You need sunscreen indoors only when direct sunlight is falling on you through glass. Laminated glass, which every windshield uses, blocks essentially all UVA. Tempered glass, which most side windows and many skylights use, lets a substantial fraction through. So the car, the sunlit desk and the glass room are worth protecting. An interior office, a north facing window, and your laptop screen are not.
If you want the short version, that is it. Below are the measured numbers behind each of those rules.
A burn is an alarm. It is also a UVB event, and glass removes it. What glass does not reliably remove is UVA, the longer wavelength that does not redden skin and does the slower work: pigment, laxity, the fine lines that arrive without an incident to blame. The exposure you can feel is the one you already respect. This is about the exposure you cannot.
The distinction is not glass. It is which glass.
Laminated glass is two panes bonded around a plastic interlayer, and it blocks essentially all UVA. Tempered glass is a single toughened pane, and it lets a substantial fraction through.
Every windshield in the country is laminated, because it is required to be. Side windows usually are not. That single distinction explains almost everything that follows.
The car
In a 2024 survey of thirty four vehicles from model years 2015 to 2025, windshields attenuated 99.25 percent of UVA. Driver side windows managed 88.78 percent.
That gap has narrowed, which is worth saying plainly, because the figure most often quoted comes from a 2016 study that found side windows blocking around 71 percent. Side glass has improved. It is still the weak side of the car, and most of what you will read on this subject has the geometry backwards.
The association people reach for next is the one about left sided skin damage in American drivers, and it deserves care. Large registry data do show a modest left sided excess in melanoma. But the effect is inconsistent, it appears on the leg as well as the arm, and it has been found in countries where the driver sits on the right, which is the opposite of what a driving explanation would predict. The honest summary: the pattern is real, the cause is not settled.
What is settled is simpler.
- Keep the windows up. Measured arm exposure runs at three to four percent of ambient with windows closed, rises to roughly a quarter to a third with them open, and past sixty percent in an open convertible.
- Apply before a commute or a long drive. Include the left side of the neck and the left hand, which sit in the unprotected field.
- Do not assume a newer car is safer. In that survey, mileage predicted protection better than model year did.
- Check the rules before adding film. Windshields and front side windows have minimum visible light transmission requirements that vary by state.
The desk
The trigger is not the presence of a window. It is whether direct sun is falling on you.
If you can feel it on your face, or see the window casting your shadow, you are sitting in a real UVA field, and thirty to sixty minutes of it through ordinary glass is a meaningful dose. Apply in the morning, and once more after lunch.
If you work in an interior room, or your window faces north, or you are simply not in the beam, then indoor sunscreen is not necessary, and any brand telling you otherwise is selling rather than explaining. Australia's Cancer Council says the same. The cheapest correction here is not a product at all. Turn the desk ninety degrees. Close a sheer.
The trigger is direct sun on skin, not the presence of a window
The screen
No.
Devices emit blue light at something like a thousandth of the intensity of sunlight, and lack the wavelengths that drive pigment. The best available systematic review found no established hazard to pigmentation, redness or melasma from device use, and its authors state directly that blue light sunscreen is unnecessary for indoor device work away from a window.
We could sell you something here. We would rather you spend the money once, on the exposure that is actually happening. If you have melasma, the window and the car are what to manage. Not the laptop.
The sunroom and the skylight
Treat any glass room where direct sun lands on you as outdoors without the warning. Skylights may be tempered or laminated and you cannot tell by looking, so assume the permissive case and apply as you would outside. You will not burn in there, and that is precisely why the exposure goes uncounted.
The grey day
Cloud is a real reduction, and it deserves an honest number rather than a slogan. By the EPA's own factors, scattered cloud transmits 89 percent of ultraviolet, broken cloud 73 percent, and full overcast 31 percent. A hazy day costs you about a tenth. A properly grey day cuts it by roughly two thirds.
The stronger seasonal argument is not about cloud at all. UVB collapses in winter while UVA persists, so the proportion shifts hard: in one long running measurement series, the UVA to UVB ratio roughly doubled from summer to winter. In January, nearly everything reaching your skin is the wavelength that ages it.
Why this is the argument for a daily habit
Americans spend something like 87 percent of their lives inside buildings and another 6 percent inside vehicles, and drive on average about an hour a day.
And the line you have heard, that 80 percent of sun damage happens before eighteen, is false. The real figure is under a quarter. Which means that at thirty five, most of your lifetime exposure is still ahead of you, and most of it will arrive in ordinary light, through glass, on days that never felt like sun.
This is the entire case for a sunscreen you can wear every single morning without negotiating with it. Not a thick one you save for the beach. Shield is broad spectrum SPF 45 built to go on like a serum, disappear on every skin tone, and hydrate as it protects, so daily wear takes no discipline at all.
Sunscreen is not for the beach. It is for the commute, the window seat, the grey afternoon.
