Electric scooters are clogging up big city sidewalks and jamming up hospital rooms, too.

Using U.S. emergency room reports of e-scooter injuries, scientists estimate there were 19 injuries per 100,000 people in 2018, compared with just 6 injuries per 100,000 in 2014. That’s a 222 percent jump in four years, scientists report January 8 in JAMA Surgery. The spike comes as companies increasingly offer rental scooters as easy, fast and cheap alternatives to cars.

Because the researchers relied on U.S. hospital records, they don’t know the reasons for the crashes; alcohol, speed or reckless driving could all have been at play. Information about helmets wasn’t available either, but other studies of e-scooter riders suggest that the protective gear isn’t common.

Adults aged 18 to 34 accounted for about a third of the estimated 14,651 e-scooter injuries in 2018. That same year, these young adults also sustained 44 percent of the 1,374 e-scooter injuries that warranted admission to a hospital.

The rise in these injuries mirrors the rise of e-scooters in recent years. In 2018, there were more than 85,000 electric scooters for rent in about 100 U.S. cities, the National Association of City Transportation Officials estimates.