All-electric vehicles have been touted as the key to a bright, pollution-free future, but a new study has indicated that plug-in vehicles may not be as green as they seem.

The crux of the issue is where the electric car gets its power--if burning coal provides the energy, a "green" all-electric vehicle generates enough pollution to contribute to 3.6 times more soot and smog deaths than gasoline does, said the study published Monday in the Proceedings of the National Academy of Sciences. 

Department of Energy data says that 39 percent of the country's electricity comes from coal, with West Virginia, Wyoming, Ohio, North Dakota and Illinois notching the highest percentages of coal-generated electricity.

In the study, the researchers examined "the air quality-related human health impacts of 10 [alternative fuel] options, including the use of liquid biofuels, diesel and compressed natural gas (CNG) in internal combustion engines; the use of electricity from a range of conventional and renewable sources to power electric vehicles (EVs); and the use of hybrid EV technology."

Vehicles that make the air dirtier increase pollution, making all-electric cars worse for the environment than traditional gas-powered options.

Natural gas is a greener power source since it will allow an EV to produce half as many pollution-related problems as a traditional fuel-powered car, the study found. Hybrids and diesel engines are also healthier options, releasing less greenhouse gas and resulting in fewer pollution-related deaths.  

"It's kind of hard to beat gasoline," said study co-author Julian Marshall, an engineering professor at the University of Minnesota, as quoted by the Associated Press. "A lot of the technologies that we think of as being clean ... are not better than gasoline."

The study additionally found that EVs trap more carbon dioxide, a gas that scientists say is contributing to climate change.

While producing electricity may contribute to global warming, the development of electric vehicle technology will be handy when EVs run on a cleaner grid, according to study co-author and fellow University of Minnesota engineer professor Jason Hill.

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