Snow blindness: what it is, how fast it happens, and the sunglasses that prevent it

Snow blindness is a sunburn of the cornea, the clear front surface of the eye, caused by ultraviolet light reflected off snow. A few hours on bright snow without UV-blocking lenses is enough. The symptoms, gritty painful eyes, watering, light sensitivity and sometimes blurred vision, appear six to twelve hours after the exposure, which is why people are surprised by it that evening. It heals in one to three days and it is entirely preventable: lenses that block UV and a frame that covers the sides, because on snow the UV comes from below as well as above.

What it is

The medical name is photokeratitis. Ultraviolet light damages the outermost layer of cells on the cornea, the same way it damages skin. The cornea is dense with nerve endings, so once the damaged cells slough off, the surface is raw and it hurts. Because the damage takes hours to develop, there is no warning at the time.

The same injury happens to welders who look at an arc without a shield, and to people who use tanning beds without goggles. Snow is the most common outdoor cause because of how much UV it reflects.

Why snow

Three things stack:

1. Snow reflects UV. Fresh snow bounces up to about eighty percent of the ultraviolet that hits it back upward. Grass reflects a few percent; sand around fifteen; water around ten. On snow, your eyes receive UV from the sky and nearly the same again from the ground.
2. Altitude adds more. UV increases by roughly ten percent per thousand metres, because there is less atmosphere above you to absorb it.
3. Cloud does not save you. Most UV passes through cloud. Overcast days on snow cause snow blindness too, and are more dangerous because people take their glasses off.

The timeline

Most cases resolve completely. Severe or repeated cases should be seen by an eye doctor. Treatment is rest in a dark room, cold compresses, artificial tears, and not rubbing; contact lenses come out until it has healed.

Prevention: the lens

Any lens that blocks UV prevents the burn. The label to look for is UV400 or 100% UVA/UVB; our UV400 vs polarized post explains why "UV protection" without a number is a weaker claim, and on snow that difference matters. Darkness is separate: a dark lens without UV blocking is worse than none, because the pupil opens behind it.

Polarization is a comfort feature here, not a safety one. It cuts the glitter, which reduces squinting over a day; it does nothing about UV. Our skiing post covers when polarized helps on snow and when it hides ice.

Children are at more risk than adults, because a child's eye lens passes more UV; our kids' sunglasses post covers what to look for.

Prevention: the frame

On snow, UV arrives from below and the sides. A flat frame that shields only the front leaves those open. For a long day on snow, or any time on a glacier or at altitude:

• Glacier glasses. Dark, UV-blocking lenses with side shields, sometimes a nose guard. The purpose-built tool.
• Goggles. Seal the whole eye area. The choice in wind, snowfall or cold.
• Wrap sunglasses. Close-fitting frames with wide temples. Adequate for short exposure.

An ordinary rectangular frame with a UV claim, like our T01, protects the front and is fine for a spring afternoon or a lift line. It is not enough for a full day on a glacier; we would rather say that than sell it for one. The frame maker states "UV protection" without a cut-off wavelength, and for serious snow use you should ask any seller for UV400 in writing.

Frequently asked questions