It’s unspoken, but every driver gliding around town behind the wheel of a Prius is thinking the same thing: “I’m saving the planet. What are you doing, you dirty-fossil-fuel burner?”
What’s implied is that hybrid or electric-car drivers are also saving human lives, since the fuel-burning internal combustion engines that power conventional vehicles emit carbon dioxide and fine particulate matter including acids, organic chemicals as well as dust and soil; this pollution has been linked to respiratory and heart problems and cancer.
But, according to a new study published in the journal Environmental Science & Technology, it turns out that the use of electric vehicles may not be that clean after all, particularly in the world’s most populated country, China.
In the study, Christopher Cherry, an assistant professor of civil and environmental engineering at University of Tennessee, and his colleagues found that in terms of air pollution, electric vehicles were more harmful to public health per kilometer traveled than gasoline-powered cars. That’s right — the electrically powered cars turned out to be dirtier than those with internal combustion engines.
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How could that be? Cherry says there’s been an implicit assumption that because electric cars don’t burn fossil fuels, they’re cleaner for the environment and safer for people, but that doesn’t take into account how the electricity they use is generated. In China, that would be from — you guessed it — fossil fuels. About 85% of the country’s electricity is powered by fossil fuels, of which 95% is coal.
“It’s tricky comparing electric vehicle emissions with emissions from internal combustion engines, because you can’t compare the emissions,” says Cherry. “With gasoline engines, a 1-1 change in emissions results in a 1-1 change in health outcomes because the emissions are released in the same place where people inhale them.”
That’s not the case with electric cars, whose dirty emissions are released at the electricity-generating power plant, while the vehicle is used elsewhere. It’s this disconnect that has given electric vehicles an apparently cleaner bill for health, but Cherry says that once you factor in how many people within the range of electricity generating power plants are affected by emissions, the story gets a little dirtier. In China’s case, pollution from electricity plants is spreading exposure to potentially harmful particulates in the air from urban populations to those in more remote rural regions.
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Kilometer per kilometer, electric cars in China beat out conventional vehicles as among the worst environmental polluters. On average, the fine particulate emissions per passenger-km are 3.6 times greater for electric cars than for gasoline cars. That’s better than for diesel cars but on par with diesel buses, which can spread their environmental impact across the number of passengers they carry. “If we compare gasoline car emissions to electric car emissions, the electric cars look very, very bad,” says Cherry. “So the point is that you have to consider the emissions exposure when the exposure source is far apart — the electrical power plant as opposed to the tailpipe of a car.”
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The problem is that the Chinese government, in a well-intentioned effort to promote more eco-friendly power use, has been pushing electric cars, motorcycles and scooters in recent years. The effort has been so successful that electric bike ownership is surging at 86% annual growth. There are now 100 million electric bikes on China’s roads, and they outnumber gas-powered cars 2-to-1.
The good news is that while electric cars didn’t fare so well in reducing emissions, electric bikes and scooters — which typically use one-tenth the electricity of the cars — did a lot better. The researchers found that e-bike usage improved air quality and environmental health by displacing the use of larger, more polluting vehicles.
There’s also some hope in China’s changing energy policies; cities in the southwest have adopted cleaner electricity generating power sources, and generally release fewer emissions than those in the northeast. Cherry notes also that electricity generation in the U.S. is cleaner than it is in China, which means that the impact of electric car use in the two countries can’t be compared. But the results highlight an important lesson not just for China but for anyone eager to scale up alternative energy production as a way to benefit both man and the planet.
“China has a lot more room for improvement in its power sector, and the lowest hanging fruit would be to clean up its power sector first,” says Cherry, rather than focusing on lowering vehicle emissions. Once that happens, he says, “electric cars will have room to gain on conventional cars in the long run.”
Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.