Sheltering indoors may feel like a safe escape from the sun’s harsh rays, but many are unaware that ultraviolet (UV) radiation can still find its way through windows. Up to 74% of UVA rays the type that penetrates deeper into the skin can pass through standard glass. Even glass treated with tints, coatings, or multiple panes may still allow about half of these rays to filter inside.
This matters because UVA exposure contributes to premature skin aging and the development of skin cancer, which remains the most common form of cancer in the United States. Understanding UV radiation helps explain why indoor exposure is still a concern.
UV rays are a type of invisible radiation emitted by the sun. They are categorized into UVA, UVB, and UVC rays based on their wavelength and energy. UVB rays, which are shorter and more intense, are responsible for sunburn and direct DNA damage. Fortunately, these rays are largely absorbed by glass. UVA rays, however, have longer wavelengths and lower energy, enabling them to pass through most types of glass and reach indoor spaces.
The degree of exposure indoors largely depends on your positioning relative to the sun. If you’re sitting or lying in direct sunlight filtering through a window particularly in the early morning or late afternoon when the sun’s angle is lower you could still be absorbing significant UVA exposure. On cloudy days, UVA rays can still penetrate clouds and windows, though at reduced levels.
Exposure doesn’t just happen in homes and offices. Cars can also be a source of indoor UV radiation. While windshields are often treated to block UVA, side and rear windows typically are not, making them a vulnerability. There have even been cases where individuals who regularly drive long hours developed more visible aging or sun damage on the side of the body closer to the window.
Some studies suggest that consistent, small doses of UVA over time may contribute to an increased risk of melanoma and other skin cancers, even among those who spend the majority of their time indoors. This cumulative exposure repeated day after day over decades can build up significantly.
For those who work next to windows receiving direct sunlight, especially individuals with lighter skin tones or photosensitive conditions like lupus or melasma, preventive measures are essential. Wearing a broad-spectrum sunscreen with an SPF of 30 or higher, even when indoors, can help shield your skin. Additionally, UV-protective window films or shades can further reduce the amount of harmful rays entering your environment.
Taking steps now to minimize incidental sun exposure may help lower long-term health risks because even behind glass, your skin isn’t entirely safe from the sun.
