Electric scooters might not be as environmentally friendly as billed.

A new study by researchers at North Carolina State University found that e-scooters generally produce more greenhouse-gas emissions per mile than standard diesel buses with high ridership, MIT Technology Review first reported.

The study looked at the lifecycle of e-scooters—including production, shipping, charging, collection, distribution and disposal—to calculate how they compare with other modes of transportation.

It found around two-thirds of the time, scooters contribute more emissions per mile than other alternatives—such as high volume busing, electric mopeds, electric biking, biking and walking. However, cars produce about twice the amount of carbon dioxide per mile compared to scooters.

To determine scooter emissions, the study focused on the vehicle's components, such as aluminum frames, lithium-ion batteries and electric motors, and assessed how raw materials for those parts are extracted and delivered. It calculated that the average lifespan for rental scooters is 28.8 days.

In a survey of Raleigh, North Carolina scooter riders, nearly half of all respondents said they would have biked, walked, taken a bus or skipped the trip if scooters weren't available.

Data from Portland Bureau of Transportation's preliminary 2018 e-scooter findings report show that the availability of e-scooters in the city did decrease car driving somewhat. Thirty-four percent of Portland scooter riders surveyed said said they took an e-scooter instead of using a car. However, the survey didn't ask whether scooter riders would have utilized other transit options if scooters weren't available.

A spokesman for PBOT, Dylan Rivera, says the city is "certainly concerned about the life cycle impact of our transportation choices, including e-scooters."

He says the agency is offering incentives to scooter companies that demonstrate ability to lessen emissions in Portland, "such as using e-bikes or other low-emission vehicles to pick up and rebalance scooters."

"This study suggests that as e-scooters are built with more durability, their total impact on carbon emissions will go down," Rivera says. "And the study confirms that, if we can continue to replace automobile trips with e-scooter trips today, we can immediately reduce carbon emissions from the transportation sector."