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Science & Nature
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The Invisible Shield: Why You Can't Get a Sunburn Through a Window (But Still Risk Sun Damage)
Your window blocks over 90% of UVB rays, preventing sunburns. But it lets through most UVA rays, which cause skin aging and cancer. You're safe from burning, but not from the sun's more subtle, long-term damage.
Have you ever sat in a sun-drenched spot by a window, wondering if you should be worried about getting a tan or even a sunburn? It’s a common thought, but the answer is more complex than a simple yes or no. While you can rest easy about turning lobster-red, your window isn't a complete force field against the sun. In fact, standard window glass is a selective filter, letting in one type of harmful radiation while blocking another.
The Sun's Invisible Rays: A Quick Refresher
To understand what your window is doing, we first need to understand what's coming from the sun. The sun emits ultraviolet (UV) radiation, which is primarily categorized into two types that reach the Earth's surface: UVA and UVB.
- UVB (Burning Rays): These have a shorter wavelength and are the primary cause of sunburn. They are also the main driver for most skin cancers and are essential for Vitamin D production in the skin.
- UVA (Aging Rays): These have a longer wavelength and penetrate deeper into the skin. They are the main culprit behind photoaging—wrinkles, fine lines, and sunspots. They also contribute significantly to the development of skin cancer.
The Great UVB Block
Here’s the good news: your window is an excellent UVB blocker. According to the Health Physics Society, standard glass does an incredible job of filtering out the sun's burning rays.
Standard window glass, according to the International Ultraviolet Association, will allow UVA to pass through while filtering out the UVB spectrum. An ordinary pane of glass filters out about 97% of UVB rays.
This is why you won't get a sunburn sitting by a window, no matter how bright it is. It's also why you can't produce Vitamin D indoors, as that process relies on the very UVB rays the glass is blocking.
The Sneaky UVA Problem
While the UVB is stopped at the door, UVA rays are largely invited in. The same standard pane of glass that blocks 97% of UVB only blocks about 37% of UVA rays. This means that for hours every day, you could be exposed to the radiation responsible for long-term skin damage without any of the warning signs of a burn.
A dramatic and now-famous example of this was published in the New England Journal of Medicine, showing a truck driver who had spent 28 years on the road. The left side of his face, constantly exposed to the sun through his driver-side window, was severely wrinkled and damaged compared to the right side. This condition, known as unilateral dermatoheliosis, is a stark visual testament to the cumulative power of UVA radiation through glass.
This is also why you notice furniture, photographs, and carpets fading over time in sunny spots. That bleaching effect is the work of persistent UVA rays breaking down chemical bonds in the dyes.
So, What Does This Mean for You?
While you don't need to apply sunscreen to read a book by the window for an hour, it's important to be aware of the risks of prolonged and repeated exposure, especially in cars or offices where you spend hours in the same spot day after day. Your windows are doing a great job protecting you from immediate burns, but the silent, long-term damage from UVA is still a factor.
For those with significant concerns, such as people with photosensitivity or a high risk of skin cancer, solutions like applying UV-blocking films to windows or investing in laminated or specially coated glass can block a much higher percentage of UVA rays. For most of us, though, the key takeaway is simple: the glass protects you from a burn, but it doesn't make you invincible to the sun.