Bird boasts that its dockless electric scooters allow customers to “cruise past traffic and cut back on CO2 emissions—one ride at a time. Its rival Lime claims the vehicles “reduce dependence on personal automobiles for short distance transportation and leave future generations with a cleaner, healthier planet.”

But the mere fact that battery-powered scooters don’t belch pollution out of a tailpipe doesn’t mean they’re “emissions free,” or as “eco-friendly” as some have assumed. The actual climate impact of the vehicles depends heavily on how they’re made, what they’re replacing, and how long they last.