Indoors All Day? What Window Glass Does and Doesn't Block

Staying indoors feels like a day off from sunscreen. Glass is more selective than it looks, so here is what it actually filters and what slips through.

What Glass Filters, and What It Doesn't

Window glass is very good at blocking UVB, the ray behind sunburn. In lab measurements, every type of glass tested blocked UVB completely^[23]. UVA is a different story. Window glass blocks all UVB but not all UVA^[24], and how much UVA gets through depends on the type and colour of the glass. In one lab test, clear non-laminated glass let through about 63% of UVA, while grey laminated glass let through under 1%^[24]. Laminated glass, green glass and sunlight-control film blocked UVA entirely in another study^[23].

Why You Will Not Feel It

Because UVB is blocked, sitting by a window does not make you feel sunburnt, which is exactly why it is easy to dismiss. But UVA rays penetrate deep into the skin and are linked to premature aging^[19], and UV exposure drives the breakdown of collagen^[7] behind fine lines and a duller, looser look. Skin does not need to burn for UV to add up over time.

Where This Shows Up in Real Life

• A desk near a window, or a sunny corner at home: how much UVA reaches you depends on the glass, and you cannot tell the glass type just by looking.
• Driving: car windscreens are laminated, but side and rear windows are usually non-laminated glass^[24], so the side of your face and the arm nearest the window are the parts most exposed.

An Honest Rule of Thumb

None of this means every indoor moment is a risk. Laminated, tinted or film-coated glass blocks much more, and you may already be well covered in some rooms^[23]. But since you cannot see the difference, a daily sunscreen habit is the easiest way to avoid guessing.

• If you sit near a window for long stretches, or commute in a car every day, wear sunscreen as the last step of your morning routine.
• Do not forget your neck and the side of your face nearest the window.
• On a day spent deep indoors, well away from windows, skipping it is a reasonable call.