Why wear sunscreen inside?

What gets through the glass?

Windows filter out most UVB, the rays mainly responsible for sunburn, but UVA can get through. Window glass is a barrier to some sunlight, not a cancellation of sunlight. UVA reaches deeper into the skin, and repeated small exposures can cause changes even without an obvious burn. UVA is present throughout the day, including morning and late afternoon, so exposure is not limited to the hottest hours. The reason to protect yourself is the exposure that adds up over time, not just a single sunny afternoon.13

Does every indoor day need the same protection?

A desk beside a sunny window and a room away from windows are different exposure situations. Moving away from windows and direct sunlight reduces exposure; keeping natural light does not require your living room to become a storage cupboard. A sunny conservatory needs precautions similar to being outside, particularly if your skin is sensitive to sunlight. Driving also counts: front windshields shield you from some UVA, while side windows, rear windows and sunroofs do not block UVA effectively.12

How to apply sunscreen for an indoor day

1. 1Choose broad spectrum sunscreen, which covers UVA and UVB, with SPF 30 or higher. Water resistance is also useful if your day includes sweating or time outside.
2. 2Apply sunscreen before sunlight exposure. Allow about 15 minutes before going outdoors rather than waiting until you are already in the sun.
3. 3Cover exposed skin, including your face, ears, neck and hands. Clothing can cover other areas, so the amount you need depends on how much skin is exposed.
4. 4Use enough product. For the face, use at least 1 teaspoon. Most adults need about 1 ounce to cover all skin not covered by clothing; that is a body amount, not a face amount.

If preventing dark spots is a priority, tinted sunscreen containing iron oxide adds protection against the sun's visible light. Broad spectrum coverage still matters because the tint and UV protection do different jobs.4

How often should you reapply indoors?

If you spend the day near a window, use a two hour reapplication interval. If you stay indoors away from windows, reapplication every four to six hours is sufficient general guidance. Those situations should not share an automatic two hour alarm. Once you go outdoors, reapply every two hours and immediately after swimming or sweating. A short trip outside still exposes skin to UV, including in winter and on cloudy days.45

Can window film help?

UV window film can block more than 99% of UVA and UVB when applied to home, office or vehicle windows. Clear versions allow natural light through. Look for film specifically marketed for UV protection rather than assuming that a dark tint does the same job. Film is another layer of protection alongside sunscreen and clothing.1

What about indoor lights and sunlight sensitivity?

Ordinary home and office lighting produces very low UV levels and generally needs no special precautions. Photosensitivity, meaning a condition caused or worsened by sunlight, can change that advice: some affected people react to artificial UV or visible light too. Certain medicines, including tetracycline antibiotics and thiazides, can cause sunlight sensitivity. If sunlight repeatedly triggers a rash or an unusually strong sunburn reaction, get medical assessment. Sunscreen alone does not completely prevent every photosensitive rash; protection needs to match the condition.26