Ultraviolet light has become suggested by many as being the answer to microbial growth in HVAC systems. But is it all that its made out to be? This article goes over the principles of operation and performance issues of UV. You be the judge if UV may be in your future.

Principles of Operation

Ultraviolet light is, of course, part of the overall light spectrum. It is below violet in terms of wavelength, to near the x-ray band.

Wavelengths of light are given in terms of the length of the individual waves. Short wavelength ultraviolet light then, refers to light that is between 200 and 265 nanometers. A nanometer is 1X10 ñ9 meters, or a decimal point followed by 8 zeros and a one.

It has been found that UV light disinfects. How well it disinfects depends on many things, which we will discuss. The best particular wavelength of UV light for disinfection is 254 nanometers, as this is where the best germicidal effect occurs.

Just as incandescent light bulbs have a certain energy rating, so do UV bulbs. To provide germicidal action, they must produce enough energy from the source point (the bulb itself) to the receipt point or the micro-organism to provide disinfection.

Once this occurs, in principle microbes may be eliminated. Simple enough ; or is it?

Reaching the target

Light travels in straight lines, unless of course it is going around a massive object like the earth, in which case it will bend slightly. Because it travels in straight lines, it is apparent that when a UV lamp is placed in an air handler, it will not penetrate into the cooling coils.

As seen in the sketch, light can only reach small portions of the coil. Coil sections that are at an angle to the light will not receive UV light, and thus no germicidal action can occur in these areas. Primarily, very little of the coil area can be exposed to the UV light unless many lamps are used. Generally, this is not cost effective.

Another issue is insulation. Insulation in most air handlers is made of fiberglass. Microbes can, and do, proliferate under the surface of the fiberglass, where UV light cannot reach. Again, if the UV light has no exposure on the microbes, germicidal efficacy is non-existent, and microbes will continue to grow as usual.

We can see then, the very nature of light, to travel in a straight line, limits the effectiveness of the UV light source.

UV Destruction Energy

When micro-organisms do get exposed to UV light, there is a certain amount of energy required to provide for germicidal kill. Similar to chemical germicides, contact time is important. Contact time refers to concentration of the killing agent vs. exposure time of the microbe to the agent.

The intensity of UV light is measured in micro-watts per square centimeter and there are 2.54 centimeters per inch. Kill rates for microbes can range from 3,000 microwatt seconds per sq. centimeter all the way up to 10,000 and higher depending upon the microbe.

Why is this data important?

Well, UV light bulbs also deteriorate with time, producing less concentration output. They are the best when new, and with time, become even less able to perform their function.

Even worse, dirt on the bulb, moisture, and anything else that happens upon the bulb decreases effective light output concentration.

This means that a bulb may not provide the killing power it should, even in the limited area of exposure. Over a few months time, it may be likely the bulb surface may become dirty.

But there's more.

Decreased energy

The intensity of all light decreases as the square of the distance from its source.

For example, if you measure a light intensity of 16 watts per square centimeter at 0.5 meter, it will measure only 1/4 as much at one meter, and a 16th as much as two meters. In other words, only two meters away from the source, it is 1/16th the power.

So energy output decreases like a rock as you travel away from the source. This makes microbes feel great!

More on contact time

Consider that air in a typical air handler is moving at 400 to 500 cfm, which is quite quickly.

How much contact time does the UV light have with a microbe traveling past it at this speed? Not much. So how effective is UV light on particles that happen to pass it? Not much.

The real benefit of UV light is that for those hard surfaces that are close by and which can be bathed in UV, it will provide germicidal effect.

Some manufacturers are suggesting that long lengths of UV light bulbs be placed longitudinally down ducts, so that the airborne particulates are actually given more contact time.

You be the judge

UV light is proven to be effective in killing certain microbes.

How effective depends upon the placement of the bulb, how new the bulb is, how far away the source point (the bulb) is from the receipt point, and whether or not there is dust or debris on the bulb's surface. With air velocities as they are, UV is not likely to be a major force in decreasing airborne germs. And with all light traveling in a straight line, UV will not penetrate coils to provide any degree of germicidal activity.

You be the judge.