For some time now ultraviolet (UV) lights have been promoted as a solution for microbial contamination in Heating, Ventilating and Air Conditioning (HVAC) systems. How effective they are depends on a number of variables. In this article I will attempt to evaluate UV lighting’s effectiveness in dealing with certain types of microbial contamination in HVAC applications.
It is pretty well known that UV light is capable of killing micro-organisms including fungi (mold), bacteria and viruses. A lesser exposure to UV light, failing to kill the organism can render them unable to reproduce, thereby eliminating or reducing amplification and exposure. What are some of the factors that determine just how effective UV lights inside a ventilation system can be?
1) First of all, controlling mold inside the HVAC system and specifically the degree of occupants’ exposure to mold contamination is a major point for selling UV installation. The ability to withstand exposure to UV light is considerably higher with fungal spores than bacterial cells or viruses. And some fungal spores are more resistant than others.
2) The above statement opens up two other factors for consideration. One item is the intensity of the UV lighting that microbes are subjected to. So the number of lamps and their combined intensity must be considered to determine if they are to be effective in killing or “neutering” the type of microbes of major concern.
3) The second factor brought into focus by #1 above is the length of time microbes are exposed to the UV light. This requires air speed and placement of the UV light to be taken into consideration.
4) On a sunny day, the amount of shade that is available to us will help determine just how sunburned we might become. The same is true for microbes and UV light. Shade is provided to these little creatures by dust particles in the air. The more dust available; the more shade and thereby the more surviving and intact microbes.
5) Reflected UV light, if sufficiently intense, can have just as much “killing” power as direct light, so the material surrounding the lighted object can make a difference. Shiny metal reflects well; black material not so much. Shielding such items as gaskets and fiberglass insulation with sheet metal in the vicinity of the lighted object should add to the efficiency of the UV light.
6) Some other situations that can moderate the effectiveness of UV lights are air temperature and humidity. Low pressure lamps, which are in most cases used in HVAC systems, operate better at relatively high temperatures and low humidity of the ambient air in which the lamp is placed. So placement of the UV lamp or lamps makes a difference.
The favored placement of UV lighting is upstream of the coil “bathing” the coil with UV rays. This is quite effective for killing microbes that land on the face of the coil, but generally air movement is so fast that at least more resistant microbes, like many mold spores will pass through unscathed. They can pass by the surface of the coil going completely through or possibly landing in the shaded portion of the coil safe from the deadly light outside.
There is some downside to the use of UV lights. Just as sunlight will cause deterioration of materials derived from organic sources; UV lights will do the same. Materials such as rubber, plastic or any other items that are petroleum based can suffer damage from UV lighting so should be checked regularly for integrity if illuminated. Since human bodies are organic, caution should be observed when dealing with UV lights. Damage to eyes can occur by looking directly at the illuminated bulb. Skin and eyes should always be protected when servicing or changing UV light bulbs.
So my advice with regard to UV lighting inside HVAC systems is like anything else. It needs to be evaluated for cost versus benefits and monitored by some knowledge, including that of the installer. Installation and maintenance of UV lighting is relatively expensive.
Placement of the UV light to bathe the face of the coil is probably the best idea and high intensity is required for more killing power. Lamps placed in more than one location in the airstream might be beneficial for increasing effectiveness. And keep in mind that UV lighting is not a cure all. It will not kill all microbes that pass through the light. If properly installed, it is a positive in terms of overall reduction of microbial contamination. Once installed, the life of the bulb is usually considered to be one to two years. At a minimum the bulb should be cleaned on a regular basis in order to maintain effectiveness. The lamp should not be illuminated when servicing or cleaning.
Even more important than UV lights, from my estimation, are high quality filtration that is changed regularly and keeping the coil clean. Regular changing of filters based on functionality and thorough cleaning of the coil on a yearly basis is a good practice. That and controlling moisture through proper sloping of the drain pan and functioning of the remainder of the condensate disposal system will go a long way toward controlling microbial growth and therefore occupant exposure.