You sit by a window, the sun outside, you're in the shade and you think your skin is safe. It isn't. Glass protects you from only half the problem - and the less important half at that. Dermatologist Dr. Carlos Morales Raya explains why hours spent behind glass, in a car or by a big window, quietly age the skin without you ever burning.

The issue is the two kinds of ultraviolet radiation. UVB rays - the ones that cause burns and help produce vitamin D - are almost entirely blocked by glass. That's why behind a window there's neither redness nor vitamin D. But UVA rays pass through glass without trouble and reach the skin even indoors. "UVA rays pass through glass much more easily and reach the skin even inside," says Morales Raya.

And UVA is exactly the culprit behind aging. It breaks down collagen and elastin, increases oxidative stress in cells and causes pigment spots. The result over years: wrinkles, sagging, spots and accelerated aging - often stronger on one side of the face, the one closer to the window.

The most famous example is the case of a truck driver whose face was exposed to the sun through the side window on one side only for years - the difference between the two halves of his face is so drastic that it's used in textbooks. One half looks twenty years older than the other. That's not a burn, it's quiet damage piled up over hours.

The dermatologist's conclusion is simple: people who spend hours driving or working by big windows should use protection there too, not just at the beach. Especially those prone to spots and pigmentation. Sunscreen isn't only for summer and the sand - it's for everywhere daylight reaches, including the other side of the glass behind which you think you're safe.