(CNN)As the only American to have climbed all 14 of the world's peaks over 26,000 feet above sea level — and the fifth person to do so without using supplemental oxygen — high-altitude mountaineer Ed Viesturs has come closer to the sun here on Earth than most of us ever will.
Snow blindness can sneak up on you, but prevention is easy
Among the things the 61-year-old always brings on big adventures is an essential accessory for anyone spending time outdoors this winter: protective eyewear.
"I've been on mountaineering expeditions where we had to actually help someone down off a mountain because they suffered snow blindness at high altitude," said Viesturs, referring to the medical condition known as photokeratitis.
A temporary but painful condition, it occurs when exposure to ultraviolet rays creates inflammation on the surface of the eye.
"It happens to people when you don't realize how bright and sunny it is and you're not wearing sunglasses," Viesturs said. "Maybe you took your sunglasses off because you're filming or you just forgot them for a while."
While he has not suffered the condition himself, Viesturs has helped many others reeling in pain from it during expeditions. "The best description I've heard is that it feels like someone is pouring sand in your eyes," he said.
Even if you're more focused on snowshoeing or simple hikes than bagging big peaks this winter, snow blindness is something that can creep up on you in much milder conditions — and much closer to sea level — too.
The easiest way to think of photokeratitis, said Dr. Brian Zaugg, a cornea specialist with the John A. Moran Eye Center at the University of Utah in Salt Lake City, is like getting a sunburn on your eyes.
"People don't realize their eyes are covered in skin like the rest of their body," he said. "So they're just as prone to getting a sunburn."
"Most people, in the winter, they don't cover their eyes," he said. "They cover their whole body because it's cold, and they leave their eyes uncovered. So they're susceptible."
Often painful, the condition can be accompanied with blurry vision and watery eyes, among other symptoms, and affects the eyes' corneas and conjunctiva (the latter is the clear tissue over the white of your eye).
"Those are the parts that get the burn," Zaugg said.
Despite its moniker, snow blindness doesn't only occur in snowy and icy conditions.
"It happens at the beach as much as on snow," he said. "The UV rays reflecting off snow and ice can create a double exposure (from sunlight overhead and the reflection on a surface) and that's when it's risky — it's the same effect as staring at water."
Ultraviolet radiation levels increase by 10% to 12% with every 1,000-meter increase in altitude, according to the World Health Organization.
Exposure at higher altitudes is more damaging since the air is thinner, making the UV exposure more intense, Zaugg said, but photokeratitis can happen at lower altitudes, too.
Despite the name, it can even be caused by the glare off pavement and sand.
"We think of higher elevations causing more problems," Zaugg said. People used to living at sea level who take a ski trip to higher elevations, however, can be more prone to getting photokeratitis because they may not think to wear eye protection.
The good news is that a little eye protection goes a long way — and you hardly need to spend hundreds of dollars on sunglasses.
Indigenous cultures from the Arctic, including the Inuit and Inupiat, carved snow goggles from things like whale baleen and caribou bone, with narrow slits to peer through to reduce exposure to glare from snow and ice.