Answer to Question #11543 Submitted to "Ask the Experts"

Category: Ultraviolet Radiation

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


Where I live, it's a common practice to use germicidal lamps for indoor disinfection but without adequate protection. From outside, one can often see clearly that germicidal lamps are in operation in rooms without drapes. I occasionally walk about 10 meters (m) away from such rooms for as long as one minute. I'm not sure if each time the windows are shut.

Should I worry about the effect of exposure to germicidal lamps? Could you suggest a safe distance and duration for this kind of exposure?


The short answer to your questions is that you do not need to worry about any exposure you would receive from these types of lights. These lights do not pose a danger to you at the distance you walk by them nor are they dangerous for an extended period of time at that 10-m distance.

Plate glass panes typically used in home construction block the most hazardous types of ultraviolet (UV) light. The bulbs used in germicidal lamps use a special quartz glass and special materials to produce UV light. The UV light produced is specifically geared toward the type of light that is most dangerous to bacteria, viruses, and other germs in air. The more intense (brighter) the UV light, the better its germicidal actions. The bulbs also are constructed to give off a bluish light because humans cannot see UV light, and the bluish tint indicates the bulbs are lit.

In response to your question regarding walking by the rooms at a distance, UV light is a form of energy, and the intensity of the energy decreases with the distance from the energy's source. This decrease in intensity occurs rapidly with distance. For example, consider a room with a single lightbulb. The room is brighter nearer the bulb, and the brightness decreases as you move farther from the bulb. Generally, for every 2 m you are from the germicidal lamps, the intensity decreases to about 25% of the intensity at 1 m. At 4 m, the intensity is down to about 6% of the 1-m intensity. Therefore, when you walk by the germicidal lights at about 10 m, the intensity of the lights to which you are exposed is less than 1% of the initial amount. This also means that if you wanted to receive the same amount of energy at 10 m as you would receive at 1 m, you would have to be in the 10-m location for almost 100 times longer than you would at 1 m.

To summarize, the radiation exposure you are receiving from the germicidal lights at a distance of 10 m is not a health hazard, and you could possibly stand at that distance from the bulbs for an extended period of time. If the windows are shut, then your exposure to the harmful ultraviolet light is minimal.

Paul Charp, PhD

Answer posted on 5 April 2016. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.