Q: What can you tell me about special lights used to treat mold?

A: The lights you’re referring to are known as ultraviolet lights. Those that generate light waves in the “C” band are referred to as UV-C lights. UV lights can be very helpful in keeping air conditioning equipment clean. Without being technical here, I can say that the light will certainly have an effect on bacteria and mold spores. But I have to qualify that statement too.

UV lights are becoming very popular inside of air conditioning systems. They appear to be very effective when placed near the A/C coil, which is the surface designed to cool the air. As air is passed over the coil it loses moisture, which then drips down into a condensate pan. Historically the moisture on the coil and in the pan were the first place to support the growth of bacteria and sometimes mold. Today, with a UV light installed near the coil, the chance for bacteria and mold to grow on the surfaces of the coil and pan have dropped. But there is also more to this story.

UV lights are very effective at killing certain types of bacteria. Because of that, they have been used for many years in hospital settings as a method of controlling bacteria. UV lights are not however, as effective at killing mold spores. In fact, the spore must be present in the “kill zone” of the UV lights for an extended period for the light to have any real effect on mold. And that just isn’t fesible.

Picture this: A mold spore slips past the air filter and then heads through the coil. Since it’s traveling on air currents, it is going to go past the UV light in a fraction of a second. If the UV light was efficient, it would kill the spore during that “light contact time.” But the reality is that the limited exposure time of the light does not kill spores. This is supported in reasearch performed by Penn State. They modeled microbial decay using various powers of UV lights. The results showed that the mold spore survival rate varied depending on the power of the light. In one example, some spores had to be exposed for up to 1,500 seconds (that’s 25 minutes) before they were killed.

The use of UV-C lights next to an air conditioning coil is currently the best application for residential use. In that location, the light will ensure that the coil should remain clean and free of a buildup of bacteria and mold for some period of time. As the bulb weakens however, it’s ability to clean is lessened. That could mean that annual maintenance or the replacement of the bulb is mandatory.

UV-C lights are relatively safe. Those in use in hospital settings are usually directed upward toward the ceiling. This ensures that children and pets are not looking directly at or otherwise exposed to the rays. When used in a residential setting the lights are mounted inside of an HVAC system near the coil. This is an area where few homeowners venture and so, there is again little consumer exposure.

Mary Biagotti is a certified industrial hygenist with EFI Global's Boston office. EFI Global is a full-service engineering, environmental and fire investigations firm serving commercial, industrial, institutional and government entities.

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