Answer to Question #11911 Submitted to "Ask the Experts"

Category: Lasers, Infrared, and Ultraviolet Radiation

The following question was answered by an expert in the appropriate field:

Q

I have seen ultraviolet (UV) lights that kill bacteria in fish tanks. The light does not touch the water but shines down into the tank or sometimes is submerged. I am in ninth grade biology and writing a paper on UV-C light. My question is: If a UV-C light shines down into a container of water from above, could a human safely drink from the container? Or if a waterproof UV-C light is submerged in a water glass, could a human drink from it? Theoretically would there be any danger in either instance?

A

The short answer to your question is that the water would be safe to drink.

In general, there are many forms of energy, some we can see (visible light), some we cook with (microwaves), some we heat with (infrared), and some we cannot completely see (UV). UV light is further divided into three types, abbreviated UV-A, UV-B, and UV-C.

UV-A and UV-B lights are used mostly for plant grow lights and tanning beds (note that we do not recommend that tanning beds be used by individuals). UV-C light is the most energetic of these forms of UV light and is invisible to humans. The reason you see the bluish glow from these types of bulbs is a function of the manufacturing and physical characteristics of the quartz glass forming the bulb. Under no circumstances should you stare at a UV-C light source for any amount of time.

When UV-C light shines on water or if the bulb is immersed in the water, some of the light penetrates the water and is absorbed by germs, such as bacteria and viruses, in the water. When UV-C light is absorbed by these germs, they are killed, sterilizing the water.

The water itself is not harmed by the UV-C, and the UV-C light does not remain in the water. When you drink the water, there is no UV-C in the water. In fact, some public water suppliers use UV-C light to sterilize their drinking water, and you can also purchase UV-C light systems to sterilize your own home's water or air. There are also small portable UV-C devices used by hikers and backpackers to sterilize their water when hiking. One example is the SteriPEN. These devices work well but be sure to have extra batteries!

In summary, shining a UV-C light source on water, or placing a UV-C bulb in water, does not harm the water so the water is safe to drink.

Paul Charp, PhD

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