Quick Answer: Laminated windscreens block roughly 96-98% of UVA, but ordinary side and rear windows are usually tempered glass, which blocks most UVB while letting a large share of UVA through. UVA is the wavelength that drives pigmentation and photoageing without causing a sting, so you accumulate damage on the window side of your face without ever feeling burned. Over years, the difference becomes visible.

If you have ever looked at a photo and thought one side of your face looked older, you were probably right.

And if you drive on the left in your country, it is the other side. That is the giveaway.

What is actually different about the two types of glass?

The construction, not the tint.

Windscreens are laminated. Two layers of glass bonded to a plastic interlayer, primarily for safety — the interlayer is what stops the screen shattering into the cabin. That interlayer also happens to be a very effective UVA absorber. A laminated windscreen typically blocks well over 95% of UVA as a side effect of being safe.

Side windows are usually tempered. A single layer of heat-treated glass that crumbles into blunt pieces on impact, which is what you want for a window you may need to break to get out. Tempered glass blocks UVB well — that is a property of ordinary glass — but transmits a substantial proportion of UVA, with figures commonly reported in the 40-70% range depending on thickness and tint.

This is why the sensation is misleading. UVB is the wavelength that produces the visible, painful burn on a short timescale. It is largely gone. UVA is the longer wavelength that penetrates deeper into the dermis, drives pigment production and degrades collagen, and it produces almost no immediate sensation.

So you sit in a car for an hour in bright sun, your face never stings, and you conclude nothing happened. Something did happen — it just happened on a timescale of years rather than hours.

How much exposure are we actually talking about?

More than most people estimate, because it is the regularity rather than the intensity that matters.

A 40-minute commute each way, five days a week, is roughly 65 hours of one-sided daylight exposure a year — and that is before school runs, errands and holidays. It is not intense exposure in the way a beach afternoon is, but it is delivered to the same few square inches of skin with unusual precision, week after week.

The dermatology literature contains a well-known set of photographs documenting exactly this pattern in long-distance drivers: markedly deeper wrinkling, thickened skin and uneven pigmentation on the window side of the face, with the other side comparatively unaffected. It is the clearest natural demonstration of cumulative UVA exposure that exists, precisely because the same person acts as their own control.

The pattern shows up on the left cheek and temple, the left side of the neck, and the left forearm and back of the hand in right-hand-traffic countries, and mirrored where people drive on the left.

Late summer and early autumn are quietly the worst season for this, which surprises people. The sun sits lower in the sky, which means it comes through side windows at a much more direct angle rather than passing overhead. A September morning commute can deliver more direct facial exposure through a side window than a July midday one.

What Nobody Tells You About Windows and UV

Your office window behaves the same way. Ordinary building glass is not laminated either. If you sit beside a window all day, particularly one facing the sun for part of the day, you are getting the same one-sided UVA delivery on a longer timescale. The desk-by-the-window seat is not neutral.

Tint does not necessarily mean UV protection. Darkening the glass reduces visible light and heat, and it may or may not include UV-blocking film. These are separate properties, and a factory tint that makes a window look protective can transmit UVA freely. If it matters to you, the specification to look for is UV rejection, stated as a percentage, not the darkness of the tint.

Clouds barely help. A meaningful proportion of UVA passes through cloud cover, which is why the overcast-day assumption is one of the more reliable ways to accumulate damage without noticing.

SPF alone does not tell you about UVA. The SPF number is a UVB measure. UVA protection is indicated separately — a UVA circle logo, a PA+ rating, or the term broad spectrum, depending on where you are. A high SPF with weak UVA protection is precisely the wrong product for a commute.

Car air conditioning makes it worse, not better. Not because of UV, but because dry moving air over one side of your face reduces the sensory cues you would otherwise use — the warmth and slight tightness that normally prompt you to think about sun. Comfortable skin and exposed skin are not the same thing.

Your arm gets more than your face. The left forearm resting on the door or the sill is usually the single most exposed area, and the one nobody protects.

What is worth actually doing about it?

Small, specific things, because the exposure itself is small and specific.

Make sunscreen part of getting dressed, not part of going out. The commute exposure only gets covered if application is unconditional. A broad-spectrum SPF 30 or higher applied every morning regardless of plans covers the drive, the walk, the window seat and the errand you did not plan.

Do not forget the boundaries. Ears, the side of the neck, and the back of the hands are where the driving pattern shows first and where application is most often skipped.

Sunglasses do more than you think. They protect the thin skin around the eye, which is where crow’s feet asymmetry becomes visible, and they reduce squinting, which is a mechanical contributor to the same lines.

Consider UV film on side windows if you drive a great deal for work. It is inexpensive, and specifications above 99% UV rejection are standard. Check your local regulations on visible light transmission before choosing a darkness level.

Move the desk if you can. A window seat is genuinely pleasant and genuinely a source of daily UVA. Sitting perpendicular to the window rather than beside it changes the geometry considerably.

And keep the risk in proportion. This is general information, not medical advice. One-sided facial photoageing is a cosmetic and cumulative issue, but the same exposure pattern is also relevant to skin cancer risk, and anything asymmetric, changing, or that will not heal deserves a professional look rather than a change of sunscreen.

FAQ: Sun Through Glass, Answered

Do I need sunscreen indoors near a window?

If you spend hours a day beside a bright window, yes — and it is a small enough cost that it is rarely worth agonising over. If you are in an interior room without direct daylight, the practical answer is no, though a moisturiser with SPF removes the decision entirely.

Does a sunroof count?

Yes, and it is often the least protected panel in the car. Some are laminated, many are not, and the exposure lands on the forehead, nose and the parting of the hair — three areas that already burn easily and are rarely protected properly.

Will one side of my face ever catch up?

Some of the difference is reversible in the sense that pigmentation can even out over time with consistent protection, and retinoid-type treatments have reasonable evidence behind them for photoageing generally. Structural changes to collagen respond much more slowly and incompletely. The realistic goal is stopping the divergence rather than reversing it, and that starts working from the first consistent week.

TL;DR:

• Laminated windscreens block 95%+ of UVA; ordinary tempered side windows transmit a large share of it.
• UVA causes pigmentation and collagen breakdown without the sting of a burn, so the damage accumulates unnoticed.
• The pattern lands on one cheek, temple, neck and forearm — and low autumn sun through side windows is worse than high summer sun.
• SPF measures UVB; you need a separate broad-spectrum or PA rating for UVA, and tint is not the same as UV rejection.
• Fixes are small: daily unconditional sunscreen including ears and hands, sunglasses, UV film if you drive for work, and rethinking a window desk.

The reason this one goes unaddressed for decades is that it never hurts. Every other sun exposure gives you a signal, and this one gives you nothing until it has already shown up in photographs. Treating the car as an outdoor environment, rather than an indoor one with a view, is the whole adjustment.