You’ve seen the billboards: “Natural gas: The clean alternative.” But is it?
Maybe for heating houses, but not necessarily for fueling city buses, according to James Hammitt, professor of economics and decision sciences at the School of Public Health.
Hammitt 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.
“Basically everybody knows diesel buses are dirty,” Hammitt says. “So there’s a lot of interest in finding cleaner ways to run them.
“There’s lots of attention on compressed natural gas in certain cities, but natural gas buses are a lot more expensive than diesel, and there is emission-control equipment available for diesel systems that can make them much less polluting.”
The study, which was funded by the International Truck and Engine Corp., looked at four types of emissions: primary particulate matter, or “the black stuff you see coming out of the tailpipe”; carbon dioxide; methane, which is released only by natural gas buses; and nitrogen oxide, known as NOx. Each pollutant is important in the environment and in human health, but for different reasons. Carbon dioxide and methane contribute to global climate change, and breathing fine particles – either the primary particulate matter released from the tailpipe or the secondary particulates that form when NOx reacts with other gases in the atmosphere – can lead to respiratory disease and death.
“There is a huge amount of epidemiological evidence to show that these fine particles kill people,” Hammitt notes. Many studies have shown a correlation between daily air-pollution levels and daily deaths from heart attack, though the exact mechanism for this is unknown. It is believed that deaths from all particles in the air – from sources including not only buses and trucks but wood fires, steak cooked on a grill, and power plant and factory emissions – number about 50,000 a year in the United States alone. Compare that with the 40,000 deaths a year caused by motor vehicle accidents.
NOx emissions pose an additional threat: “NOx plus hydrocarbons in the atmosphere plus sunlight equal smog,” says Hammitt, who adds that smog, or ground-level ozone, contributes to respiratory irritation, headache, exacerbation of asthma, and perhaps even death.
Using data gleaned from the Environmental Protection Agency and other government organizations, private testing companies, and manufacturers, Hammitt asked three questions: Which buses pose the greatest hazard to human health, which cause the most damage to the environment, and finally, which are the most cost-effective? What he found may surprise you.
“Which bus you think is better depends on what you care about,” he says. Conventional diesel buses are the cheapest to run, but emit the most particulate matter. Emission-controlled and natural gas buses both offer 70 to 80 percent lower emissions of primary particles; in terms of NOx, natural gas is a 30 percent improvement on both kinds of diesel, though overall it does slightly worse on CO2 and methane. But how does this translate into health effects?
“To compare these different pollutants,” Hammitt says, “we studied the epidemiological literature that tells how many deaths would be caused and how many cases of chronic bronchitis and other disease.” He 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; therefore, according to QALY measurements, it’s more tragic when a young person dies than when an older person does. The QALY approach also says the years a person is well count more than the years he or she is sick; that is, a year lived with chronic bronchitis may equal, say, seven-tenths of a year lived in ideal health.
“Basically, QALYs provide a way to combine mortality and morbidity effects,” says Hammitt, “because obviously we care about both of them.”
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.
But here’s the rub. 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.” That translates to one healthy year of life for nine U.S. citizens, at a cost of $1.7 million per QALY gained.
Hammitt’s conclusion is that it’s a good idea to use emission-control equipment on new vehicles and retrofit it onto old ones, but “more extreme measures to reduce air pollution, such as natural gas, don’t look particularly attractive by comparison, because even though they’re somewhat cleaner, they cost so much more. If we spent money on emission-control diesel, we’d get more health for the money.”
The current policy in cities throughout the United States is mixed, but a number of metropolitan areas are scooping up the new natural-gas buses. Overall, 10 percent of U.S. buses use natural gas, though that figure varies by city; almost three-quarters of the Los Angeles fleet, for instance, is natural gas. But L.A. may be jumping the gun.
“It’s important to look at all the consequences of a decision,” says Hammitt. “Natural gas is cleaner than diesel, that’s right – but that’s not sufficient. Because what do we give up to get that cleaner air? We could get a lot more health by spending that money in other ways, like cancer screening and more stringent controls on old power plants.”