(CNN) "My friends told me I looked like a human missile," said Brandon Nelson, a 32-year-old firefighter, as he recounted an accident last month on an electric scooter in San Diego, one of hundreds released onto that city's streets by Bird, a California-based rideshare company.
Heading to a hotel across town with five other firemen, he struck a bump in the road and flew off the scooter. Like most riders, he wasn't wearing a helmet, and he lost consciousness after hitting the pavement.
At the UCSD Medical Center, Nelson was treated for a concussion, a broken nose and lacerations across his face, leaving him with wounds that have yet to heal a month later.
"I was in the trauma room for maybe 20 minutes when a 28-year-old man was wheeled in with a head injury," Nelson said. "He had also been riding a Bird scooter and had a brain bleed, so they were taking him up to emergency surgery to relieve the pressure in his head."
Nelson, who grew up riding dirt bikes and motocross, never had an injury like this. "I'd fallen off those bikes before, but I always had a full helmet for protection," he said. "With the scooters, you're wearing whatever you wore to dinner."
The scooters have swarmed cities this past year, with companies like Bird and Lime aggressively expanding into markets across the United States as well as Europe - most recently, with Lime reaching Paris, Berlin, Frankfurt, Zurich, Madrid and Valencia and Bird entering the streets of Vienna, Brussels, Paris and Tel Aviv.
But the machines have brought with them a wave of scooter-related injuries and even two recent deaths.
One man, Carlos Sanchez-Martin, died on a Lime scooter last week after an SUV struck him in Washington, DC, dragging him for over a dozen yards and pinning him under the vehicle, according to the Washington Post. And earlier this month, 24-year-old Jacoby Stoneking died after falling off a scooter he was riding home from work in Dallas, according to CNN affiliate KTVT.
"You have a public that hasn't been aware of the inherent dangers in what they see as a fun recreational vehicle," said Dr. Sam Torbati, co-chair of the emergency department at Cedars-Sinai Medical Center in Los Angeles. "They're no safer than any other motorized vehicles, and in some ways, are more dangerous than a bicycle."
Catherine Lerer, a personal injury lawyer in Los Angeles who is representing Nelson, says she receives calls every day from injured riders. "We're not against electric scooters," she said, "but they are less stable than bicycles for a variety of reasons. They have a shorter wheelbase and they have smaller wheels, so they're going to be affected by any defects in the pavement."
Users can locate and unlock the devices using their smartphones, but scooter companies require little training for first-time riders. Bird's app, for example, just shows users a series of graphics that tell riders to do things such as "bring your own helmet" and "ride in bike lanes."
Lime, which operates in more than 70 cities across the country, offers a "How to Lime" instructional video, but the company doesn't require riders to watch it before riding. Without that guidance, the scooters, which travel up to 15 miles per hour, can take some riders by surprise.
"I didn't realize how fast the scooters went and how quickly they accelerated," said Daniela Rivera, a 25-year-old actress who lives in Los Angeles. "I got on the scooter, pressed the gas, and the scooter just completely slipped out from under me," she said. "I skid across the gravel floor and had burns all over my legs."
Rivera said she couldn't work for a week after the accident. She is a server at a restaurant and said she couldn't put pants on without bleeding through them.
There are no official statistics available on injuries from standing electric scooters, but Torbati estimates that his hospital has seen more than a hundred such injuries this year, ranging from minor scrapes to life-threatening traumas. Up to 40% of those are head and neck injuries, he said.
Even though companies recommend that riders wear helmets, few actually do, and California recently scrapped a law that required riders to wear them on scooters. Individual cities, however, may still require them.
"Most of the patients aren't wearing helmets," said Torbati. He noted that those who fall without head protection "might just have mild bruising or a mild concussion, but if they hit their heads hard enough, they can have fractures, internal bleeding, lacerations of the face and broken jaws."
Beyond helmets, a patchwork of laws across municipalities can make it difficult for riders to safely navigate new cities. "There is no uniform set of rules right now that govern the use of scooters [across] local jurisdictions," said Scott Cummings, a professor at UCLA School of Law.
"The business model of the companies is to roll out the scooters and then respond to the regulatory backlash," he said. While that business model has proved successful, it can also sow confusion among new riders.
In Denver, for example, electric scooters are classified as "toy vehicles" that must be ridden on sidewalks. But across California, that's a crime, and riders can only travel in bike lanes or on city streets.
In the UK, strict laws mean electric scooters are yet to take off. Classed as "powered transporters" under the 1835 Highways Act, they cannot be used on sidewalks and driving them on the road would require the scooters to be registered, licensed and insured and all users would need to wear a helmet.
Accidents, however, aren't always a rider's fault. "The most common call we're getting is from people who were injured because of a scooter malfunction," said Lerer. "And the number one malfunction we're seeing are brakes not working and throttles sticking."
To use the scooters, riders often have to agree to lengthy terms of service that release companies from legal liability for injuries on their devices. Lime's contract, for example, requires users to acknowledge that "the products are machines that may malfunction, even if the products are properly maintained, and that such malfunction may cause injury."
But Lime remains confident in its devices. Spokeswoman Mary Caroline Pruitt said that the company "is continuously upgrading and improving [its] products" and has "a team on-the-ground in charge of inspecting and maintaining scooters on a daily basis."
Bird says it performs similar maintenance on its vehicles, and a company spokesman said that "the true danger on our roads is cars." She noted that 129,000 pedestrians were treated in emergency rooms for crash-related injuries in 2015, according to the CDC.
Nonetheless, Lerer says that companies aren't doing enough to maintain their scooters and keep riders safe, and her law firm is planning legal action on behalf of injured riders.
"We are going to argue that the user agreements are not enforceable because the scooter companies have been grossly negligent," she said. "We feel confident that we're going to be able to defeat the user agreement."
Rivera, who still has scars on her legs, is also fighting back. "I'm never getting on one again," she said. "And I tell every person I meet to stay away from them."