Do we need sunscreen indoors?
- Standard window glass blocks most UVB but transmits most UVA - the primary driver of photoageing and pigmentation
- Car side windows are typically standard glass - significant UVA passes through during driving
- Clinical photos show dramatic unilateral photoageing on the window-exposed side of the face in people who spent years working near glass
- For most indoor days, one morning application of SPF 50+ is sufficient - reapplication is needed if you go outside
- SPF 50+ daily year-round is the simplest approach - it removes the decision entirely
The detail that matters: standard window glass blocks most UVB radiation but transmits most UVA. UVB causes sunburn and gives immediate feedback. UVA drives photoageing, pigmentation and collagen breakdown - and it does so silently, without causing redness or discomfort. The result is that the sun you experience through a window feels harmless but is still doing cumulative work on your skin.
What glass actually blocks - and what it does not
Standard soda-lime glass, used in most house windows, blocks the majority of UVB radiation but allows most UVA to pass through. Research indicates that standard glass transmits approximately 74% of UVA while blocking most UVB. This means sitting by a sunny window exposes the skin to meaningful UVA even when no sunburn risk exists.
The practical implications:
- Home windows - standard glass, transmits most UVA. A south or west-facing window in morning or afternoon sun delivers meaningful UVA to whoever sits near it.
- Office windows - usually standard glass unless specifically UV-treated. Desks positioned next to large windows accumulate UVA exposure across the working day.
- Car windscreens - modern windscreens are laminated safety glass and block both UVA and UVB.
- Car side windows - typically standard glass, transmitting significant UVA. This is where unilateral photoageing from driving accumulates.
The sun feels different through glass because the burning part (UVB) is blocked. But the ageing part (UVA) is still getting through. The absence of sunburn is not evidence that UV damage is not occurring.
Related Reading
The clinical evidence: what years of window UVA looks like
The most striking evidence for indoor UVA accumulation comes from clinical case studies of people who spent years working in positions exposed to one-sided window light.
The first case involves an Italian secretary who worked seated next to a window for 15 years. The asymmetry between the two sides of her face is significant: the window-exposed side shows considerably more fine lines, uneven texture and sagging than the protected side.
Moulin G., Thomas L., Vigneau M., Fiere A. (1994). A case of unilateral elastosis with cysts and comedones: Favre-Racouchot syndrome. Ann Dermatol Venereol, 121(10), 721-723.
The second case is a truck driver whose left side - facing the side window during decades of driving - shows pronounced unilateral photoageing: more wrinkles, deeper pigmentation and noticeably looser skin compared to the right side of his face.
Jennifer R.S., Gordon M.D., Joaquin C. & Brieva M.D. (2012). Unilateral Dermatoheliosis. The New England Journal of Medicine, 366(16), e25. doi:10.1056/NEJMicm1104059
Both cases demonstrate the same pattern: persistent, low-level UVA exposure through glass, accumulated over years, produces visible and measurable skin ageing on the exposed side. This is not an acute effect. It is the result of daily cumulative UVA - the same exposure most people experience at home or in the office without thinking about it.
"The side of your face that faces the window ages faster than the side that faces the room. Not dramatically. Not visibly in any single day. But consistently, over years, in a direction you only notice in retrospect."
Car glass: windscreen vs side windows
Modern car windscreens are made of laminated safety glass - two sheets of glass bonded with a plastic interlayer. This construction blocks both UVA and UVB radiation. The windscreen is not the concern.
Car side windows are typically standard glass and behave exactly like a house window: most UVB is blocked, most UVA passes through. For anyone who drives regularly, the side of the face closest to the side window accumulates UVA over time - which is precisely what the truck driver case study documents over a professional lifetime.
For regular long-distance drivers, applying SPF to the left side of the face (in countries where driving is on the right) is a specific practical consideration, not just a general recommendation.
How to apply SPF on an indoor day
The reapplication rules for outdoor days do not straightforwardly apply indoors. Research on indoor SPF application suggests that a single morning application provides meaningful protection throughout a low-UVA-exposure indoor day, provided you apply enough product and avoid rubbing your face frequently throughout the day.
A practical framework:
- Apply SPF 50+ every morning as the last step of your skincare routine, regardless of your plans for the day
- Indoor day with minimal window exposure - one morning application is sufficient
- Indoor day near a large sunny window - consider a midday reapplication if you are in direct sun for extended periods
- Going outside at any point - reapply after returning indoors if you were in the sun for more than a brief errand
- Driving regularly - treat the side-window-exposed side of your face as outdoor exposure
The case for daily SPF regardless of season
The simplest answer to "do I need sunscreen indoors" is to make SPF 50+ a daily morning habit year-round and remove the decision entirely. The question of whether today's indoor exposure warrants protection is one that most people will get wrong often enough - on high-UV days, near sunny windows, during an unexpectedly long lunch outside - that a daily habit is more protective than a conditional one.
SPF 50+ daily is also the clearest recommendation from a UVA perspective. In winter the UVB risk drops, but the UVA case does not change. Indoors the UVB risk drops, but the UVA case does not change. The consistent thread is UVA - and a daily SPF 50+ with PA++++ or EU UVA circle logo addresses it regardless of season or location.
NAYA Everyday Sun Cream SPF 50+ PA++++ - fragrance-free, no white cast, lightweight enough to wear every day including days you never leave the house.
Shop Everyday Sun Cream SPF 50+Frequently Asked Questions
Do you need sunscreen indoors?
If you sit near windows where sunlight falls on your skin, yes. Standard glass blocks most UVB but transmits most UVA - the primary driver of photoageing and pigmentation. For most people in most rooms, a single morning application of SPF 50+ is sufficient. If you are directly next to a large sunny window throughout the day, daily SPF is clearly warranted.
Does glass block UV rays?
Standard glass blocks most UVB but transmits most UVA. Laminated safety glass, as used in modern car windscreens, blocks both. Standard house windows and car side windows allow significant UVA through - which is why clinical photos of people who worked near windows for years show asymmetric photoageing on the exposed side.
Do I need to reapply sunscreen if I stay indoors?
For a day with minimal direct window exposure, a single morning application is generally sufficient. Research suggests one application provides meaningful protection through a low-exposure indoor day, provided you avoid touching your face frequently. If you go outside at any point, reapply after sun exposure.
Do I need sunscreen near a window?
Yes, if direct sunlight reaches your skin through the window for extended periods. Standard window glass blocks most UVB but allows most UVA through. UVA is the wavelength most associated with photoageing, pigmentation and collagen breakdown, so sitting beside a sunny window can still contribute to cumulative skin ageing over time.
Does car glass block UV rays?
Modern windscreens are laminated and block both UVA and UVB. Car side windows are typically standard glass and transmit significant UVA. Regular drivers accumulate unilateral UVA on the window-facing side of the face - documented in clinical case studies showing pronounced asymmetric photoageing in long-term drivers.
Further Reading - Sun Protection
© NAYA Skincare. All information is for educational purposes and does not constitute medical advice.
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