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Do UV Lamps Really Improve Indoor Air Quality?


I used to be a smart-ass who would say things I thought were clever but really were just annoying.  When I learned the basics of quantum physics, for example, I had a clever/annoying answer for anyone who asked me, What’s new?  E over h, I’d respond.  Hahaha!  Then we’d both laugh our heads off.  Just kidding.  They’d look at me like I just stepped out of a flying saucer, and I’d explain the “joke.”

Some of you know where my answer came from, but let me explain.  It really does bear on the subject of this article.  E/h = ν, where E is energy, h is Planck’s constant, and ν, the Greek letter nu, is frequency.  That’s the energy of photons for electromagnetic radiation with frequency, ν.  So the higher the frequency, the higher the energy of the photons.

This is important in understanding ultraviolet (UV) radiation and its use in buildings.  UV lamps are sold as an aid to improving indoor air quality (IAQ), and lately that includes killing the coronavirus that may be in the air in your home.  Let’s begin by understanding the first part first, UV radiation.  Then we’ll answer the question about whether UV lamps might help your IAQ.

Ultraviolet radiation

Electromagnetic radiation can be described by its wavelength or its frequency.  You can convert between the two using its speed, which, for electromagnetic radiation, is the speed of light.  Wavelength and frequency are inversely proportional.  When one increases, the other decreases.  Longer wavelengths mean lower frequency.  Higher frequency means shorter wavelength.  Here’s a nice illustration of the electromagnetic spectrum from NASA showing that relationship and more.

Using the equation behind my smart-ass answer, we can see that higher frequency also means higher energy.  You can do more damage with ultraviolet rays than with visible light, for example.  And since UV is the focus of this article, let’s zoom in on just that part of the spectrum.  It’s divided into three general regions, called UVA, UVB, and UVC.  UVA is the range that includes black lights.  UVB is what gives you sunburns and skin cancer.  UVC, with its shorter wavelengths and greater energy, is the part of the spectrum used in UV lamps to kill germs.

Using UVC to kill microorganisms

The thing about UVC, though, is that even though a lot of it is generated by the Sun, none of it makes it to the Earth’s surface.  It’s all absorbed by the ozone layer in the stratosphere.  (Ozone is an important part of this discussion in another way, too.  We’ll come to that in a bit.)  As a result, life here on Earth has evolved without exposure to UVC and that makes all kinds of life forms vulnerable to it if exposed.

And therein lies its usefulness.  Bacteria, fungi, and viruses can be killed by exposure to UVC.  The UVC photons are energetic enough to damage the DNA and RNA of microorganisms, destroying their ability to replicate.  Different microorganisms respond differently to UVC.  Viruses are very susceptible to it, bacteria are less susceptible, and fungal spores are the least susceptible.  What that means is you have to apply the right dose of UVC to kill the stuff you want to kill.

And yes, this does work.  This week I watched a webinar with Professor Bill Bahnfleth on the topic of what’s called ultraviolet germicidal irradiation (UVGI), and he showed some of the studies that have proven the effectiveness of using UVC to stop the spread of infectious disease.  He also gave a really good overview of using UV lamps to kill germs.  Here’s a link to the recording of the webinar, and here’s the pdf of the slides.

What UV lamps do

There are two ways that UVC irradiation kills or incapacitates the bad stuff.  It kills them in the air and it kills them on surfaces.  Also, UV lamps are generally used either out in a room, buried inside an HVAC system, or in a standalone air cleaner.  I’m going to focus mainly on in-duct UV treatment here because putting exposed UV lamps in a living room or kitchen is not something that people do with their homes.  (I hope not anyway.)  That technique is for hospitals and healthcare facilities.

Most UV lamps are like fluorescent lights.  It’s the same technology but the primary output is UVC.  In fact, it’s a specific wavelength of UVC:  254 nanometers (nm).  The mercury vapor in the lamps emits light at that frequency, and it turns out to be pretty darn good for killing germs.  The graph below shows that the peak of germicidal effectiveness occurs at 265 nm, so our mercury vapor UV lamp gives us UV light that’s about 80 to 90 percent effective relative to the peak.

UV lamps in residential HVAC systems

The first thing to say here is that it’s not common to find UV lamps in residential HVAC systems.  Some homes do have them, but it’s a small percentage so chances are you don’t have one.  Second, let’s go back to the two ways that UV lamps kill microorganisms:  in the air or on surfaces.

In the lead photo of this article, you see UV lamps installed inside an air handler, between the cooling coil and the filter rack.  The primary benefit of using UV lamps like this is to kill stuff growing on the coil or in the drain pan below it.  Air conditioning coils get wet by condensing water vapor.  When they stay wet, they tend to start biology experiments as dirt and microbes collect there.  In a system with poor filtration and leaky return ducts, it can get really bad, as you can see below (and read about here).

Keeping the coil and drain pan clean is something UV lamps are good at.  Zapping germs out of the air inside a residential HVAC system, however, is generally not as effective.  The main reason for that is that the UV lamp won’t have the intensity needed to provide a high enough dose to kill the various germs.  The air moves through most air handlers and duct systems at 500 to 900 feet per minute.  The faster it moves, the more power you need in your UV lamps to zap the speedy little germs.

So for UV lamps integrated into the HVAC system, the main benefit is to keep stuff from growing on the surfaces, especially the coil and drain pan.  It’s not going to kill much coronavirus or other baddies that get pulled into the ducts.

In addition, you have to make sure the materials that will be exposed to the UV irradiation can take the heat.  As we all know, some materials break down from UV exposure, a process called photodegradation.

The huge downside of some UV lamps

One thing that’s missing in the discussion of UV lamps a lot of times is the other wavelength that mercury vapor lamps generate.  In addition to the 254 nm output, they also produce UV at a wavelength of 185 nm.  That’s well off the peak of germicidal effectiveness (see graph above), but it’s worse than just ineffective.  That shorter wavelength, higher frequency, higher energy UVC radiation causes regular oxygen molecules (O2) to break up and then form ozone (O3).

And ozone, which is great in the stratosphere, is a pollutant here at ground level.  Here’s what the US Environmental Protection Agency (EPA) has to say about it:

When inhaled, ozone can damage the lungs. Relatively low amounts can cause chest pain, coughing, shortness of breath and throat irritation. Ozone may also worsen chronic respiratory diseases such as asthma and compromise the ability of the body to fight respiratory infections.

You don’t want ozone in the air in your home.  Yes, some companies do sell ozone generators and make claims about how the ozone will react with the indoor air pollutants and neutralize or destroy them.  Those claims are not supported by scientific data.  The EPA page on ozone generators shows what really happens.

Not all UV lamps result in ozone generation, however.  The key is the glass used in the lamp.  Soft glass or regular quartz glass allow the 185 nm UVC to pass through and generate ozone.  But when quartz glass doped with titanium is used, all of the 185 nm UVC is absorbed.  For in-duct UV lamps, UL has a standard that some companies use to get their products tested and labeled.  Here’s the label from the APCO-X UV lamps made by Fresh Aire UV.  (They’re the same company whose UV lamps are shown in the lead photo at the top of this article.)

Ozone in the stratosphere is good.  Ozone in the air you breathe is bad.

What’s a homeowner to do?

What it all comes down to is that UV lamps can help with some things, they’re not so good at others, and you need to watch out for the bad things they do.  Here’s a recap:

  • In a residential setting, their best use is to keep the air conditioner coil and drain pan clean.
  • Although they can kill viruses, bacteria, and fungi in the air, they have to be engineered properly to get the right amount of power for the air flow rate.
  • Your best bet is still source control, filtration, and ventilation for good indoor air quality, and that’s where you should start.
  • Some materials degrade when exposed to UV so you need to make sure your system won’t break down.
  • Some UV lamps generate ozone so you need to get one that’s shown to be ozone-free.

There you have it.  Ultraviolet irradiation can help with your indoor air quality at home, but probably not as much as you were told by the person selling you the UV lamps.  And if you show this article to them, they’ll tell you I’m just a smart-ass who doesn’t know anything about the real world.  I think maybe they saw me step out of that flying saucer.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia.  He has a doctorate in physics and is the author of a bestselling book on building science.  He also writes the Energy Vanguard Blog.  For more updates, you can subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn.


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This Post Has 65 Comments

  1. Allison, I always enjoy your
    Allison, I always enjoy your blogs but this one is especially good considering what we all are dealing with. I may just have to take back all the bad things I say about you. 🙂

    1. Thanks, Thomas!  I wouldn’t

      Thanks, Thomas!  I wouldn’t be so quick to take back ALL the bad things you’ve said about me, though.  I’m sure some of them are true.  ;~)

      1. @EnergyVanguard Question How
        @EnergyVanguard Question How do you feel about the UVC lamps to fight the Coronavirus in the air in bars homes or schools?

  2. I just have to say that what
    I just have to say that what sets your writing apart is the ability to bring just the right amount a physics to a practical building performance topic. Some might fall down the PHD rabbit hole and lose most of the audience, others would miss the opportunity to convey a deeper understanding that brings it all together. Looking forward to the book. (no pressure)

    1. Thanks, Andy!  I attribute

      Thanks, Andy!  I attribute that to two things:  not being the smartest physicist in the pool and having written 900 articles in the past ten years.

  3. Another great article Allison
    Another great article Allison. Thank you!

    Curious, the placement of said UV light is best between air filter and coil? Are 2 better for both sides of a wet coil?

    1. Eric, those are great

      Eric, those are great questions and point to an issue that I didn’t discuss in the article.  In-duct UV lamps should be engineered to get the best results, and I don’t know what’s the best way to do it.  I’ve seen photos showing UV lamps in various places in the system.

      Another issue I skipped in the article is the possible degradation of materials exposed to the UV.  As we know, even UVA and UVB can cause materials to break down (roofing, curtains, water-resistive barriers…).  UVC is even harsher so you’ve got to make sure you’re not going to do damage with the UV lamps.

  4. “fungal spores are the least
    “fungal spores are the least susceptible [to UVC]”

    Ah, yes, more evidence that fungi came from space!

    Also, I finally understand why I get butterflies when I stand next to the microwave. Its not because I was in love with it after all.

    In all seriousness, a great write-up! Agree with Andy.

    1. Thanks, Tyler!  I’m sure this

      Thanks, Tyler!  I’m sure this is the only article ever written about UV lamps where you can read about flying saucers and fungi from outer space.  ;~)

      1. I do have a major concern, I had one installed in may HVAC system about 2 years ago my entire family myself 42 my wife 37, my son 15 and daughter 7 have all been being diagnosed with multiple neuorlogical conditions any possible connections? Thanks Jeremy

  5. For 30 years I have been put
    For 30 years I have been put through the wringer, 1st by my former Boss and with countless exposures to Dog and Pony shows with reps for Ozone Generators to various UV light/air purifiers. Before this current crisis we had others and the reps would come into my office and out of a large portion of politeness, (those vendor reps that know me will confirm), I wouldn’t pull up some of the stuff I have, which is some of what you have links to. I would simply ask, “If this could do even 1/2 of what you are claiming, wouldn’t it be all over the News?”
    Before the Net, finding out information on Ozone was hard, but it was available with some digging but i refused to install or service the in duct Generators that my old Boss pushed. Some Network marketing guys got into it to. You could tell as soon as you walked in a home with one of them running. In-duct or free standing.

    1. Kudos to you, Robert!  There

      Kudos to you, Robert!  There’s a lot of misinformation (to put it kindly) floating around on the topic of UV lamps and other products touted for their IAQ benefits.  Keep fighting the good fight!

  6. I also attended this webinar.
    I also attended this webinar. If I understood it correctly, the picture at the beginning of this article that shows the lights between the cooling coil and filter is for a system where the filter is downstream of the coil. I have never seen this done, at least in residential systems. Is this common for commercial systems? Why would anyone put the filter downstream of a cooling coil? Were they trying to catch water blow-off from the coil?

    1. Roy, I was confused by that,

      Roy, I was confused by that, too.  I figured it was just another thing I didn’t know about commercial systems.  It seems if they did put filters downstream, they would be secondary filters as you’d still need filters before the coil.  Bill’s main focus was on healthcare facilities so maybe that’s a thing there.

  7. Allison, I believe this is
    Allison, I believe this is your finest article yet. It takes a little known topic and teaches the reader enough to understand the topic then shows where to seek more information, all in a perfect cadence with a measured dose of your signature levity. Very well done! I can’t wait for your book.

    Years ago, my nephew got involved with a direct marketing company selling the ozone generators you describe. They were marketed as electrostatic precipitators similar to electronic air cleaners sold by Honeywell and others. The problem was they didn’t have collector plates in the unit, rather, the charged dust particles would adhere to the first surface in the home they encountered. The walls became part of the filtration system.

    In order to help the kid I bought one and it sat at the end of my drafting table for months. One day he came over and plugged it in and of course I almost immediately smelled the ozone, which is an almost pleasant smell common to the copy room of the firm I had worked in. It also generated annoying static electricity!

    The thing ran for a couple of hours before my computer that was sitting next to it died. I pulled the cover off of my very expensive 8086 computer and found a hole zapped through the motherboard by the static electricity. Computers are better protected these days, but not our lungs. I have no idea why these devices are still allowed to be sold, but alas, they are.

    1. Yow!  That electrostatic

      Yow!  That electrostatic precipitator turned out to be more expensive than you thought.

      Thanks for the kind words, Mac.

  8. “Downstream Coil Surface”
    “Downstream Coil Surface” description for the first picture would indicate that one receives the air downstream of the filter as one would expect. The pdf does acknowledge UVC can be installed upstream or downstream as shown on page 50. By installing them before the coil greater UVC efficiency is gained through warmer bulb temperatures. Some believe filters can be damaged by UVC however I find that to be not the case. While not a proponent of 185 nm UVC for home use, it has its place as a valuable and chemical free method to sanitize drinking water, ponds, aquariums and drilling rig injection water. Residential applications include rainwater collection systems for household use. There is an argument that the ozone created by 185 nm air sanitizers can be neutralized by appropriate catalysts but a question remains if the catalysts are effective in the sizes sold for residential systems. Clearly the safer hvac UVC systems are the 254 nm versions used to sanitize coils and filters without generating much ozone.

    1. Thanks for the info, rjp.  I

      Thanks for the info, rjp.  I’ll do some digging around to see what I can find.

      1. I bought a UV light stand hat
        I bought a UV light stand hat also emits ozone. I usually set it up in a room in my home fir an hour and leave the house for two hours. Then open doors when I get home to air it out. Is this dangerous for me? How much ozone can I breathe without harm to my lungs?

  9. A great resource on this and
    A great resource on this and other air cleaning technologies like electrostatic filters or ozone generators with lots of references to articles that will let you chase this rabbit far down the hole if you are so inclined is ASHRAE’s position document on indoor air cleaning. It can be found here:
    Great article Allison, I was looking forward to this one!

    1. Thanks, Ethan.  Yes, the

      Thanks, Ethan.  Yes, the ASHRAE position document is a great resource.  I’ll be doing a followup article to go a little deeper into aspects of this topic that could stand to have a bit more light shone on them.

  10. Good information until you
    Good information until you got to ozone and decided to use the EPA deceits about ground level ozone. Ozone also kills viruses, including corona viruses. If ozone were really as bad as the EPA claims it is, it would be downright irresponsible for the EPA not to make ozone generators illegal. Anyone can purchase ozone generators, and do so in order to destroy harmful VOCs and viruses. I keep a low rate ozone generator running in my home with no adverse reactions and no colds of flu issues.

    1. Robin, EPA’s jurisdiction is
      Robin, EPA’s jurisdiction is the outdoor environment, not indoor air quality, thus, they do not regulate any indoor air cleaning devices. You must not live in California where no one can buy ozone generators since they enacted a ban on them over 10 years ago.

      1. Nearly all information on
        Nearly all information on ground level ozone is obtained from the EPA, including what Allyson wrote. ASHRAE, The American Lung Assoc. and every other organization merely repeat what the EPA claims about ozone. When it comes to ozone, we cannot trust the EPA claims and must dig deep to find the truth. The EPA demonizes ozone because measuring ozone levels is the way the EPA determines pollution, which is very wrong minded. Lancaster County, PA is considered by the EPA to be one of the worst polluted counties in the U.S. because of the high level of ozone. Lancaster County, PA is a dairy county that produces a lot of VOC’s from livestock and decaying vegetation. Sunlight interacting with the naturally occurring VOC’s is what produces the ozone in Lancaster County. The EPA is manipulating the demonization of ground level ozone in order to continue funding by being able to make claims such as areas like Lancaster County, PA have high levels of pollution. The EPA most certainly is the government’s last word on ozone.

        1. Robin, first, the EPA bases

          Robin, first, the EPA bases its statements on peer-reviewed scientific research.  Second, ASHRAE also looks at the research, not just summaries written by other organizations.  They also sponsor research.  Here’s a paragraph on ozone from the ASHRAE position document that Ethan Foley provided a link to in his comment above:

          Ozone is harmful for health and exposure to ozone creates risk for a variety of symptoms and diseases associated with the respiratory tract (Koren et al. 1989; Touloumi et al. 1997; Bell et al. 2004). Many products of ozone homogeneous and heterogeneous reaction processes also create risks for health, including formaldehyde, unsaturated aldehydes (produced during the reaction of ozone with ketones and alcohols), and ultrafine particles (secondary organic aerosols) (Weschler 2006).

          They cite the original research done, not the EPA.  Your conspiracy theories are not the truth.

          1. We know that ozone, UVC, and
            We know that ozone, UVC, and some other things can help destroy “germs”, but keep in mind that we (humans) are really just the biggest “germ” in the environment and are probably not immune to things that destroy germs. So my philosophy, in terms of indoor air quality, has always been to prevent bad things from entering the indoor air (source control), dilute the bad things with outdoor air (ventilation), or remove the bad things (filtration). I am against adding anything to the indoor air such as ozone, air fresheners, etc. because of possible unintentional consequences. I am even against air cleaners that are intended to break down air contaminants (like photo-catalytic oxidation) when they leave the byproducts in the air stream.

          2. Roy — I love your comment:
            Roy — I love your comment: “keep in mind that we (humans) are really just the biggest “germ” in the environment” :^}

          3. Agreed. The best solution to
            Agreed. The best solution to air pollution is outdoor air dilution. Outdoor air contains ozone completely naturally and dilutes household caused VOC’s. It doesn’t hurt to run the outdoor air through UV lighting and a particulate filter either.

          4. I’m not sure why you jumped
            I’m not sure why you jumped to conspiracy theories. Everything I posted is fact. Let’s break it down to the basics;
            Is ozone not a form of oxygen that is mostly created by natural means?
            Is ozone not one of the most powerful disinfectants known?
            Is ozone not produced in high enough quantities to be easily detected by smell during lightning storms?
            Does ozone come from any exhaust pipe or smoke stack?
            Is ozone created mostly in the presence of harmful VOC’s, and destroys itself while also destroying those harmful VOC’s?
            Does ozone not eliminate itself and not need to be destroyed?
            Is the half life of an ozone molecule not about 45 minutes at 70 degrees?
            Has not everyone in the world breathed in ozone on a regular basis?
            Is ozone not used by the hotel/motel industry and the automotive rental industry even in California, where ozone generators are banned from sale but not for use?

            When we think for ourselves and do a little research for ourselves, we are able to detect when “peer reviewed scientific research” is not what it pretends to be and that models are not scientific methods but rather educated guesses based on data input manipulated to result in what the model designer wants the model to indicate. It is the very same method used by authors of dystopian future stories that never come true because models cannot predict the constant alterations that make them mostly useless.
            We are missing out on one of nature’s most prevalent cleansers of air and water by allowing ourselves to believe that because ozone is powerful that it is dangerous at any level or not useful at lower levels. Does not a drop of bleach purify a gallon of water? Yet we would not think of drinking bleach from the bottle. And bleach is 98% inert.

          5. A simple copy/paste of the
            A simple copy/paste of the first part of the paragraph cited by ASHRAH came up with this on the first search;
            Whether directly or indirectly, all information on ozone comes from the EPA. When we decipher what the EPA actually states as fact about ozone, the EPA does admit what ozone actually is and it’s actual benefits. It just takes a lot of elimination of negative rhetoric to get to factual information provided by the EPA when it comes to ozone. The first thing we need to do is to eliminate all of the terms such as “could”, “may have”, “is likely to” etc. Those are all baseless claims. I’ll stick with Sgt. Friday and ask for “just the facts”.

          6. Oh, Robin, it’s so tiresome

            Oh, Robin, it’s so tiresome having to respond to your comments but in case anyone reads this far I feel I should.  You don’t know why I accused you of spouting conspiracy theories?  How about this:  “Nearly all information on ground level ozone is obtained from the EPA.”  Or this:  “When it comes to ozone, we cannot trust the EPA claims.”  You throw out opinions like that without any support.  Then you point to what comes up in a Google search as justification.  You ignore the fact that peer-reviewed scientific research is actually where the information comes from.  You ignore the fact that the rest of the world also considers ground-level ozone a risk to human health.

            And you think you know the science better than scientists do:  “When we think for ourselves and do a little research for ourselves, we are able to detect when ‘peer reviewed scientific research’ is not what it pretends to be and that models are not scientific methods but rather educated guesses based on data input manipulated to result in what the model designer wants the model to indicate.”

            If the world’s scientists are in agreement based on lots of research into ground-level ozone, any scientist who came up with significant, reproducible results contradicting the consensus would make a huge splash in the scientific community.  They would set off a rash of new research and analysis of old results to find out why the consensus was wrong.  If that contrarian research was correct, it wold be verified by other scientists and extended.  That’s how science works.

            In comments you’ve posted in this blog over the past several years, you have denigrated the EPA’s work in the area of refrigerants, climate change, and now ozone.  OK.  You don’t like the EPA.  We get it, Robin.  They say things that challenge your belief system.  Until you can provide solid information, though, we’re done with this conversation.

          7. One more thing, Robin. 

            One more thing, Robin.  Criticizing peer-reviewed research in the abstract is lazy.  If your points are valid, you should be able to look up some of those papers and refute them, point by point.  Because you like to speak in generalities, I’m not going to hold my breath waiting for that, though.

          8. I am just an amateur HVAC
            I am just an amateur HVAC enthusiast and occasional lurker commenting for the first time.
            Don’t blame the joke…its your delivery.
            I didn’t get the joke until you explained it. Your problem is, right after the punch line you need to say, “get it? get it? ‘NEW’…the Greek letter ‘Nu’…get it?”, all while pointing to the listener with your elbow and moving it as if you are trying to repeatedly poke him in the ribs. Say the punchline deadpan, but quickly switch to a grin on your face like you just told the funniest joke ever told.

            I have had a question I wanted to put to you for a few years, and now that I’m locked down I thought now is as good of a time as any to see if you would entertain my question. Actually I don’t know the question and I may need
            help writing it, but here is my shot at it:

            Given a house with a fixed cooling load for the next 24 hours, and a humidity level at, say 50%, with no new humidity being introduced, what is the ideal coil temperature, air speed, system that would reduce the humidity the most?

            Or perhaps another way of asking what I’m trying to understand: given a perfectly insulated box, with no heat gain, and a 50% humidity level, the temperature starts at 75 degrees and you lower the thermostat to 70 degrees, what is the ideal coil temperature, air speed unit size to reduce the humidity the greatest amount at the moment the thermostat is satisfied?

            At first I was thinking 32-somthing degrees would be the answer, just warm enough not to freeze. But then I thought, is that cooling the air too much so that it mixes with moist air and satisfied the thermostat too soon?

            And does the answer change depending on the beginning humidity? for instance you start with 40% and want to reduce that the most possible during the run cycle.

            I realize I mentioned variables that may make no difference and left variables out that may make a difference such as perhaps cubic feet of conditioned space. My hope is that you can see what I’m trying to understand from my question and address the topic sometime.

            PS: Why can’t you take the cross product of Spider-man? Because hes a SCALER! Get it…bet it…scaler? hahahahaha

          9. Correct Allison. EPA’s
            Correct Allison. EPA’s research and studies are peer reviewed very carefully. This article is excellent, but is focused mostly on residential applications. Does this information change when you apply it to larger commercial systems?

  11. Dr. Bailed, at what velocity
    Dr. Bailed, at what velocity would an air cleaner become effective? The reason I ask is because it appears some of these devices have baffles or similar to possibly reduce velocity of air as it moves across the light. Would this make the sanitation of air borne viruses more effective?

    1. Chris, that’s an impossible

      Chris, that’s an impossible question to answer without making the question more specific.  First, the effectiveness depends on what it is you’re trying to kill – viruses, bacteria, fungi.  Second, it depends on the intensity of the UV lamp.  When you know those two things, you can calculate the dose needed, which is a product of the intensity and the time of exposure.  To know what they’re designed for, you should ask the manufacturer of a particular device.

      Slowing down the air, though, would lead to a higher dose, so it’s certainly possible they do that.  But that comes with a tradeoff:  Moving the air through the device at a lower velocity means that device will treat less air so then you need to look at that factor.

      1. Allison,
        Your comment on time of exposure apparently considers only one pass air flow in the AC duct. However, most of the air moving through the system is recycled continuously. This actually exposes “the same” airborne viruses repeatedly to the UV-C radiation. The device effectiveness is therefore rather cumulative. An initial virus load could then be reduced to zero significantly faster than without a UV lamp, as long as no new shedding occurs in the space. How much faster depends on the intensity of the lamp.
        Have you seen data supporting that?

        1. I’m late to the party on this but that is an interesting question worth answering.

          In the 2019 ASHRAE Handbook-HVAC Applications, Chapter 62 Ultraviolet Air and Surface Treatment it states the following:

          “The dose is generally interpreted as that occurring on a single pass through the
          device or system. Although the effect of repeated UV exposure on
          microorganisms entrained in recirculated air may be cumulative, this effect has not been quantified, and it is conservative to neglect it.”

          With that said, maybe there is new research on this topic. In all likelihood though, multipass efficiency is probably hard to model in the real world with so many factors at play.

  12. Dr. Allison–Thanks for
    Dr. Allison–Thanks for sharing your thoughts about this. I’ve been working for an HVAC contractor in Atlanta for 20+ years. The last 6-years as Service Manager. I’ve always been wary of UV type IAQ products. I get sales calls all the time about these things and I just don’t know if they do the things they say they will.

    Filters I get, you can show the customer how good a filter works. Dehumidifiers and humidifiers, I can show you an improved and readable humidity level with a humidistat or psychrometer. But a light in a duct, that claims to go out and kill everything in the house that is bad for you, get rid of odors, and attach to duct particles to help your filter work better? Sounds a little too good to be true if you ask me.

    With all of this COVID-19 going on, I know I’m going to get phone calls from both Commercial and Residential customers asking if an option like the new REME-HALO LED is a viable option. We have a lot of light commercial customers that could potentially benefit from a product like this (Dentist offices, Daycares, Boutiques, Etc.), IF it does what it says it can do. I’d love to know your thoughts about this product or a comparable one like the Air Scrubber plus.

    Thanks for your time. I just found this blog and plan on doing a lot of binge reading over the next few days. Thanks again.

    1. Brian, the HALO-LED in-duct

      Brian, the HALO-LED in-duct UV lamp does claim to be ozone-free and has the Intertek lab verifying that.  Overall, I think they make it sound like a super duper amazing air cleaner (“proactively treats every cubic inch of air conditioned space”) and I have questions about it:  What UV wavelength(s) does it emit?  It needs to be close to 265 nm for maximal germicidal effectiveness.  What’s the pressure drop across it when you put it in the air stream?  It’s a sizable piece of equipment so it probably eats a significant amount of static pressure.

      Happy binge-reading!

      1. Hi Allison,
        Hi Allison,
        Great article, love the more in depth review. I too am an HVAC contractor in Birmingham and have always been skeptical of IAQ products, this is partically due to me being a high C on the DISC profile and not super sales oriented. I have taught my techs that a single bulb UV light is not intended to clean the air but it can help keep the coil clean and possible the plenum. We find a lot of mold/fungus growing in the supply plenums when it is ductboard or internally lined metal duct. If we do find growth we take it out and replace with metal duct wrapped on outside. Plenty of other contractors would just put a UV light in, collect a check and never check to see if it did anything. I feel that the cause of growth is due to water blowing off the coil and landing in the fiberglass, then gets mold spores and dander and then you have growth. Metal duct takes care of this and recommend this. If we have a system that does not have growth but is ductbaord, or fiberglass we recommend that UV light could prevent growth above the coil. The only downside is that the UV light could breakdown the fiberglass but I don’t know enough about that.

        We have been selling the Solaris UV PCO air pufifer lately made by Ultravation and it is similar to the Reme Halo. I wanted to know if you have done more research on these PCO and their claims and also the claims from the new ion generators (not ozone generators) that are flooding the market. These include the GPS and iWave products by NuCalgon. I have been told that that the Solaris does emit a small amount of ozone but was told it was so low it does not register as an amount that is harmful per EPA. Does such a level of ozone exist?

        Thanks for another great article!

      2. Hi Allison,
        Hi Allison,

        Thanks for your work on this subject. My wife and I both have respiratory sensitivities and live in the Pacific Northwest which has been increasingly subject to wildfire smoke and particulates, and being in a higher risk group in the midst of the COVID-19 pandemic, we are revisiting air filtration for our home. Current system includes a Carrier Infinity 25HNB6 60A320 heat pump with a Carrier Infinity 59MN7A120V241122 gas furnace (propane conversion) and a 5” MERV-11 filter (HVAC installation contractor recommended against MERV-13 due to adverse pressure drop). We ruled out alternative filtration systems during construction in 2011-13 due to concerns and contraindications regarding ozone.

        I have been looking into the RFG HALO-LED. In speaking with RFG, they indicated the UV wavelength generated by the LED lamps is 254 nm, and there is a negligible pressure drop. (According to the product documentation there is “no air flow restriction or back-press on the HVAC blower”.) As discussed in this forum, this product is certified by Intertek to be ozone free (no more than .005ppm).

        However, as I understand it, the UV lamps are not intended to do the purification work in this device; instead the UV lamps interacting with a “quad-metallic PHI” catalyst (and/or a “hybrid ceramic” catalyst) generate hydro-peroxide gas molecules when switched on, and this occurs whenever the HVAC system’s blower switches on, resulting in .01ppm to .02ppm with the PHI unit on and .00 ppm with the PHI unit off according to the product documentation. The hydro-peroxide gas molecules saturate the building’s air spaces and are said to reduce viruses (99%), bacteria (99%), odors (55%–98%), mold (97%–98%), VOCs (80%–99%) and smoke (70%) according to the product documentation. [ ] The documentation also indicates at .01 ppm, there will be approximately 177,000,000,000,000,000 hydro-peroxide gas molecules in a single liter of air, thus close to 1 micron apart. And, reportedly, naturally occurring outdoor levels of hydro-peroxides are between .01 and .03 ppm and the dangerous levels appear to be ~1.0 ppm.

        Are you familiar with the indoor application of airborne hydro-peroxide molecules and any related data on health and safety (in addition to efficacy) of such applications? What are your thoughts on this type of treatment in a residential application?

        Thanks again,

        1. I can address this. If there
          I can address this. If there existed a product that operated in this perfect arrangement presented, filling a space with exactly 0.1 ppm of an active sterilizing agent with molecules perfectly arranged less than 1 micron apart that would neutralize all viruses, wouldn’t it be in every hospital and home already? If it sounds too good to be true, it probably is. Oxidizing agents like peroxide are highly reactive and have a very short lifespan before they interface with a surface or particle that reacts with them and renders them inert. Metal duct walls and diffusers are examples of materials that will react with peroxides. If there was a concentration of peroxides in the air strong enough to neutralize bacteria, it would also neutralize the pigment in paints and upholstery, and the beneficial bacteria in your skin and sinuses. In practice, these devices rarely have a detectable amount of active peroxide that makes it into the room air, so the effectiveness is limited. It could potentially neutralize some virus in the ventilation air. Over time (months or years) it could degrade the paint and carpet around the diffusers.

          The general wisdom is to use the HVAC filters to protect the HVAC system (fan and coils) and use an air purifier in the living space to protect the occupants. Stand-alone HEPA filters can’t go wrong (although they do make noise and generate a draft).

  13. Dr Bailes: I commend you once
    Dr Bailes: I commend you once again for (1) your superb skills in communicating technical topics in ways non-experts can understand; (2) dedicating yourself to advancing knowledge withIN the industry, identifying key issues needing more research, & helping to clarify & solve practical problems & deficiencies in current HVAC equipment & practices. IMO, this/you are doing an exceptional job that is sorely needed…& in the process you are giving advice & knowledge that a consultant would charge us 100s & 100s $$ for–IF they even had the knowledge & insights you have. Much gratitude for that. NOW for my question [ha, ha]: do you have any citations of research that demonstrates that UVC not only ‘kills’ micro-orgs such as mold particles, but DISINTEGRATES THE TOXIC MYCOTOXINS INTO HARMLESS MOLECULES? [not shouting at you–capitals to emphasize the key issue]. I apologize for the length of this ‘comment’ but you recognize this is a complex topic I am raising & it takes more sentences to clearly state it. I’ve been reading a lot of research papers on molds & their mycotoxins & relevance to health & indoor air quality. What I’ve learned: (1) it’s the mycotoxins & glucans contained in/on mold cells that pose the most serious health problems for humans bcuz they can cause toxic damage to body systems; (2) many mold species produce myctoxins & all mold spp. contain glucans in their cell walls & research indicates those glucans also may pose potential adverse health effects; (3) the toxic mycotoxins are contained in/on ALL mold structures [e.g. mycelia], mold fragments, viable spores & DEAD/nonviable spores. So in light of that research, the critical health question is whether there is evidence that UVC totally disintegrates mycotoxins into harmless molecules, because even a ‘dead’/non-viable/non-living mold spore or fragment still contains toxic mycotoxins in those ‘dead’ mold particles unless it’s proven that the UVC totally disintegrates the mycotoxin compounds into harmless molecules? [Same question applies to the highly touted ‘PCO’ type ‘air cleaner’ technology–even if they’re shown to reduce active/viable SPORES, is there evidence they destroy the toxic mycotoxin compounds?] So far, I have NOT found any research showing that the mycotoxins are totally disintegrated into harmless molecules either by UVC or by ‘PCO’ type ‘air cleaner’ technology. My searches on PubMed & even GoogleScholar have so far failed to reveal any research addressing that. There’s lots of research going on internationally on trying to find ways to reduce/eliminate mycotoxins in food products [becoming a more severe problem due to climate change]. What that research has told me in simple terms: (1) mycotoxins are very difficult to disintegrate into harmless molecules; (2) some mycotoxins respond to efforts to destroy them by converting into a different type of mycotoxin instead of being totally broken down; (3) some methods have promise of disintegrating 1 type of mycotoxin under certain conditions, but doesn’t work on other types of mycotoxins [some spp of mold produce numerous diff mycotoxins & it’s also typical for multiple spp of mold & multiple mycotoxins to be present in the same food [e.g., grains, nuts, fruits, grapes, coffee]. I’d appreciate any research refs you might have run across: does UVC or PCO disintegrate mycotoxins into totally harmless [non-toxic] molecules? THANK YOU!

    1. Terry, with Alison’s
      Terry, with Alison’s permission I would like to take a stab at this. I will limit my main focus to residential use as that is the article’s focus, and any specialty manufacturing use requires specialized knowledge that I don’t claim to have.

      There is a lot of solid research on illness caused by “mold and moisture” in buildings. The research into the nature of the illnesses is spotty…some are known, such as mycosis and allergic-type responses, but many illness mechanisms are at best theories. Speaking directly about mycotoxins, we know they exist but actual research and understanding disease mechanisms just isn’t there. Most doctoral studies on mycotoxins are not reproducible, so the conclusions and treatments based on those conclusions are not currently scientifically based. We just don’t know enough about mycotoxins to make scientific claims about their prevalence, toxicity, or possible chemical transformation.

      The state of the science is that indoor mold growth should be controlled at the source. For residential purposes, that means simple things like controlling indoor humidity, avoiding water leaks or condensation caused by building envelope deficiencies, and removing excess moisture at the source by local exhaust. Inadvertent water leaks should be responded to promptly to avoid mold growth. If these practices are followed, there will not be concerns of mycotoxins in the home so anything sold to “eliminate” them is at best unnecessary and at worse snake oil.

      The only possibility I can see of these air purifying technologies providing the benefits they claim is by generating ozone, and ozone has to be extremely high concentration to destroy fungi. I would be willing to be proven wrong, but nobody has so far. Repeated requests about the mechanism of treatment from the companies I have approached are either unanswered or responded to with marketing materials with pseudo-science references.

      1. It is very difficult to keep
        It is very difficult to keep up with the changes that are made to ozone level claims. From what I am seeing, .9 PPM of ozone is effective against most fungi while there were levels of 9 PPM, ten times the amount, that was blamed on pulmonary edema in welders who were also exposed to other air pollutants. Even though the current allowable level of ozone exposure to humans by government agencies can be higher than what is required to kill fungi, viruses and bacteria, continually reduced from 10 PPM from the 1970s, the EPA continues to claim that maintaining a sanitizing level of ozone in a home is unsafe. Why?

  14. Hi Allison,
    Hi Allison,
    Nice article and timely. Thank you. You stated what I already thought in a nice way with references. My take on residential use is that it might not kill everything but it if you are not making ozone there is no harm and likely some benefit. There is no test standard for efficacy that I’m aware of, and no way to prove it is doing anything, other than anecdotal information. For now, don’t make big claims and let the buyer make his own mind. Give them a link to your article. Maybe ASHRAE could sponsor a residential study? With a test and rating standard? That would help the consumer the most. And us manufacturers.

  15. Terrific timing for this
    Terrific timing for this article. I have already shared it with contractors who have asked me why I do not recommend installing UV systems in standard residential systems. You provided more details than just saying the air does not spend enough time over the UV light.

  16. Hi Allison. I have a related
    Hi Allison. I have a related COVID question.

    Having additionally read your excellent 2014 article on the problems with closing vents, I have a bit of a conundrum in that I will be sealing off a small (8×10) room within a 950sqf house (Florida weather) that has central air with 6 supply vents. The room will have a window AC unit with the supply vent shut off (leaving 5 of the 6 registers open), and the door treated so that there’s no air escape when it’s closed, which will be all of the time.

    The purpose of this is to be able to isolate in the room from the central air circulation, while having air exchange only in and out of the window unit, so that potential exposure to persons with COVID in other rooms of the house can be controlled (at least while in the isolated room).

    My question for you is that since it has been established that closing vents has potential consequences on the operational performance of an HVAC system, what proper modifications can I make (through a knowledgeable professional) to the HVAC system that can mitigate the potential harm that closing the supply vent in this room may cause?

    Would adding a manual balancing damper as noted in the comments on your 2014 article be one solution? I don’t fully understand that but would look into it if it’s recommended.

    Also, will the fact that I’ll have the door mostly sealed when closed (i.e., no return air to the rest of the house) additionally affect the situation positively or negatively?

    Thank your very much for considering my questions,


    PS I would have commented under the other article, but the comments are closed.

    1. I have a background in
      I have a background in microbiology and am a contractor. Your best practice would be to remove the window unit and replace with a low volume, low speed exhausting fan or use it as an exhaust only unit (set on low) thereby creating a negative pressure room. Leave your central airvents open or partially open and also have a portable room AC on wheels with a standby bucket for emptying the condensate that will build up in that unit. Seal door moderately and keep this as a negative pressure room that you can use in the event of infection. Btw, stay away from the exhausting side of the window unit while the room is occupied or outfit it with a uvc lamp.
      Btw, I have a uv light installed in my return air before the evaporator and has been in for almost a month now. Air is wonderful and I have not seen a return, in shower, of mold or mildew and odors are noticeably absent. Wife even burn burgers yesterday and there was no lingering smell after a couple of hours. Now, I’m aware what ozone can do to the olfactory system so I will be investing in a ozone detector, if available, to check on regular ppm’s. Hope this info helps your bug room. Good luck.

    2. Pete, the first thing I’d

      Pete, the first thing I’d suggest it to measure (or have someone else do it for you) the total external static pressure across the air handler.  Closing one of the vents may not add enough extra resistance to cause the kind of problems in your system that I discussed in the article on closing vents.  You could also have the total air flow checked with and without the vent closed to see how much it’s affected.

      You mentioned wanting air exchange through the window unit AC, but I think few if any window unit ACs have that as a feature.  As with a split system, there are two separate air flows:  one for indoor air flowing over the evaporator coil to remove heat from the room and another for outdoor air flow over the condensing coil to dump that heat to the outdoors.  You could retrofit the AC to do that, but it wouldn’t be easy.  A better method would be to do what Terry M. suggested and install a small exhaust fan (maybe 20 cfm).  That would put the room under negative pressure, pulling in air from other parts of the house.

      But you mentioned in your comment that the purpose of this room is to prevent “potential exposure to persons with COVID in other rooms of the house.”  It seems better to have an isolation room for the infected person, but if you want to do it the other way, you’d want that room under positive pressure.  Instead of a small exhaust fan, you’d want a small supply fan, blowing outdoor air into the room.  In either case, you’ll need to be careful with the amount of outdoor air you introduce to the house because you can end up growing a lot of mold in Florida.

      1. Thank you Allison and Terry.
        Thank you Allison and Terry.

        Based on this conversation, I’m going to install a small supply fan to bring air into the room next to an AC unit to cool the room off (going with a small window unit instead of a portable unit that would be too loud).

        The supply fan I’m looking at is 110cfm. Is that too much flow coming from outside? Here’s the fan in question:

  17. Kudos to you people. Your
    Kudos to you people. Your article is a bit “scientific” for me, I get cross-eyed when someone talks volts and wattage, but there is enough there in English that I still get information I can use. thanks so much !!! MJo

  18. Just found your articles and
    Just found your articles and find them interesting. I thought you might find the cited paper below of historical interest. It is a review of the history of UVGI.

    In a chicken-egg kind of situation, in 1935, UV was used to prove the airborne infection route and at the same time proved the effectiveness of UV on airborne pathogens.

    Reed NG. The history of ultraviolet germicidal irradiation for air disinfection. Public Health Rep. 2010 Jan-Feb;125(1):15-27. doi: 10.1177/003335491012500105. PMID: 20402193; PMCID: PMC2789813.

  19. Thanks, Bobby, for your
    Thanks, Bobby, for your response above. I’m posting as a new comment since the subject appears relevant to the whole UV discussion thread. /* admin: please feel free to move post if appropriate. */

    While this newer technology — LED UV lamps interacting with a “quad-metallic PHI” catalyst to generate hydro-peroxide gas molecules circulated by the HVAC system — is well out of my field of computer science, I agree the section of RFG literature describing a perfect lab scenario (1 liter of air with .01 ppm hydro-peroxide gas molecules arranged less than 1 micron apart) sounds at best like overly exuberant marketing copy. The literature also states .01ppm to .02ppm with the unit/blower on, but doesn’t indicate the conditions or locations of these metrics. All of this is supposed to neutralize—to varying degrees—viruses, bacteria, mold, odors, VOCs, etc. The RFG product also claims to be ozone-free with Intertek certification (less than or equal to .005ppm).

    Interesting points about oxidizing agents having a very short lifespan before being rendered inert though of much greater concern is your comment about adversely affecting beneficial bacteria in your skin and sinuses, not to mention in paints and upholstery.

    I’m trying to reconcile the marketing claims of this newer technology with actual science and some recent anecdotal data, e.g., I have an acquaintance, a medical doctor, who installed an RFG device a couple of years ago and another acquaintance who recently installed the Aerus device and both are happy with the results. Additionally, there appear to be other devices employing similar UV/oxidation technologies that claim to be used in medical, hospitality and cruise ship applications, including those mentioned in posts by J. Braun and Will at regarding the Aerus line of products (one reportedly with FDA clearance). Also, another device was mentioned by Tyler Kime, the Solaris UV PCO, in this discussion thread.

    I’m wondering if there is any newer data on any these more recent approaches, and if they mitigate any of the concerns you have raised. I had the pleasure of leading diverse technical teams over many years and have found divergent points of view can lead to innovative solutions. I’m hoping to stimulate further discussion of what potentially could be an efficacious, and hopefully safe, emerging technology versus what happened with ozone-generating equipment of years past.

    Also, I should mention that my interest in this subject is personal rather than professional (my family has respiratory sensitivities).


    1. Bruce, thanks for your
      Bruce, thanks for your question. Let me first state that my intention was not to call out other technologies as quackery or outright harmful. There are so many on the market now that I don’t even try to keep up with the market claims and actual studies performed. Many technologies have been effective in industrial applications for years. To me it comes down to the accurate scaling of the technology to the home it is intended to be installed in, and the marketing claims. Hydroxide and hydroxyl radical technologies are often quite effective at removing odors, and they are used in industrial applications to break down VOCs. The truth is that most of this function happens very close to the source. If the unit is designed to produce 0.02 ppm inside the air handler, it may very well do that. But unless you are running a sterile clean room environment with inert duct materials, many of those particles will have reacted with airborne or surface matter before making it to the diffuser in the living space. Hydroxide will live longer than hydroxyl radicals, but it can only be used in very low concentrations and those concentrations will diminish quickly over time and distance from the source.

      Another technology worth mentioning is ionizers. They have some effect on VOCs (not always favorable) and the action on particulate is by means of causing particles to cling together and (theoretically) fall out of the air onto the carpet in the middle of the room, right in front of your vacuum cleaner. Realistically, it causes particles to cling to curtains, clothing, artwork, furniture, and any other surface. Air is cleaner until you touch the curtains or sit on the sofa, then (worst case scenario) you might get a concentrated dose of everything pulled out of the air for the past few months. Electrostatic precipitators have this same effect. Both result in fewer airborne particles but no significant reduction in allergy symptoms.

      When it comes to killing biological pathogens, there are many effective ways to do that. The problem is that humans are just a collection of biological matter that are also as susceptible to harmful chemicals or energies. In short, if it kills airborne bacteria, it will also harm humans. If reactive particles are being distributed to the living space in concentrations effective to kill thick-walled airborne bacteria, they will most likely produce unwanted health effects when inhaled. In most cases this is merely academic because humans don’t go inside air purifiers where the reactions take place.

      Very few things harm mold spores without being lethal to humans, but spores usually aren’t a concern without a moisture source. You can keep airborne mold low by addressing moisture. If there is a mold source, you can’t purify your way to a clean environment. Viruses are a witches brew and I won’t even guess about effectiveness without test data, and viruses are hard (and expensive) to culture. And with molecular biocides there is often great variation between effectiveness on one virus and another (Polio vs. measles vs. coronavirus vs. flu).

      In terms of how effective a particular product is in a particular home, I can’t say without extensive data. Not only would I need test results, I would also need the conditions the test was performed in and evaluate how closely those conditions resemble the proposed home and HVAC system. In anecdotal experience, VOCs causing odors or irritation are a lot easier to remove than bacteria or allergens. Air purifiers have had little effect in my experience on biological agents. (I would love to be proven wrong on this! Just a bit jaded by outsize promises.) HEPA filters are effective on mold spores and bacteria and apparently viruses. Source control and physical filtration are proven and time tested and are used at EVERY hospital and lab. Other technologies can also work for specific purposes, but there is no one-size-fits-all. Maybe the best resource is looking at various environments where the equipment is installed and seeing if it is having the desired effect. In addition to acquaintances, look for online reviewers and try to contact them. Talk to multiple contractors who have installed them, if possible. Whatever the selection, just don’t rely on it for virus control to the extent that you relax proven methods.

  20. I know your articles
    I know your articles specifically talks focus mainly on in-duct UV treatment but with the current pandemic, what would be your recommendation for travelers who are looking for all options to disinfect hotel rooms?

  21. Wow. It looks like someone
    Wow. It looks like someone took a very long trip on the confirmation bias train. Allison, kudos for keeping a level head with your replies. The average person has never had this amount of information available to them in human history. It’s a damn shame so many people aren’t equipped to sift through it in a scientific way, even dismissing peer reviewed research and moving on until they find what they want to see. There’s a perfect meme I saw. It depicts Ralph Wiggum from the Simpsons sitting in front of a computer, typing “what I want to be true” into youtube and the caption reads “I’m a resurcher”.

    Anyway, thank you for taking the time to write this blog and attempting to promote the best information available at the time.

  22. Dear Allison, Thank you for
    Dear Allison, Thank you for your very clear and objective summary of this topic. I appreciate the time you took to develop this article, which is very helpful to the residential consumer.

  23. For companies who would like to have the effectiveness of their UVC in-duct products tested, who do you recommend having them contact?

  24. Thank you for the article.

    As a homeowner with a White-Rogers electronic air cleaner installed in 1989 should it be shutoff and the two washable cells replaced with a 16 x 25 Merv13 filter? I wonder if the Merv13 will reduce the airflow too much. Does the White-Rogers generate excessive ozone if it’s properly maintained?

    Thanks for your input.

  25. what about LED UV lamps? Any problem with Ozone generation?

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