Sunscreen indoors: what window glass blocks, and what it quietly doesn't

The honest answer to “do I need sunscreen indoors” is: usually yes if you sit near a window, no if you spend the day in a blind-drawn room — and the reasoning is more interesting than either camp on the internet admits. The answer turns on which kind of ultraviolet light we are talking about, and glass treats the two kinds differently.

What glass actually stops

Window glass is a decent UVB filter and a poor UVA one. UVB, the burning wavelength, is absorbed almost entirely by standard panes. UVA — the longer wavelength tied to pigment changes and the slow breakdown of collagen — passes through ordinary glass in large measure. This is not fringe science; dermatology groups including the American Academy of Dermatology note that most of the photoaging signals people spend money on later trace back to UVA exposure, and UVA does not respect the wall between your office and the street.

The distinction matters because the sun’s damage ledger has two columns. One is sunburn, which indoor life largely escapes. The other is cumulative UVA dose, which does not require you to feel anything at the time.

Where indoor protection actually earns its keep

Two situations justify desk-side sunscreen. The first is the window seat: a workstation within a few feet of untreated glass delivers a steady low-grade UVA dose across years, and side windows in cars and trains compound it for commuters. The second is scattered light. On overcast days, a large share of ground-level UV is diffuse — bounced from sky and cloud rather than arriving in a direct beam. Sitting beside a large window on a cloudy afternoon still means sitting in that scatter. Shaded outdoor porches work the same way.

The exemption is real, though. A room where curtains stay drawn from morning to evening, with no glass in your line of sight, removes the exposure almost entirely. That person can skip the step without a lecture.

The dosing problem nobody solves

Sunscreen labels promise an SPF measured at a dose of two milligrams per square centimeter of skin. On a face, that is conventionally a quarter of a teaspoon — a strip of product along two full fingers. Published surveys of application behavior keep finding that real-world users apply somewhere between a quarter and a half of the test dose, and protection falls off roughly in proportion. A labeled SPF 30 used thinly behaves closer to an SPF 10.

This is the uncomfortable arithmetic behind “just wear sunscreen daily.” The habit is worth building, but the number on the bottle assumes an amount most people never dispense. A practical compromise: dispense the full two-finger strip indoors on window days, and treat reapplication as something you owe yourself before the commute home rather than a ritual every two hours at a desk. Our guide to mineral and chemical sunscreens covers how to pick the texture you will actually use at that quantity, and our skincare hub tracks the rest of the routine around it.

The working answer

Daily sunscreen at the full dose belongs to window seats and commutes; a sealed, curtained room earns the skip. The question is not “indoors or outdoors” but “where does the glass sit” — and, once you have answered that, whether you are willing to use the amount the label was tested at.