Snowblindness is a sunburn of the cornea. It develops without warning, hurts intensely 6–12 hours later, and is entirely preventable with the right lens category. Here is everything you need to know.
Snowblindness — photokeratitis — is one of the most reliably preventable injuries in mountain sports and one of the most consistently underestimated. Every year, experienced mountaineers, ski tourers and hikers develop a condition that produces intense, incapacitating eye pain through an entirely avoidable mechanism: UV radiation from snow-reflected sunlight burning the corneal epithelium while they were having too good a day to worry about eye protection.
The reason snowblindness is so frequently underestimated is its delayed onset. The UV damage occurs silently during the day. The pain begins 6–12 hours later, typically at night or early morning, when the inflammation from the corneal UV burn peaks. By then, the person is in camp or the hut, far from the cause, often unable to identify what happened. The connection between the sunny day on the glacier and the agony that followed is not always made.
The Physics: Why Snow is Uniquely Dangerous for Eyes
At sea level on a clear summer day, the sky provides UV radiation from above. At 3,000m altitude on a snow-covered glacier on a clear day, a mountaineer is receiving UV from three directions simultaneously:
- Directly from the sun above — increased by 10–12% per 1,000m altitude due to reduced atmospheric filtration
- Reflected from the snow surface — snow reflects 80–85% of incident UV, effectively doubling the UV exposure compared to an identical altitude without snow cover
- Scattered from the sky — blue sky on a clear day scatters significant UV from all directions
The combined UV exposure on a high alpine glacier on a clear day is comparable to looking directly at the sun for extended periods. The cornea, with no melanin-based UV protection mechanism (unlike the skin), absorbs this energy without any protective response — no pain, no discomfort, no warning signal. The damage accumulates invisibly throughout the day.
Lens Categories: The Number That Matters
Sunglasses and goggles are rated by lens category (0–4) based on light transmission percentage. The category determines what terrain they are appropriate for:
| Category | Transmission % | Appropriate for |
|---|---|---|
| Cat 0 | 80–100% | Indoor or very low light; no UV protection implication |
| Cat 1 | 43–80% | Overcast days; low-light conditions |
| Cat 2 | 18–43% | Average sunlight; standard outdoor use |
| Cat 3 | 8–18% | Bright sunlight; beach and high-altitude hiking |
| Cat 4 | 3–8% | Extreme high-altitude glacier terrain; mandatory above 3,000m on snow |
Standard sport sunglasses sold in outdoor shops are typically category 3. For glacier travel and high-altitude snow above 3,000m, category 4 is the appropriate choice — not a recommendation, a requirement. Category 3 glasses in glacier conditions still allow sufficient UV transmission to cause photokeratitis over a full day’s exposure. Category 4 lenses are the only adequate protection for sustained high-altitude snow travel.
Fashion sunglasses with dark lenses and no CE/EN category marking provide no guaranteed UV protection. A dark lens without a UV filter causes a more dangerous situation than no glasses at all: the pupil dilates in response to the reduced light transmission, allowing more UV to reach the retina while the lens absorbs none of it. Always verify the category marking and UV 400 certification before using sunglasses for mountain terrain.
Frame Geometry: Why Side Protection Matters
Wrap-around glacier glasses or goggles with side shields prevent UV from entering around the lens edges. Standard flat-lens sunglasses allow significant UV entry from the side — particularly relevant when the reflected UV source (the snow surface) is below and to the side rather than directly in front. True glacier glasses have leather or silicone side shields that close the gap. Climbing goggles provide complete enclosure. For any route above the glacier snowline in full sun, glacier glasses with side protection are the appropriate eyewear.
Recognising and Treating Snowblindness
Symptoms and time course
The onset sequence of photokeratitis is predictable:
- Hours 0–6 after exposure: no symptoms; the UV damage has occurred but inflammation has not yet peaked
- Hours 6–12: gritty sensation in the eyes (“sand in the eyes”), beginning photophobia (sensitivity to light), mild redness
- Hours 12–24: intense pain described as “eyes full of broken glass”; severe photophobia; tearing; eyelid spasm (blepharospasm); inability to open the eyes in any light; temporary vision blurring
- Hours 24–72: gradual resolution; most cases recover fully within 24–48 hours of peak symptoms if the eyes are protected from further UV exposure during recovery
Field treatment
Snowblindness is self-limiting — the corneal epithelium regenerates within 24–72 hours. Field treatment is symptomatic:
- Total darkness or eye patching: the most important measure; further light exposure prolongs the inflammatory response; the person should remain in complete darkness or with eyes tightly closed
- Cold compresses: clean cold cloths over closed eyes reduce inflammation and provide comfort; replace when they warm up
- Analgesics: ibuprofen 400mg every 6 hours provides anti-inflammatory effect and pain relief; paracetamol for additional pain management if ibuprofen is insufficient
- Lubricating eye drops (artificial tears): reduce the gritty irritation and comfort the inflamed surface; available without prescription
- Do not rub: rubbing an inflamed cornea causes additional epithelial damage and extends recovery time
- Topical anaesthetic eye drops (tetracaine, oxybuprocaine): provide immediate pain relief but impair the corneal protective blink reflex and delay healing — they are used for examination in medical settings, not for extended symptomatic relief
The improvised eye shield for snowblindness in the field — if the person must continue moving before proper darkness is available — is a piece of dark material (a buff, a fabric strip cut from clothing) with tiny slits cut every 20mm to allow minimal light transmission for orientation while blocking the majority of UV. This is the traditional Inuit snow goggles design: a piece of bone, antler or wood with narrow horizontal slits. It works. Dark fabric held against the eyes with minimal slit opening is more protective than standard sunglasses in snowblindness treatment because it allows virtually no light rather than just reduced light.
Other Snow Environment Eye Injuries
Ice crystal impact
High-wind conditions on exposed ridges can drive ice crystals into the eyes at sufficient velocity to cause corneal abrasion. Goggles provide complete protection; sunglasses do not. For any route with forecast high winds above the snowline, goggles are the appropriate eyewear.
Conjunctivitis from UV and wind
Alpine conjunctivitis — red, irritated, discharging eyes from combined UV and cold wind exposure — is distinct from infectious conjunctivitis. Treatment: lubricating drops, UV protection during recovery. Chloramphenicol antibiotic drops are appropriate if bacterial infection is suspected (purulent discharge, unilateral presentation), but are not necessary for non-infectious alpine conjunctivitis.
Foreign body (wind-driven debris)
A foreign body on the corneal surface produces intense, localised pain with blepharospasm and excessive tearing. If the foreign body is visible on the white of the eye (not on the cornea): gentle irrigation with clean water, or gentle removal with the moistened corner of a clean cloth. If the pain persists after attempted irrigation or the foreign body is on the cornea: do not attempt removal; patch the eye and evacuate for ophthalmological assessment. Corneal foreign body removal requires slit-lamp examination and appropriate instruments.
The Spare Pair: Non-Optional Equipment
Sunglasses break. They are lost. Lenses scratch to opacity. A glacier day without an adequate spare pair of glasses or goggles is a gamble. The field solution: carry a second pair of minimum category 3 glasses in the pack on any snow route. The second pair weighs 30–60g. The consequences of needing them and not having them include evacuating a person who cannot open their eyes — which is an avoidable mountain rescue that begins with a pair of glasses forgotten at the hut.
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