is relevant to both the ECD and compressed natural gas (CNG) technologies. Although the exact results will change over time as both the ECD and CNG technologies evolve (and as investigators publish new data), the framework we presented provides a useful way to compare these and other alternatives. Our emissions estimates were based on an exhaustive review of the literature available at the time of our analysis, a process that was overseen by our Advisory Panel. Second, McClellan and Lapin (2) point to the wide ranges we reported for the number of Quality Adjusted Life Years (QALYs) saved by ECD and CNG as evidence that quantifying health benefits in this way is not useful. But the upper- and lower-bound estimates of the health benefits of substituting 1000 ECD or CNG buses for conventional diesel buses for 1 yr (0-41 QALYs saved by ECD and 0.01-65 QALYs saved by CNG) reflect the simultaneous use of all worst-case assumptions and all best-case assumptions, respectively. For the case of mortality, the 60-fold difference in exposure-response functions mentioned by McClelland and Lapin (2) is the range between extreme assumptions and does not represent a confidence interval. Hence, it is more likely that the true QALY savings are closer to the central estimates (6 QALYs saved for ECD and 9 QALYs saved for CNG). In addition, many of the uncertain assumptions in our analysis push the QALY savings estimates for ECD and CNG in the same direction. For example, assuming a smaller association between particulate matter (PM) exposure and mortality
4824
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 20, 2003
decreases the estimated number of QALYs saved by both ECD and CNG. As a result, the ranking of the two technology’s benefits is not as uncertain as suggested by McClellan and Lapin (2). Because the health benefits are uncertain, McClellan and Lapin (2) suggest de-emphasizing them and instead focusing on “the much more certain and measurable reductions in emissions”. However, because ECD emits less of some pollutants and CNG emits less of others, it is not clear how to choose between them based solely on emissions. How can one compare the importance of reducing NOx, PM, SO2, and greenhouse gas emissions without using common metrics to quantify health damages and economic impacts? Failure to evaluate health and economic effects using common metrics would also make it difficult to identify which of the uncertain impacts are most consequential for choosing between technologies and must be researched further (PM induced increases in total mortality) and which are less important for decision-making (cancer risks and climate change). Staying with more certain intermediate measures, like emissions, would be easier but would not aid in efforts to identify technologies that can best improve public health in a world with limited resources.
Literature Cited (1) Cohen, J. T.; Hammitt, J. K.; Levy, J. I. Environ. Sci. Technol. 2003, 37, 1477-1484. (2) McClellan, R. O.; Lapin, C. Environ. Sci. Technol. 2003, 37, 4823.
Joshua T. Cohen,* James K. Hammitt, and Jonathan I. Levy Harvard School of Public Health Harvard University Boston, Massachusetts 02115 ES030552S
10.1021/es030552s CCC: $25.00
2003 American Chemical Society Published on Web 09/20/2003
