24-06-2025
Staying Cool Inside? UV Rays Can Still Harm You
June 23, 2025 – Sheltering indoors helps beat the heat, but are UV rays still sneaking through those windows?
Probably. Up to 74% of UVA light can penetrate standard glass. Even reflective, tinted, coated, or double-paned glass can still allow up to 50% of UVA through, said Peter Littlewood, PhD, a professor of physics at the University of Chicago.
With skin cancer remaining the most common cancer in the U.S., that matters – and here's why.
The basics: Most of the sun's rays are visible light – the colors we can see. But some are invisible ultraviolet (UV) rays, which can damage our skin.
Within the UV spectrum are UVA, UVB, and UVC rays – divided by wavelength. "The shorter the wavelength, the higher the energy," said Littlewood.
UVB rays are shorter and more intense, causing sunburn and direct DNA damage. When their high-energy photons hit a window, the glass molecules absorb them.
UVA rays are longer and less intense, but they penetrate skin more deeply. Their lower-energy photons don't interact much with the glass's molecular bonds, so most UVA passes right through.
Both contribute to skin aging and skin cancer, though "there's much more UVA than UVB," Littlewood said. (UVC rays don't reach the earth's surface, but they're used in artificial sources like germicidal lamps to kill viruses and bacteria.)
Where you're at risk: Figuring out your indoor UV exposure can be tricky. To be exposed indoors, you must be in direct sunlight – like a cat or dog stretched out in a sun patch on the carpet. Much of the time, you're shaded inside – and safe – because the sun is high overhead and sunlight isn't directly shining through windows.
But in the early morning or late afternoon, the sun sits lower in the sky and comes directly through windows, increasing your UV exposure. Even on cloudy days, UVA still gets through clouds and windows – just at reduced levels – so you can still get some UV exposure inside, Littlewood said.
Sun exposure can also be a problem in your car.
Many front windshields, especially in newer cars, are made with a UVA-blocking polymer laminated between two sheets of glass, said Michael David McGehee, PhD, a professor of chemical engineering and materials science at the University of Colorado. Side and rear windows may not have this treatment (though they may be tempered or tinted, which helps). In 2012, The New England Journal of Medicine published a case of a truck driver who had visibly aged more on the left side of his body – the side closer to the driver's window. Other research suggests airline crews may have greater melanoma risk, possibly due to the increased UVA exposure through aircraft windows at higher altitudes.
But the risk may be real for less extreme occupations, too. A 2024 study of U.S. indoor workers (radiologic technicians) linked melanoma risk with lifetime ambient UVA exposure – the kind we get from our environment every day, such as through windows.
These small daily exposures add up. A study published in May found that the skin cancer burden among older adults has surged over the past three decades – likely due to "prolonged lifetime cumulative risk factors, such as the accumulation of intermittent sun exposure," the authors wrote.
What to do: If you spend a lot of time next to windows that get direct sun – say, you work next to a large south-facing window – it's a good idea to protect yourself, especially if you have lighter skin or a photosensitive disorder like melasma or lupus, said Anthony Rossi, MD, a dermatologist at Memorial Sloan Kettering Cancer Center. Wear sunscreen that is SPF 30 or higher and broad spectrum. (Understanding how sunscreens work can help you choose the right one.) You could also apply a UVA-blocking window film or install UV-filtering shades.
