Answer to Question #11934 Submitted to "Ask the Experts"

I work in an intensive care unit. We have started using the Xenex LightStrike™ xenon machine to help kill infectious organisms during the disinfection process after patients have left the room. Although the literature states that ultraviolet (UV)-C light cannot pass through glass, when the machine is operating, with glass doors closed and curtains drawn, the light is very harsh to my eyes.

I have searched the literature and cannot find definitive information, except that UV-C light comes from the sun. However, one website mentioned that when pulsed, the energy of UV-C is 100,000 times greater. I have also heard that when used in operating rooms, cataracts have been reported. But again I cannot find information.

My question is: Is this dangerous?

Very bright light could be harmful to your eyes especially if you stare at the light for an extended period of time. Furthermore, there may be an elevated risk for developing cataracts if there is continued long-term exposure to UV-C lights. To understand these risks requires explanations regarding what pulsed light sources are and why they are used, why the UV-C light might be harsh to your eyes, and whether UV light is capable of inducing cataracts.

The energy of light will vary with the wavelength of the light: The shorter the wavelength, the more energetic the light. Furthermore, one theory of light energy is that it is carried in packets. The more packets, the higher the energy for a given wavelength of light and the brighter the light. When the light is pulsed (rapidly turned on and off), a greater number of energy packets can be generated during the brief time the light is on, with less heat produced than if the light remains on continuously. This is why pulsed light energy can be much greater than normal UV-C lights.

The ability of glass to block the dangerous UV-C light wavelengths has been documented many times in the scientific literature. The ability of glass to block this type of light is one reason UV-C-producing lights are encased in quartz instead of glass, as the UV-C will penetrate through quartz. Other forms of UV and visible light are also produced by these bulbs which is why these bulbs appear bluish. Most of these non-UV-C wavelengths can penetrate standard glass and other thin materials such as curtains. Your eyes may be more sensitive to these non-UV-C frequencies, especially since they are brighter and have higher, pulsed energy.

The human eye can see only a very small part of the vast range of wavelengths that are possible and that can be produced by the sun or light sources such as those you describe. The lens of the eye is comprised of various types of proteins that allow us to see the visible light. All other wavelengths either pass through the eye without being seen (for example, x rays or radio waves) or are absorbed by the lens of the eye and are not seen by humans. Cataracts of the eye form when the lens materials, mostly proteins, absorb various wavelengths of light. These wavelengths, when absorbed, can induce changes to the lens proteins. The proteins can become clumped together, occluding the lens and reducing the ability of light to penetrate the eye. The clumping proteins become the cloudy materials forming the cataract. The more energetic forms of light (UV-A, UV-B, and UV-C) can change the lens proteins through this chemical reaction.

UV light from the sun and UV light from pulsed sources can have the same wavelengths and the same energy. The difference between the two sources is the number of packets delivered over time, with the sun perhaps delivering more packets than most pulsed light sources (other than high-powered laser systems). Therefore, the sun-produced UV light and UV light produced by bulbs would behave similarly. To protect your eyes, as well as the eyes of your coworkers, a thicker curtain or a curtain of different material or darker color could be used to block more of the light.

For additional information on light-induced cataracts, please visit the National Institutes of Health National Eye Institute website.

In summary, the harshness of the pulsed light following its passage through standard glass and curtains is perhaps the result of its brightness. Because the light is very bright, it could be harmful to your eyes especially if you stare at the light for an extended period of time. Furthermore, you may be at an elevated risk for developing cataracts if there is continued long-term exposure to these lights.

Paul Charp, PhD