Not all ultraviolet (UV) radiation from the sun stays outdoors. You may think that windows prevent the sun from harming your skin, but the truth is that UVA rays can pass through window glass and reach you indoors, at home and on the road, putting you at risk for premature aging and skin cancer.

There are two types of UV radiation: UVB rays, which mainly cause sunburn, and UVA rays, which mainly cause tanning and skin aging but can also lead to sunburn. Both types of UV rays increase your risk for skin cancer.

Window glass does a good job at filtering out most UVB rays. However, UVA rays are longer wavelengths that pass through windows and clouds year-round. And while slightly less intense than UVB, UVA rays penetrate your skin more deeply. Exposure to UVA causes genetic damage to cells on the innermost part of your top layer of skin, where most skin cancers occur.

Many people don’t realize that sun exposure and skin damage can happen while indoors. But when you consider how sunlight can fade home furnishings and flooring over time, it’s easier to understand how the sunlight that hits you inside your home or car can also harm your skin.

This type of exposure is known as incidental sun exposure. It’s dangerous because of the way it adds up over time, and compounds your overall sun damage and increases skin cancer risk.