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.


  1. Thanks, a very useful article. I need to do a little more research on using UVC to treat filtered rain water. Like the whole house water filter systems, I am sure, there are UVC systems that can be tied in.

  2. UVC lights changes any disease-causing bacteria and virus and fungus — pathogens — to harmless.

    At the risk of pulling a ‘AOC’, how does the wavelength know the difference between health-causing and disease-causing?
    [For reference, see the meme with the dear child questioning her drinking-mug’s magical ability to keep hot drinks hot and cold drinks cold. In the meme, she wonders “How does it know!”]

    1. A good question, LargeMarge… I will await the opinion of an expert on this, but my best assessment would be that the UVC lights do not differentiate. This is a matter of the greater good or the lesser of the evils.

      1. If I may drop in my scientific understanding; bacteria, mold spores, viruses, pollen, etc etc are essentially draped on everything, everywhere and all the time. We LIVE awash in a sea of “micro life”. Personally, I’m glad I grew up in the country drinking unfiltered well water as I now rarely ever get colds or the flu (so I presume that my immune system got strong facing everyday cooties). One would apply this UV-C technology on those specific surfaces and items that you suspect of contamination, NOT to frequently sterilize your entire world “just in case”. So, I run my keys, phone, every-day-carry stuff thru my UVC box when I get home. Also, I’d expose my bathroom/toilet for 30 minutes if a family member had an upset stomach. And I’d use my portable UVC wand on its internal timer in a hotel bathroom while I went to dinner. You simply can not kill all cooties, nor should you, but you CAN focus on likely problems.

  3. Good article on the value of sunlight. Bottom line: get your sorry carcass outside in the sun & fresh air and quit worrying about the comments of the talking heads and so called “experts”, whose opinions change daily. Mostly cause they are on the goobermint payroll and of one sort or another and don’t have a clue how the real world works anyway. Try your best to live by Gods Law and our Constitution…. You live – you die…. That’s just the way it works….

  4. Again many thanks, Dr. David J! There is a lot to learn, and your article has expanded my knowledge base.

    I had a special appreciation for your closing statement: “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.” I am oft heard using the following expression: “Prevention is the only game in town.” You are right. If we can prevent illness, we should do that — every time — for ourselves, and so that we can support and assist all those around us (being best able to do so when in good health).

  5. Thanks, Dr. J, for this solid overview! I’ll repeat my concern from yesterday that many “Germicidal” LED lamps sold online are bogus. Fortunately, the glass tube lights (low pressure mercury vapor; “LP Hg”) are legit and work as per Dr. J’s guidance above.

    The YouTube channel “bigclivedotcom” has several excellent videos about the bogus LED “germicidal” lights (typically either the fake versions of wands similar to what is described above, or “corn cob” lamps covered in 100-150 LEDs) which only emit nice blue light, but zero UV-C wavelengths. If they have 100+ LEDs but cost only $30-50, they’re almost certainly fake. BigClive suggests using a green banana to test crudely for UV-C; if the skin turns dark after 10-20 minutes of exposure, they DO produce UV light! Note that a true UV-C LED *must* have a tiny metal ring around the LED or they are phony. I have photos of my spectrometer tests of both these real and fake lights, if only I knew how to post them… There ARE real Germicidal LED lamps out there but beware the numerous frauds!

    I have a plastic cooler lined with aluminum foil, a metal rack to hold PPE, cell phone, etc items up off the floor so the light can bounce all around them, a 15W LP Hg bulb in a socket, and a darkroom timer. Furthermore, I have a battery-operated LP Hg “wand” that I use on shopping cart handles, ATM keypads, gas pump controls etc.

    Covid may be mostly transmitted through the air, but sometimes surfaces are a concern too. Use this technology carefully and it works (for Covid or whatever other cooties, including cleaning home when a family member has some bug; as per Dr. J’s description). Best wishes

  6. Thanks to the Doc for his 2 part article on UVC light. Well done and needed.

    I am very familiar with the use of UV light in mechanical applications. It works very well in HVAC systems to aid in limiting bacterial and viral loads. It even helps, although not proven by documentation, with breaking down allergens.

    To address a comment on ozone (O3), O3 is naturally occurring. Every time lightening strikes, O3 is generated. Ever been close to a strike? If you notice, the air smells extremely “clean”. That is O3 cleansing the air. The separation of the third molecule back to O2 is volatile to organic matter, thus will have a detrimental effect. Example. Over 3Kmmg in a 200sf room with a 1 hr constant exposure will ruin natural rubber, kill small bugs and have weird effects on plants. It is a respiratory irritant to humans temporarily, but I’m still here. By the way, O3 has a very short conversion time back to O2, so concentration use and ventilation afterwards is safe.
    In moderation it is an extremely beneficial gas.
    I can personally attest in using it in mold remediation, destruction of organically derived scents (smoke, urine, etc.), and for over a year now, produce extension. I have rechargeable O3 generators in multiple refrigerators and fresh produce lasts way longer. Taste is retained. As long as the product can be exposed to the interior environment, (not in a container) deterioration is inhibited. A word of caution, if you are saving fruit, i.e.. grapes, for homemade wines, it will destroy the natural yeast that would normally ferment the fruit.
    If you do things like that.

  7. I’m sorry but my science/math abilities are limited for an old guy. Is there a simple formula to show how long it takes to cover an “X” amount of area at “X” inches of distance using “X” watt UV light? I have an LED rated at 60W and use it for groceries, package deliveries and my shoes before they come into the house.Got very little info when purchased other than 30min in 500sq ft room for sanitizing.

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