Answer to Question #12082 Submitted to "Ask the Experts"

We believe my husband has photodermatitis. I am trying to learn if ultraviolet (UV) rays come into the house with ambient light or must you be exposed to direct sunlight? If it comes through the windows with just the ambient light, is there anything reasonable we can do to lessen the radiation?

In brief, some types of UV light can pass through window glass.

UV light is divided into three classifications called UV-A, UV-B, and UV-C. Of these three classes, UV-A and UV-B are associated with sunburns and tanning, and UV-A is thought to suppress the immune system. (See this website for more information on photosensitivity.) UV-A and UV-B are also associated with tanning beds, and there are hazards associated with the overuse of these beds. UV-C is produced only by specialized lights and the sun. When produced by the sun, UV-C does not penetrate the earth's atmosphere.

Standard window glass, according to the International Ultraviolet Association, will allow UV-A to pass through while almost 100% of the UV-B and UV-C light is blocked. Therefore, some UV light will enter your home and potentially affect your skin. Some of these effects could include increased freckles and increased sensitivity to sunlight that could result in rashes such as photodermatitis.

Today, methods exist to reduce the ability of UV-A to penetrate glass. One such method is to add a solar window film to the interior of your windows. These films block the UV-A and may also keep your house cooler as they also will reflect some heat. In fact, I have installed solar window film on my home's windows. Many of these solar window films can be installed by the homeowner as they require only water and a few drops of dishwashing soap in a spray bottle. If you do decide to add solar window films, verify that the film is a solar film, as not all films will reflect or stop the penetration of UV-A light.

You did not specifically ask about automobile glass, but that also is an issue for exposure to UV-A. The Skin Cancer Foundation states that windshield glass is specifically treated to remove the UV-A; however, the side windows and rear window will allow the UV-A to penetrate the interior. Therefore, care should be taken in vehicles, as well. Window tints can be applied to automobiles, but be aware that states have regulations on tinting.

In summary, typical window glass such as that found in homes will block most of the UV light from entering your house with the exception of UV-A light. Exposures to UV-A light can affect your skin if you have extended exposures to the light. The addition of solar window films to the interior of the windows can reduce most, if not all, of the UV-A from entering your house.

Paul A. Charp, PhD