The picture for other pollutants is more complicated. The study found that on average, air would be cleaner. But in some areas of the country, such as those downwind from coal plants and having certain atmospheric characteristics, there could be a slight increase in levels of mercury, ozone, or particulate matter in the air by 2030, due to increased demand on power plants from plug-in hybrids. The best results would be in areas that use the cleanest electricity, such as in the Northwest, which relies heavily on hydroelectric power.
One implication of the study, says Luke Tonachel, an NRDC analyst, could be that federal incentives for encouraging the sale of plug-in hybrids should be adjusted by region. In some areas, the study shows, conventional hybrids may do almost as well as plug-ins at reducing greenhouse-gas emissions, while resulting in overall cleaner air. It would make sense to promote plug-ins more in other regions, where electricity sources are cleaner.
The report makes clear that turning to plug-in hybrids will mean changing the way the government thinks about vehicles. The standard miles-per-gallon EPA ratings will only give a part of the picture. What’s needed, Tonachel says, is a new standard that conveys the impact of both burning liquid fuels and using electricity, such as a measurement of the overall amount of carbon that using a car will release.