People who own all-electric cars where coal generates the power may think they are helping the environment.

电动汽车的用电来自燃煤发电厂，使用电动汽车的人可能认为他们保护了环境。

But a new study finds their vehicles actually make the air dirtier, worsening global warming.

The controversial study raises major questions over the future of 'green' cars.

'Unfortunately, when a wire is connected to an electric vehicle at one end and a coal-fired power plant at the other end, the environmental consequences are worse than driving a normal gasoline-powered car,' said Ken Caldeira of the Carnegie Institution for Science, who wasn't part of the study but praised it.

Driving vehicles that use electricity from renewable energy instead of gasoline could reduce the resulting deaths due to air pollution by 70 percent, it concluded.

Ethanol isn't so green, either, the researchers claimed.

'It's kind of hard to beat gasoline' for public and environmental health, said study co-author Julian Marshall, an engineering professor at the University of Minnesota.

'A lot of the technologies that we think of as being clean ... are not better than gasoline.'

The key is the source of the electricity all-electric cars need.

If it comes from coal, the electric cars produce 3.6 times more soot and smog deaths than gas, because of the pollution made in generating the electricity, according to the study that is published Monday by the Proceedings of the National Academy of Sciences.

They also are significantly worse at heat-trapping carbon dioxide that worsens global warming, it found.

The study examines environmental costs for cars' entire life cycle, including where power comes from and the environmental effects of building batteries.

But if the power supply comes from natural gas, the all-electric car produces half as many air pollution health problems as gas-powered cars do.

And if the power comes from wind, water or wave energy, it produces about one-quarter of the air pollution deaths.

Hybrids and diesel engines are cleaner than gas, causing fewer air pollution deaths and spewing less heat-trapping gas.

But ethanol isn't, with 80 percent more air pollution mortality, according to the study.