UVA and your eyes: sunglasses, UV400, cataracts and pterygium

Skin at least tells you when it has had too much sun. The eye has no equivalent alarm for UVA, no pain receptors that respond to it, and no capacity to repair the damage in the way skin does. It also has an unfortunate optical property: under some geometries it concentrates ultraviolet rather than merely receiving it.

Where UVA stops inside the eye

The eye filters ultraviolet in stages, and the stages divide the bands neatly:

In an adult, the crystalline lens is an efficient UVA filter — and it pays for that role. It cannot replace its proteins, so oxidative damage accumulates across a lifetime, progressively yellowing and clouding it. The lens protects the retina by absorbing the damage itself.

Children are different, and it is not a minor difference. A young lens is far clearer and transmits substantially more UVA to the retina; its filtering capacity develops through childhood and adolescence. Combine that with larger pupils and far more hours outdoors, and children receive both a higher ocular dose and a larger share of it reaching the retina. Sunglasses for children are not a scaled-down version of adult advice — the case for them is stronger.

What the exposure is linked to

• Photokeratitis — sunburn of the corneal epithelium. Acute, intensely painful, and mostly a UVB and UVC effect. It is a snow and water condition, because those surfaces reflect UV up under the frame. It heals in a day or two.
• Pterygium and pinguecula — fibrovascular growths on the conjunctiva, strongly associated with cumulative outdoor UV exposure and dramatically more common near the equator and in outdoor workers. Pterygium can encroach on the cornea and distort vision.
• Cortical cataract — the epidemiological association with cumulative UV exposure is well established. The strongest evidence implicates UVB, with UVA's contribution biologically plausible via oxidative damage but less firmly quantified. The lens is where UVA stops, so it is where any UVA effect would be expected.
• Eyelid skin cancers — the periocular skin is thin, almost always uncovered, and rarely gets sunscreen. Basal cell carcinoma of the eyelid is common enough that the eyelids account for a notable share of all skin cancers.
• Retina and macula — a plausible concern rather than a settled one in adults, since the lens absorbs most UVA before it arrives. It becomes a genuine issue after cataract surgery if the implanted intraocular lens lacks a UV filter, and in children whose natural lens filters less.

Peripheral light focusing: why frame shape beats lens darkness

This is the piece of ocular optics that reframes how you should shop for sunglasses.

Light arriving from the temporal side — from beside your head, at a shallow angle — strikes the cornea at grazing incidence and is refracted across the anterior chamber, converging onto the nasal limbus on the far side. The intensity at that focal point can be many times the incident intensity. This is the Coroneo effect, and it explains a fact that would otherwise be arbitrary: pterygium overwhelmingly forms on the nasal side of the eye, and nasal-side cortical cataract changes cluster there too.

What to prioritise, in order:

1. UV400 rating — non-negotiable, and cheap.
2. Wrap-around or close-fitting frames, especially on snow and water.
3. Large lenses covering the brow and cheek approach.
4. An anti-reflective coating on the back surface, which stops light from behind bouncing off the inner lens face into your eye.
5. A brimmed hat, which cuts ocular UV further and is complementary to eyewear rather than an alternative.
6. Tint colour and darkness — comfort and contrast only. Last on the list.

The standards, decoded

Two traps are worth naming. First, the filter category is about darkness, not ultraviolet: a category 3 lens is dark enough for bright sun, which tells you about visible transmission and nothing about UV. Second, "blocks 100% of UV" often means 100% up to 380 nm. That is not a lie, but it is not UV400 either — and the missing 380–400 nm is precisely UVA-I, the longest-wavelength, deepest-penetrating portion of the band. The same 380-versus-400 gap appears in window film specifications, and it is worth checking in both places.

The dark-lens problem

Your pupil responds to visible light, not to ultraviolet. Put a dark lens in front of your eye and the pupil dilates to recover the lost brightness. If that lens filters UV, fine — less UV arrives and the wider pupil is harmless. If it does not, you have widened the aperture without reducing the ultraviolet, and more UVA reaches the lens and retina than would have with no sunglasses at all, when you would at least have been squinting.

The mechanism is straightforward and widely accepted; hard clinical outcome data comparing unrated dark lenses to bare eyes are, in fairness, limited. But the practical conclusion is the same under either reading: an unrated dark lens is the one purchase to avoid. Clear UV400 lenses are fine. Dark UV400 lenses are fine. Dark, unrated novelty sunglasses are the failure case.

UV-blocking contact lenses

Many soft contact lenses now incorporate a UV absorber, rated in two classes: Class I blocks at least 90% of UVA and 99% of UVB, Class II at least 70% of UVA and 95% of UVB.

They are a genuinely useful adjunct, because they sit on the cornea and protect the pathway to the crystalline lens directly, without the gaps a frame leaves. But they cover only the cornea. They leave the conjunctiva, the limbus, the sclera and the eyelids exposed — and the limbus is exactly where peripheral light focusing deposits its energy and where pterygium forms. Manufacturers say this themselves: UV-blocking contacts supplement sunglasses, they do not replace them.

When it matters most

Ocular UV exposure does not track the sun's height the way skin exposure does — and the reason is geometric. With the sun high overhead, your brow and lids shade the eye well. With the sun low, it shines almost straight into the eye, and low sun is exactly when the spectrum is most UVA-dominated.

The highest-exposure situations are therefore:

• Snow — 80–90% reflectance, from below, under the frame. The single worst case.
• Water and bright sand — the same effect at lower intensity, plus glare off the surface.
• Early morning and late afternoon, when the sun is low and aimed at your eyes rather than your scalp.
• Driving into a low sun — the windshield blocks nearly all UVA, but the open side window does not.
• At altitude, where the baseline is already elevated.

Winter, low sun and snow put all of these together at once, which is why a February morning on a mountain is a more demanding environment for your eyes than an August noon at sea level — even though the UV Index will be a fraction of it. Checking the UVA Index, with the surface set to snow, gives you a far better sense of the actual load.