Does Glass Block UV Light?

How Much UV Light Does Glass Really Filter?

Glass blocks UV-B light, but does not usually filter much UV-A.
Glass blocks UV-B light, but does not usually filter much UV-A. Astronaut Images, Getty Images

You may have heard you can't get a sunburn through glass, but that doesn't mean glass blocks all ultraviolet or UV light. Here's what you need to know:

Types of Ultraviolet Light

Ultraviolet light or UV is a term that refers to a relatively large wavelength range between 400 nm and 100 nm. It falls between violet visible light and x-rays on the electromagnetic spectrum. UV is described as UVA, UVB, UVC, near ultraviolet, middle ultraviolet, and far ultraviolet, depending on its wavelength.

UVC is completely absorbed by the Earth's atmosphere, so it doesn't pose a risk to your health. UV from the Sun and man-made sources is mainly in the UVA and UVB range.

How Much UV Is Filtered by Glass?

Glass that is transparent to visible light absorbs nearly all UVB. This is the wavelength range that can cause a sunburn, so it's true you can't get a sunburn through glass.

However, UVA is much closer to the visible spectrum than UV-B. About 75% of UVA passes through ordinary glass. UVA leads to skin damage and genetic mutations that can lead to cancer. Glass does not protect you from skin damage from the Sun. It affects indoor plants, too. Have you ever taken an indoor plant outside and burned its leaves? This happens because the plant was unaccustomed to the higher levels of UVA found outside, compared with inside a sunny window.

Do Coatings and Tints Protect Against UV-A?

Sometimes glass is treated to protect against UV-A.

For example, most sunglasses made from glass are coated so they block both UVA and UVB. The laminated glass of automobile windshields offers some (not total) protection against UVA. Automotive glass used for side and rear windows ordinarily does not protect against UVA exposure. Similarly, window glass in homes and offices does not filter much UVA.

Tinting glass reduces the amount of both visible and UVA transmitted through the glass. Some UVA still gets through. On average, 60-70% of UVA still penetrates tinted glass.

Ultraviolet Light Exposure from Fluorescent Lighting

Fluorescent lights do emit UV light, but usually not enough to cause a problem. In a fluorescent bulb, electricity excites a gas, which emits UV light. The inside of the bulb is coated with a fluorescent coating or phosphor, which converts the ultraviolet light into visible light. Most of the UV produced by the process is either absorbed by the coating or else doesn't make it through the glass. Some UV does get through, but the UK Health Protection Agency estimates UV exposure from fluorescent bulbs is only responsible for about 3% of a person's exposure to ultraviolet light. Your actual exposure depends on how close you sit to the lights, the type of product that is used, and how long you are exposed. You can reduce exposure by increasing your distance from the fluorescent fixture or wearing sunscreen.

Halogen Lights and UV Exposure

Halogen lights release some ultraviolet light and usually are constructed of quartz because ordinary glass cannot withstand the heat produced when the gas reaches its incandescent temperature.

Pure quartz does not filter UV, so there is risk of UV exposure from halogen bulbs. Sometimes the lights are made using special high-temperature glass (which at least filters UVB) or doped quartz (to block UV). Sometimes halogen bulbs are encased inside glass. UV exposure from a pure quartz lamp can be reduced by using a diffuser (lamp shade) to spread out the light or increasing distance from the bulb.

Ultraviolet Light and Black Lights

Black lights present a special situation. A black light is intended to transmit ultraviolet light rather than block it. Most of this light is UVA. Certain ultraviolet lamps transmit even more of the UV part of spectrum. You can minimize the risk of damage from these lights by keeping your distance from the bulb, limiting exposure time, and avoiding looking at the light.

Most black lights sold for Halloween and parties are pretty safe.