UV protection for your eyes: sunglasses, snow and altitude

A sunny day on a glacier under a deep blue sky – and by evening your eyes sting as if there were sand in them. That is often how snow blindness starts. Your eyes take in UV radiation just like your skin, but there is no sun cream for them. Here you can read why mountains and snow raise the dose so markedly, and how to recognise sunglasses that genuinely protect.

Why altitude and snow increase your UV dose

According to the World Health Organization (WHO), UV levels rise by roughly 10 per cent with every 1,000 metres of altitude. Climb from the Swiss Plateau to 3,000 metres and, by this rule of thumb, you receive roughly a quarter more UV. Then there is the ground: snow can reflect as much as 80 per cent of UV radiation, and fresh snow almost doubles your exposure, says the WHO. Grass, soil and water, by contrast, reflect less than 10 per cent, and dry beach sand about 15 per cent.

Clouds are not reliable protection either. Dense cloud generally lowers UV levels, but thin clouds have little effect – through scattering they can even increase them. A modelling study by the universities of Lausanne and Geneva with MeteoSwiss (2019) adds a further point: on a cloudless summer day, 51 to 78 per cent of the UV dose around the eyes and face came from diffuse radiation – from the whole sky rather than straight from the sun. So even with the sun at your back, your eyes still catch a fair share. The WHO advises taking protective measures once the UV index reaches 3.

Snow blindness: sunburn of the cornea

The medical term is photokeratitis, an inflammation of the cornea. The American Academy of Ophthalmology (AAO) likens it to a sunburned eye. The catch is the delay: according to EyeWiki, the AAO’s clinical encyclopaedia, symptoms appear 30 minutes to 12 hours after exposure – often only in the evening or at night. Typical signs are pain, a gritty feeling, tearing, sensitivity to light, redness, swollen eyelids and blurred vision; rarely, vision is lost temporarily. Artificial UV sources such as sunbeds or arc welding can trigger photokeratitis too.

According to the AAO, symptoms usually ease gradually within a day or two, and EyeWiki notes that the corneal surface normally heals within 24 to 72 hours. According to the WHO, lasting damage is not normally expected. What helps, according to the AAO and EyeWiki:

• take out contact lenses straight away and leave them out until your eyes have healed
• go into a darkened room
• apply cool, damp cloths and use artificial tears
• do not rub your eyes

Warning signs: severe or worsening pain, clearly reduced vision, symptoms that are not clearly improving after one to two days, or symptoms in only one eye need prompt assessment by an eye doctor. Snow blindness usually affects both eyes; if only one is involved, EyeWiki notes that a foreign body may be the cause. Looking directly at the sun can also damage the retina – damage that, as the AAO notes, is longer-lasting and more serious than the corneal irritation. Sunglasses do not protect against that, as the Swiss Federal Office of Public Health (FOPH) stresses. According to the AAO, this kind of damage usually causes no pain, because the retina has no pain nerves. Visual symptoms such as a blind spot in the centre of your vision, distorted lines or changes in colour vision usually appear within four to six hours, though some people only notice them after 12. Even if nothing hurts, have this checked promptly by an eye doctor.

Damage that adds up over the years

Beyond acute irritation, the WHO lists several long-term effects of UV radiation: cataract, in which the lens of the eye becomes cloudy; pterygium; and cancer in and around the eye. UV may also be involved in age-related macular degeneration. Worldwide, the WHO estimates that 15 million people are blind due to cataract, and some 10 per cent of these cases may be caused by UV exposure. You can read more about clouding of the lens in our guide to cataract.

A pterygium is a growth of conjunctival tissue that can spread over part of the cornea. The FOPH explicitly recommends sunglasses with side protection partly to help prevent pterygium and cataract, because reflected UV can otherwise reach the eyes from the side.

Filter categories 0 to 4: what the label tells you

The standard ISO 12312-1:2022, a Swiss national standard since January 2023, sorts sunglasses into five filter categories. They describe glare protection – how much visible light gets through:

For everyday use the FOPH recommends category 2 or 3, and category 4 with side protection for extreme conditions. Important: category 4 lenses are too dark to recognise traffic lights reliably and are therefore not permitted on the road; the label then shows a crossed-out car. So for the drive home from the glacier, you need a second pair. Broad temples and side shields are not suitable on the road either, says the FOPH, because they restrict your field of view.

Tint is not the same as UV protection

How dark a lens is gives no reliable indication of its UV protection – according to the FOPH, that depends on the material, not on colour or tint. Look for the CE mark together with ‘100% UV’ or ‘UV 400’. ‘100% UV’ means the lens blocks practically all UV radiation up to 380 nanometres; ‘UV 400’ extends this to wavelengths up to 400 nanometres.

Dark lenses with inadequate UV filters can even do harm: behind them the pupils widen and we squint less. A Brazilian modelling study (Scientific Reports, 2024) examined 214 lenses from unbranded sunglasses, only one of which met the ISO standard. With 8 of them, according to the authors, the risk would have been slightly higher than for unprotected eyes. The bigger factor was not the pupil but the wider field of view, because the involuntary squint is lost.

For the Swiss market, a test commissioned by the FOPH (2024) is reassuring: all 57 CE-marked sunglasses tested met the requirements for 100 per cent UV protection, and expensive models were optically no better than cheap ones. However, 4 of the 30 adult pairs failed the ageing test. The standard is meant to secure UV protection for two years at up to two hours’ wear a day, and scratches weaken it – so old, scratched sunglasses should be replaced.

Shape matters too. In the Lausanne and Geneva study, close-fitting goggles of the kind worn in snow gave almost complete protection, whereas the widely used medium-sized sunglasses shielded noticeably less at the sides. The FOPH advises lenses that reach up to the eyebrows and cover the full width of the face. Brown or grey tints are good for clear vision; blue and red ones reduce contrast. UV-blocking contact lenses only cover part of the eye and do not replace sunglasses; with prescription lenses, it is worth asking whether they offer 100 per cent UV protection.

Children’s eyes need extra protection

Children’s eyes have less pigment and a clearer lens than adults’ eyes, so UV radiation penetrates deeper, as far as the retina. In adults, the cornea and lens together fully absorb UV between 280 and 365 nanometres. According to the FOPH, children’s and adolescents’ eyes also have a narrow additional transmission window around 320 nanometres, which only closes by the age of 30. The WHO likewise describes children and adolescents as particularly vulnerable because of the structure of their skin and eyes. Light-coloured eyes are also more sensitive than dark ones, because the iris contains less pigment.

In practice, a hat alone is not enough, says the FOPH. In its test, all baby and children’s sunglasses offered full UV protection, but with two children’s models red signal colours could not be reliably recognised, and one category 4 baby pair lacked side protection.

Quick check before your next mountain trip

• CE mark plus ‘100% UV’ or ‘UV 400’ on the label
• category 3 for mountains and beach, category 4 for glaciers and high mountains – but not for driving
• side protection or goggles (not for driving), plus a wide-brimmed hat
• wear sunglasses on cloudy days too, especially on snow and near water
• replace scratched or old pairs
• never look directly at the sun, even with sunglasses on

If your eyes do not settle after a day in the snow, you notice a growth on the white of your eye, or your vision changes, an eye examination makes sense. In our ophthalmology service, examining the front and back of the eye is part of the basic check-ups.