The first time Christopher Crescitelli contracted what he calls “VR arm syndrome,” he stopped playing one of his favorite virtual reality games for five days. “My arms feel disconnected from my body, almost like they were my virtual arms,” he says. “When I first started experiencing this, I thought, ‘Did I break myself?’”

Crescitelli is a VR designer and developer, the owner of Las Vegas–based Dreamland Entertainment, and the founder of VR Fest, a sprawling multiday event now in its fifth year. Yet even Crescitelli, a person with more than 10 years of experience with VR, says he occasionally has to force himself out of the virtual realm to ensure he doesn’t suffer from headset fatigue (or VR arm syndrome).

Virtual reality is a modern-day beacon of escapism — a way to fully immerse yourself in other worlds — and it’s seeing unprecedented applications. VR-based exposure therapy, for instance — in which patients are exposed to the sources of their fears — is already changing how some psychologists treat patients. VR is finding myriad applications in health care. And there’s seemingly no limit to its use in games and, yes, porn.

The market, no surprise, is exploding, with some industry groups estimating a $60 billion global market by 2022. In late 2017, sales of virtual reality headsets exceeded 1 million in one quarter, with Sony, Oculus, and HTC leading the pack. In 2018, as prices for virtual reality hardware drop even further, nearly 8 million units are expected to sell.

As business booms, however, people who are using the tech are reporting a growing number of physical side effects — like VR arm, but worse: eye strain, dizziness, headaches, nausea, and even dissociative experiences. VR companies recommend that people take frequent breaks and moderate their VR time when they’re first starting out. “As you become accustomed to the virtual reality experience, you can begin increasing the amount of time you use Daydream View,” reads one line of the health and safety information included with Google’s VR platform.

But what happens when it’s your job to build these escapist technologies? The potential health risks for everyday consumers are compounded for those who make VR products for a living.

“We’re on the lookout to see if there are any long-term consequences.”

When VR bigwig Jeremy Bailenson founded Stanford University’s Virtual Human Interaction Lab, in 2003, two items were even more important than the VR equipment he was using: “We had to keep a bucket in the lab and a mop nearby,” Bailenson says. Today, he institutes a strict 20-minute limit on headset time for people in his lab.

These health effects produce unique challenges for VR developers. “We have to understand not just the good but also the downsides of this technology. There are a lot of questions we need to answer,” Bailenson says. “The whole point of VR is it takes you out of your space, but you can’t be doing that for many hours a day.”

As VR has gotten more sophisticated, so too have its health implications. In the 1990s, virtual reality creations were computer generated without any expectation that what was being developed would look like reality, according to Stephanie Riggs. An experienced VR developer and instructor, Riggs got her start two decades ago working with the late Randy Pausch, the former Carnegie Mellon University professor and VR pioneer.

“When we were developing 20 years ago, we custom-built all of our input and optic devices. If you were Godzilla stomping around blowing fire, we built a blow device — we weren’t pushing a button,” Riggs says. “Now we have things that do kind of trick the senses.”

In today’s VR environment, that can mean seeing an expansive field that stretches for miles even though the screen projecting the image is just centimeters away. It can mean sitting in a moving vehicle in the virtual world while remaining stationary in real life. It can also mean performing movements — walking forward, catching a ball — by pushing buttons on handheld controllers, which is how newer VR systems tend to allow for navigation of a virtual space.

Suddenly rotating around a virtual environment using handled controllers or quickly looking left and right in the VR space without any concomitant physical movement in the real world tend to physically affect Jonathan Yomayuza, VR technical director at the Emblematic Group, a creative firm based in Southern California. “When it’s very jerky and swift, quick movement, I actually get really nauseous,” he says. “After 10 minutes, it’ll kick in, and then I’ll be feeling sick for about an hour and a half.”

The feeling Yomayuza describes is common among people who work with or use VR. Donning a headset through which images are played preserves optic flow—the visual cues of the world that come through our eyes—but impedes the vestibular cues, the functions of the inner ear that help us maintain our balance as we walk around. This sort of sensory conflict is the source of cybersickness, the onset of nausea and dizziness that some people feel in virtual reality.

“People get that uneasy feeling because what’s visually happening to them isn’t matching what’s physically happening to them,” Riggs says.

At the Emblematic Group, a team of 14 VR developers typically spends about six hours per day working on the computer and two hours a day inside headsets. Normally, that headset time is broken into smaller chunks, at intervals of fewer than 30 minutes, although Yomayuza says he can sometimes spend upwards of 45 minutes within a VR simulation.

Limiting one’s time in the virtual world is important for mitigating eye strain and headaches. Put on a pair of goggles and you’ll see a three-dimensional image that isn’t actually in 3D, but rather is displayed on a flat surface. This leads to vergence-accommodation conflict, a term that refers to how the eye zeroes in on an object.

When we fixate on something, our eye movement is the vergence; accommodation is when the lenses inside our eyes focus light on the retina to make an object appear in sharper focus. In the real world, the two processes are yoked together and happen involuntarily and simultaneously. In a headset, that relationship is broken.

“People get that uneasy feeling because what’s visually happening isn’t matching what’s physically happening.”

“The light comes from the screen, but the content might be behind the screen or in front of the screen,” says Marty Banks, a professor at the UC Berkeley School of Optometry. “For the most part, we think that if you take the headset off and go back to normal visual activity, that’s a good thing to do. The eye strain and headache will go away. Not instantly, but we’re on the lookout to see if there are any long-term consequences.”

Most developers think the physical side effects of virtual reality go away with increased usage.

“When I was first using virtual reality, I couldn’t do every game that had me moving the VR headset without me moving,” says Lee Vermeulen, a game and VR developer from Toronto, Ontario. “That’s pretty common for people playing VR for the first time. Over time, it became less of an issue. My mind just adapted toward it.”

But the psychological consequence of spending extended time within a VR simulation is an open question — one that researchers are actively probing. In 2016, two philosophers hired by the European Commission to study the ethical implications of virtual reality released the first code of ethics for consumer VR. The code, which was published in the journal Frontiers in Robotics and AI, concluded that “behavior while in the virtual environment can have a lasting psychological impact after subjects return to the physical world.”

Online, it’s easy to find anecdotal evidence of follow-on effects, such as anxiety or a strong feeling of sadness after exiting VR. Some news articles cite a 2006 study by clinical psychology researcher Frederick Aardema that found, in one writer’s words, “that VR increases dissociative experiences and lessens people’s sense of presence in actual reality.”

Albert “Skip” Rizzo, a virtual reality leader since the 1990s and director of the Institute for Creative Technologies at the University of Southern California, says he hasn’t seen or heard of psychological effects that lead to cognitive or emotional upheaval among developers. “I just don’t hear people saying, ‘I worked in VR, it messed with my mind, and now I can’t relate to real people anymore.’”

Others are less sure. On his personal blog, the Toronto developer Vermeulen recounts an experience from 2014 where, at a conference, he tried a demo of VR technology being developed by video game company Valve. “[L]ooking back my memories of it are like I was actually there and not just viewing pixels on a screen,” he writes. “It was better than real life — people will get lost in this and not want to leave. Nothing else gives the same escapism.”

Vermeulen goes on to note that once the demo was over, he knew it was over — and yet he couldn’t quite shake it. He had to touch the walls around him and eventually sit down to overcome his “weird existential dread” and recognize that the demo was truly finished. “It was so incredibly weird that it got me worried about the tech in general… that the line will blur and the entertainment we consume… will be something our brains think we actually experienced.”

Vermeulen says that, over time, these effects have waned. He does, however, offer a word of caution for the future, as VR technology gets ever better. “I think distinguishing VR from reality is going to be a lot more difficult,” he says. “I don’t know if it’s really a problem. I’m not saying it really is. But that’s something that could come up.”

Some VR developers are working on solutions that would make clear where virtual reality ends and reality begins. It’s a literal framing challenge. The content we consume on our smartphones and computers, for instance, is always framed — there’s a line separating what we’re viewing from the world around us. But stepping through that frame, as one does in VR, changes how we perceive what we’re interacting with.

“I think distinguishing VR from reality is going to be a lot more difficult.”

At the Emblematic Group, Yomayuza says his team already builds framing references — an immovable foreground or overlay — into its products to denote for the viewer that they’re inside a virtual environment.

Even so, there are still scenarios that trigger physical feelings of nausea and dizziness and who knows what else over time.

“At the end of the day, it comes down to the person using the headset and how sensitive they are toward it,” Yomayuza says. “The thing we make sure to tell people: They’re free to take it off whenever they want. If it triggers something, just take off the headset.”