the Evil Line

Plug-in hybrids are good, but not perfect:

For a plug-in owner in California, where most electricity on the grid is generated by low-pollution facilities, driving a PHEV might cut emissions of carbon dioxide by one-third compared with driving a regular hybrid.

But if the same PHEV were charged in the Midwest, where coal-fired power plants supply the electricity, reduction of CO2 emissions would be nil. Nitrous-oxide emissions (which form smog) would fall slightly, but sulfur-dioxide emissions (which contribute to acid rain) would quadruple.