A Primer on UVC Light – Part 2, by Dr. David J.

(Continued from Part1.  This concludes the article.)

Area Disinfection

For area disinfection, an unshielded UVC light bulb is put on a lamp and controlled either by remote control or timer. Many are purpose-built, but it is certainly possible to buy a UVC bulb and place it on your own unshaded lamp and use a generic remote control or timer.

The UVC rays will neutralize microorganisms in the air and all surfaces that the light strikes. It can also be used to disinfect strategically placed masks, clothing and other items. In my opinion, the strength of area UVC light is to purify air of respiratory pathogens, any surfaces disinfected are a bonus. Area treatment with UVC often requires that the space be aired out for several minutes after treatment.

UVC for area disinfection has been used for some time for infection control in ambulances, emergency rooms, veterinary clinics and other health care facilities. It is particularly useful in hospital burn units. With the COVID 19 pandemic it has seen more widespread use in dental and medical offices as well as some correctional facilities. It must be used in an unoccupied space.  I could see if being very useful in a home setting where one is caring for an ill family member. A bedroom could be disinfected while the patient is in the bathroom, and the bathroom disinfected when the patient returns to the bedroom. Any area where people congregate- offices, house of worship, daycare etc., could be disinfected when the area is cleared of people and animals.

The important question then is, how long does it take to disinfect a room? The answer, of course is, it depends. The air close to the lamp is disinfected quickly (seconds), the air further away takes longer. But how much longer?

I will try to keep it simple:

UVC light loses it strength according to the inverse square law (The same formula applies to dissipation of sound and to dissipation of nuclear radiation fallout.)   It gets technical, but the strength of the UVC rays drops by the square of the distance from the source. The bottom line is the light strength drops quickly with distance. To be effective in a large space, in a reasonable amount of time, the bulb must have high wattage. To completely cover a larger room, a UVC light may need to be moved several times.

The intensity of UVC light is measured in millijoules (mj) or milliwatts (mw) per square centimeter. The dose of UVC is then measured by the intensity per second.   Different pathogens are inactivated at different doses.

The virus on most people’s mind is the COVID19. According to the International Ultraviolet Association, COVID 19 is neutralized at between 10-20mj/cm2, a fairly low dose. They also say that 1000-3000 mj/cm2 to compensate for light blockage should really do it. There are other estimates available, but 1000-3000 mj/cm2, is a large dose. Any bacteria or virus that receives that dose is certainly harmless. MRSA is inactivated at about 50 mj/cm2 and C-diff is inactivated at about 100 mj/cm2. Here is a chart that shows dose to inactivate various pathogens, there are many others available:

So, this technical explanation doesn’t really answer the question.  How long does it take to disinfect a room? Fortunately, there is a simple solution. Dosimetry cards.   These are fairly inexpensive cards that change color with UVC dosage. Some are marked with the pathogen neutralized, others with the energy dosage. They can be taped up to parts of the room and checked after time intervals (1minute, 5 minutes, 10 minutes etc.) For the frugal, they can be cut in half or quarters. They can be purchased form CUREUV, American Ultraviolet and other manufacturers.  They can also be used to evaluate other UVC devices, wands and ovens.

When testing a UVC area light with dosimetry cards, you may find that to totally clear a room, the light needs to be moved to different spots, several times during the disinfecting process.

There are also meters with probes that measure UVC intensity. Unlike the cards, they are reusable. I personally own a Solarmeter model 8.0, that is very well made and reliable, but that is probably much more than most people need.

I strongly recommend using dosimetry cards, at least initially to evaluate your UVC device. I have tested a number of UVC devices for friends and coworkers and found that many did not produce UVC light and others at such a weak output that they were impractical.

Area UVC lights can be purchased from the same companies that produce the dosimetry cards as well as many amazon vendors. The higher wattage the better.

UVC Boxes or Ovens

An example of a small UV oven is the very popular phone soap, a small box that cell phones and similarly shaped items can be placed to be disinfected. Phone soap is a well-designed oven with bulbs on the top and bottom, so all surfaces are exposed to light. I have tested it, and the strength is enough to disinfect. UVC ovens are probably the safest and most convenient item to use. The casing protects the user from any UVC exposure and the area around it is not tied up. In my opinion, these are very useful prolonging the life of PPE, especially the scarce N95 masks. It can also be used for tools, utensils and eyewear.

There are a number of models available, many are marketed for nail and beauty salons. Many models only have bulbs on top surface and do not apply the light evenly. This can be compensated for by flipping the item over and repeating the cycle. These ovens generally have a timer, but the 5,10 15-minute settings are arbitrary and do not reflect actual times to disinfect.

I own this unit from Tool Klean. It only has bulbs on the top, the bulb facing part of the wire tray measures at 400 mj/cm2 and the downward reflective surface facing at 20mj/cm2. Using the 3000 mj/cm2 guide line, the bulb facing area is disinfected in 7.5 seconds, and the reflective facing section in 150 seconds. The timer is preset for 5 minutes.

Handheld wands

Hand held devices are the most versatile. They can be battery or corded. They also require the most care in not accidentally exposing the rays to the eyes. Small battery-powered wands are very convenient but may take a long time to be effective due to their low output. I own the GERMAWAY UV, a corded 18watt light, that looks like a normal trouble light used around the house.   We use it to disinfect faucet handles, toilets, computer keyboards, eyeglasses, cellphones and keys. It is also effective in disinfecting bedding and stopping bathroom mold growth on surfaces.   Depending on the size of the room, it could also be used to disinfect air and as previously mentioned, water in a basin. At 1 inch the output in 1900 mj/cm2, so it’s very quick.

I find it particularly helpful with my chicken flock. I believe my chickens are much cleaner than commercially raised livestock. However, there are numerous reports of salmonella outbreaks in backyard flocks, and while I am careful, it is one my concerns. I use UVC light to disinfecting eggs (no washing). I also use my handheld light to disinfect their water fonts and coups as a final step in cleaning. In addition, it is also used to disinfect my workspace when butchering them.

Lights in ventilation systems and air filtration units

The last method that I will mention in this article, is UVC bulbs placed in HVAC duct work. The bulb installation is well within the range of a DIY project. Once installed, except for an annual change out of the bulbs, no additional effort is required.

When placed close to AC coils, this is a very effective method of controlling mold in air conditioning units. It is also useful in disinfecting bacteria and viruses traveling through HVAC ductwork. The limitation is that the dosage of UVC light is dependent on the velocity of the air travel and may not be enough to completely purge the air of dangerous microorganisms.

Some closing comments

This is article intended to be primer and not a complete course in UV light. Many UV topics were not covered, including a type of UV called Far UV, that seems to hold a lot of promise. UVC bulbs are a hot commodity right now, but they are available. Those readers that are handy can purchase the bulbs and make their own wands, ovens and area lights. Please shop carefully as demand has led some merchants to chart very high prices. UVC is a simple, cost-effective way of limiting the spread of disease. If you are not sick, then you can be a better asset to your people during the upcoming trials we are all going to face.