E-scooters have a very environmentally-friendly image. Tiny and battery-powered, they seem like a convenient, responsible alternative to making short trips in a car, but new research suggests that the big picture is a lot more complicated.

A team of researchers from NC State took an in-depth look at the overall environmental impact of the e-scooter trend, examining the emissions output associated with building, shipping, and charging the scooters, as well as the travel habits of individuals who often use them. The results paint a troubling picture of the seemingly green transportation option.

E-scooters themselves are zero-emission vehicles, and that’s great. However, as the researchers explain, the manufacturing process that actually produces the scooters is far from pollution-free, while shipping and distribution of the scooters adds even more to their carbon footprint.

But perhaps the most striking thing the study reveals about the e-scooter trend is that the majority of those who rely on them for their transportation needs aren’t doing so in lew of a car ride. In fact, in a survey of e-scooter riders, the team found that 49 percent of them would have biked or simply walked if the scooters weren’t readily available.

The most damning aspect of e-scooter proliferation seems to be the fact that companies rely on other vehicles — gas-burning trucks and vans — to collect the scooters and redistribute them to the locations where they are picked up by users.

“We found that the environmental impact from the electricity used to charge the e-scooters is fairly small – about 5% of its overall impact,” Jeremiah Johnson, co-author of the study, said in a statement. “The real impact comes largely from two areas: using other vehicles to collect and redistribute the scooters; and emissions related to producing the materials and components that go into each scooter.”

Put simply, e-scooters may be an Earth-friendly alternative to firing up your car for a trip to the store, but walking or biking proves to be a far more environmentally conscious.