Dermatologists can often guess your commute from your skin: the driver’s-side asymmetry — more sun damage on the window arm and face-side — is a documented pattern with a physics explanation and a two-dollar fix. The car window chapter of sun protection.

The Window Truth

Car glass blocks most UVB (you won’t burn through a closed window) but standard side windows pass substantial UVA — the aging-and-deeper-damage wavelength (the sunscreen guide’s two-ray briefing, automotive edition). The windshield is treated; the side windows mostly aren’t. Hence the asymmetry.

The Damage Pattern

The clinical observations: more photoaging and skin issues on drivers’ window sides across studies of commuting populations — the left arm, left face and left neck in left-drive countries logging decades of unnoticed UVA. The commute was an exposure; nobody filed it.

The Fix Menu

The countermeasures by effort: SPF on the window arm and face-side as morning default (the daily-SPF habit already covers it — the arm just joins), the UV arm sleeve for long drives (the cycling accessory’s second career — glovebox-stored, two dollars, zero sunscreen reapplication), and window UV film for the committed (legal tint rules vary; the film blocks the UVA the factory glass ignored).

The Passenger Footnote

The pattern applies to any regular window seat: the passenger-side commuter, the window-desk worker (office glass runs the same UVA math — the desk by the sunny window is a mild exposure site), and the road-trip rotation. The fix travels: SPF as default, sleeves in the glovebox, and the asymmetry stays a dermatology anecdote instead of your chart note.

UVA passes the side glass, the window arm keeps score, sleeves and SPF settle it. The driver’s tan line was never a tan — it was an exposure log, and the glovebox sleeve closes the account.