There is a famous clinical photograph — published in the New England Journal of Medicine — of a truck driver's face after 28 years on the road. The window side is deeply wrinkled, thickened, and sagging. The other side looks decades younger. He never sunbathed. The damage came through glass, on the job, one commute at a time.
That photo compresses the whole story of this article: ordinary window glass stops the ultraviolet rays that burn you, and lets most of the rays that age you pass straight through. If you think of sun protection as a beach-day concern, your left arm and the window side of your face disagree.
The physics: two kinds of UV, one kind of glass
UVB (280–315nm) is the short, high-energy wavelength that causes sunburn. Standard soda-lime glass — windows, windshields' side glass, office panes — absorbs nearly all of it. This is why you don't burn through a window, and why the absence of a burn convinces people they're safe.
UVA (315–400nm) is longer, penetrates deeper into skin, and is the primary driver of photoaging: collagen breakdown, wrinkles, loss of elasticity, and pigmentation. Standard glass transmits the majority of it. Studies of automotive side windows have measured UVA transmission ranging from about 44% up to 79%, depending on the glass. Only laminated glass — required for windshields, optional everywhere else — blocks UVA well.
| Glass type | UVB blocked | UVA blocked | Where you meet it |
|---|---|---|---|
| Windshield (laminated) | ~100% | ~96–99% | Front of every car |
| Car side/rear windows (tempered) | ~100% | 21–56% (avg ~29% in one 29-car study) | Driver's window — your right arm and face |
| Standard home/office pane | ~97%+ | ~25–50% | Desks by windows, sunrooms |
| Airplane windows | ~100% | partial — UVA measurable at altitude | Window seats (UV is stronger at 35,000 ft) |
| Low-E / UV-filtering film | ~100% | ~95–99% | Newer buildings, aftermarket film |
The evidence written on people's skin
- The NEJM truck-driver case: unilateral dermatoheliosis — one-sided photoaging from cab-window UVA over 28 years
- US drivers show measurably more skin cancers and precancerous lesions on the LEFT side of the face and left arm; in right-hand-drive countries the pattern flips sides
- A study of office workers found higher photoaging scores in those seated within ~2 meters of unfiltered windows
- Pilots and cabin crew show elevated melanoma rates — altitude UV through cockpit glass is one suspected contributor
What this means for a normal day
Add up an ordinary Tuesday: a 25-minute commute each way, a desk within sight of a window, lunch by the cafe glass, the drive to pick up the kids. None of it feels like sun exposure — no heat warning, no burn, no reminder. But UVA is nearly constant through daylight hours, penetrates cloud, and accumulates silently. Dermatologists estimate the majority of visible facial aging is driven by UV, and for people who 'don't spend time in the sun,' window-transmitted UVA is often the main dose.
What actually protects you
- Broad-spectrum sunscreen, daily — 'broad spectrum' is the phrase that means UVA coverage; the SPF number alone only describes UVB. Applied in the morning, it covers the commute you weren't thinking about
- Choose SPF 50 broad-spectrum for the buffer — real-world application is always thinner than the lab standard
- UV-filtering film on car side windows (~$100–200 installed) blocks 99% of UVA — the highest-value upgrade for anyone who drives daily
- At the office: sitting 2+ meters from single-pane glass meaningfully cuts the dose; modern low-E glazing largely solves it
- The window seat on a plane: enjoy the view, close the shade on long daytime flights
Glass gives you the feeling of shelter and keeps the sunburn out. The aging rays never noticed the window.
Common questions
Can you tan or burn through a window?
Burn: essentially no — glass blocks the UVB that causes sunburn. Tan: yes, slowly — UVA triggers pigment darkening, which is why long-haul drivers develop one-sided color. If skin can darken through glass, it is receiving aging doses through glass.
Do I need sunscreen if I work from home?
If your desk faces or sits near an unfiltered window and you're there through daylight hours, a morning application of broad-spectrum SPF on face, neck, and hands is a reasonable habit. Interior rooms away from glass: genuinely fine without.
Does my car's windshield protect me?
The windshield, yes — it's laminated and blocks nearly all UV. The side windows are the gap: tempered glass that lets 40–80% of UVA through. Your window-side arm and face take that dose on every drive.
Is UVA really that harmful if it doesn't burn?
Yes — differently harmful. UVA penetrates to the dermis where collagen lives, drives most visible photoaging and pigmentation, and contributes to DNA damage linked to skin cancers. The absence of a burn removes the warning, not the harm.
Do sunscreens protect against UVA equally well?
No — only 'broad spectrum' labeled products passed the FDA's UVA test. A high SPF without that phrase can block the burn while leaving substantial UVA through: the window-glass problem, in a bottle.
Sources
- New England Journal of Medicine — unilateral dermatoheliosis case report (truck driver, 2012)
- JAMA Ophthalmology study measuring UVA transmission through automobile side windows (29-car sample)
- Research on left-sided predominance of skin cancers in US drivers; FDA broad-spectrum test requirements (21 CFR 201.327)