James Hammitt, professor at the School of Public Health, and his colleagues have spent the past three years doing risk analyses of buses with conventional diesel engines and emission-controlled diesel engines versus buses that run on natural gas. To compare, the researchers used a conventional measurement known as the quality-adjusted life-year (QALY). It says essentially that if someone dies, we count the loss in terms of how many years he or she would have lived otherwise. The bottom line? The pollution from 1,000 conventional diesel buses will result in the loss of 16 QALYs annually, in the entire population of the United States. For 1,000 buses with emission-controlled diesel engines, QALYs lost will be about 10, at a cost of about $2,000 more per bus. That’s a cost of $300,000 per QALY gained by going from conventional to emission-controlled diesel engines. The pollution from natural gas buses results in nine, or about half, the lost QALYs – but each of the 1,000 buses cost about $15,000 more a year to run. “So for those 1,000 buses,” Hammitt points out, “we’re spending $15 million a year, and saving nine QALYs.” The study was funded by the International Truck and Engine Corp.