The recommendation to wear sunscreen every single day, including indoors, has become a standard piece of skincare advice. The motivation is good. But the blanket “always wear SPF indoors” position deserves more nuance than it usually receives, because indoor UV exposure varies enormously depending on your circumstances.
What kind of UV actually gets through windows
Standard window glass (soda-lime glass, the kind in most homes and offices) blocks virtually all UVB radiation. UVB is the short-wave ultraviolet responsible for sunburn and most direct DNA damage to skin cells. A regular glass window is close to a complete UVB filter.
What standard glass does not block significantly is UVA. UVA (320-400 nm wavelength) penetrates window glass reasonably well. Depending on the glass type and any UV-filtering treatments, somewhere between 25-75% of UVA passes through standard untreated window glass. UVA penetrates deeper into the skin than UVB, reaching the dermis where collagen and elastin are located. It is the primary driver of photoageing (wrinkles, pigmentation, loss of elasticity) and contributes to skin cancer risk, though its role in direct cancer initiation is less established than UVB.
Laminated glass (the kind in car windshields) blocks both UVA and UVB very effectively. Car side windows, however, are typically tempered glass and block UVB but not UVA. Research published in JAMA Ophthalmology in 2011 found significantly higher rates of left-side skin cancers (the driver’s window side) in US drivers, which is direct evidence of meaningful UVA exposure through car side windows.
When indoor SPF genuinely matters
If you sit near a window and direct or significant indirect sunlight falls on your skin for extended periods, UVA exposure is real. Someone sitting at a desk with south-facing windows in a sunny spring, for example, will receive meaningful cumulative UVA exposure over a working day. For this person, daily SPF makes practical sense even while working indoors.
Long car journeys, particularly as a driver, expose the left arm and left side of the face (in right-hand drive countries) to substantial UVA through the side windows. Regular commuters and professional drivers have a clear practical case for SPF application on exposed skin even without leaving the car.
Photosensitive individuals (those with conditions like lupus, polymorphous light eruption, or those on photosensitising medications like tetracyclines, certain antifungals, or some blood pressure medications) have an additional reason for careful UV protection regardless of their location.
When indoor SPF is less critical
If you work in a windowless office, spend most of your indoor time away from windows, or live in a climate with low winter UV and spend mornings and evenings indoors, the daily SPF recommendation for indoor use becomes much less urgent. Applying and removing SPF products every day has a minor cost to the skin barrier (most cleansers, even gentle ones, cause some barrier disruption) and adds time and expense. For people with minimal indoor UV exposure, this cost-benefit calculation tips toward skipping it on days when they will not go outdoors.
Estonia and other northern European countries have UV Index levels below 3 (the threshold above which protection is recommended) for roughly six months of the year (October through March). During this period, even outdoor exposure in winter provides minimal UV, and indoor exposure is essentially irrelevant for skin damage purposes.
The practical recommendation
Rather than a simple “wear SPF indoors every day,” a more accurate framework is:
- Sitting near windows with direct sun exposure for extended periods: yes, SPF makes sense.
- Regular car travel, especially as the driver: yes, particularly on the window-side arm and face.
- Working in a windowless space or spending most of the day away from windows: SPF is most relevant on days when you go outdoors, even briefly.
- UV Index 3 or above (spring through autumn in northern Europe): daily SPF when any significant outdoor exposure is expected, even just a walk to lunch.
- UV Index below 3 (winter in northern countries): outdoor SPF less critical; indoor SPF essentially unnecessary.
The case for daily SPF use is strongest during high-UV months, for people with significant window exposure, for those with photosensitive conditions or a family history of skin cancer, and for anyone using photosensitising actives in their routine. For everyone else, matching SPF use to actual UV exposure rather than applying it reflexively indoors year-round is a reasonable approach.
Where you do wear SPF, wearing it properly matters more than the specific SPF number. A well-applied SPF 30 of an appropriate amount beats a minimal application of SPF 50 every time.