USA Today is reporting that the expected introduction of plug-in hybrid electric vehicles could cut U.S. gasoline use but could increase deadly air pollution in some areas. That's because a plug-in's lower tailpipe emissions may be offset by smokestack emissions from the utility generating plants supplying electricity to recharge the big batteries that allow plug-ins to run up to 40 miles without kicking on their gasoline engines.

Plug-ins, called PHEVs, are partly powered, in effect, by the fuel used to generate the electricity. About 49 percent of U.S. electricity is generated using coal, so in some regions a plug-in running on its batteries is nearly the equivalent of a coal-burning vehicle. The trade-off is one that even plug-in backers acknowledge. It could undercut the appeal of vehicles that appear capable of using no gasoline in town and hitting 50 to 100 mpg overall fuel economy. If large numbers of plug-in hybrids were being recharged with power from the least-sophisticated coal plants, "There is a possibility for significant increases of soot and mercury," says a report by environmental advocacy group Natural Resources Defense Council. Soot particles can make it hard to breathe, especially for asthmatics. Mercury is toxic. Sulfur dioxide also may be an issue. A study by the Minnesota Pollution Control Agency found plug-ins also could result in more sulfur dioxide (SO2) emissions. SO2 is toxic in large amounts and is a component of corrosive acid rain. The Minnesota study found that use of PHEVs would lower most emissions compared with other vehicles, but that resulting SO2 emissions would be more than double those from gasoline vehicles and about three or four times greater than from driving a regular hybrid.

The Minnesota study also found that PHEVs would emit more carbon dioxide (CO2) than driving a conventional hybrid. CO2 is a greenhouse gas thought to contribute to global warming.