Since you are reading an ActivePure blog, you already know we're the best of the bunch. However, you shouldn't have to take our word for it without understanding other technologies. In this article, we will explain nearly every type of passive and active air purification technology on the market so you can make your own decision. We'll also explain what is special about ActivePure's proprietary innovations.
Passive vs. Active: the Basics
Air purifiers can be sorted into two broad categories: passive and active.
Passive purifiers require that the air first pass through an internal mechanism. You might notice a problem with this though. If the air has to flow through a filter to be treated, what's to stop it from first flowing into your lungs? This is particularly important in light of the COVID-19 pandemic's Delta variant. A single infected person can spread the disease to the entire room before the passive purification has a chance to remove the virus. Examples of passive purifiers include HEPA filters, carbon filters, and most UVGI.
Active purifiers, by contrast, are proactive. If they have a philosophy, it's "The best defense is a good offense." Active Purifiers reduce contaminants in the air without needing to first draw the air through their mechanism; they go to the contaminants rather than waiting for contaminants to come to them.
Active purifiers are relatively new (compared to old standards like passive HEPA) but certain types demonstrate their effectiveness in study after study. Examples of active purification include ion generators, photocatalytic oxidation, and ActivePure's proprietary technology.
Purifiers may either be installed into the ductwork of a building, or they may take the form of a portable unit.
Some Vocab Notes
Fluid: You may be used to thinking of the term "fluid" as synonymous with "liquid." But in science jargon, "fluid" technically means both liquids and gases, including the air we breathe.
Ion: We'll let Merriam-Webster explain this one; an ion is "an atom or group of atoms that carries a positive or negative electric charge as a result of having lost or gained one or more electrons." Ions which gain an electron are negatively charged. Ions which lose an electron are positively charged. Each ion is attracted to particles of the opposite charge. Ionization is the process of creating ions.
Polarize: Molecules may be polarized, either naturally or because they pass through an energy field which gives them a polar charge. A dipolar molecule has a slightly negative charge at one end and a slightly positive charge at the other.
In the context of air purifiers, bipolar ionization refers to products that produce both positive and negative ions simultaneously. This can be confusing at first, as polarization and ionization are not the same thing. While both charge contaminants, polarization bestows charge at the molecular level, while ionization bestows charge at the atomic level through the loss of electrons. In this context, "bipolar" doesn't refer to polarization, but to producing two simultaneous but opposite charges.
VOCs: Volatile Organic Compounds. A category of odiferous, carbon-based gasses, some of which are harmful to human health.
Particulates: Particulate matter pollution. A combination of solid and liquid droplets consisting of everything from smoke to dust to pollen. They can be quite dangerous to human health, depending on the size.
Types of Passive Purifiers
Passive purifiers work on the principle that air must be drawn through them in order to be treated. Because of this, occupants in the room have a chance to breathe in contaminants before they pass through the filter. Passive purifiers fall into six general categories.
Fibrous Filter Material
This is what most folks think of when they hear the term "air purifier" (unless, of course, they are familiar with ActivePure. Then *we* become the archetypal filter). As you might expect, fibrous filters push air through many densely woven fibers.
The most well known fibrous filters are HEPA (High Efficiency Particulate Air) filters. HEPA filters are usually made from fiberglass strands 1/75th the size of a human hair. These fibers catch particles in three different ways. They catch large particles simply by virtue of those particulates being too big to pass between the fibers. They catch small particulates because Brownian motion (the natural random movement of molecules in a fluid) pushes these small particulates into the fibers, where they stick. They catch medium-sized particulates because the fibers form narrow, twisting pathways that are too difficult for these speeding particulates to navigate.
Of these three sizes, medium particulates are the most difficult for the HEPA filters to catch. For this reason, in order to be classified as a True HEPA filter, the filter must remove "at least 99.97% of dust, pollen, mold, bacteria, and any airborne particulates with a size of 0.3 microns (µm)," per the EPA.
Fibrous filters also have a MERV (Minimum Efficiency Reporting Value) rating that measures what percentage of particles they catch between 0.3 and 10 microns. The EPA recommends that home filters have a MERV rating of at least 13, if the HVAC system can accommodate it. To qualify as a HEPA filter, however, the filter must be MERV 17 or higher.
Advantages: You might think because ActivePure is an active technology that we don't like HEPA filters. Nothing could be further from the truth; because of their positive impact in medium and large particulate removal, we put them in many of our own machines to supplement our proprietary technology. This has several advantages. First, it prevents your lungs from being the filter for these particles. Second, the filters also keep particles from building up on our ActivePure cells, which would reduce their efficacy.
HEPA filters are very effective at removing dust, pollen, and other allergens, as long as the fan pumping air through them is the correct size for the room.
Disadvantages: HEPA filters can have some challenges when it comes to removing ultra-small pathogens. When HEPA filters do catch viruses, they do it passively-that is, the virus must first pass through the filter. Viruses and bacteria may still remain active on the filter itself after being caught; (this doesn't necessarily mean they are likely to reinfect anyone, but it's still gross).
Fibrous filters do nothing at all against VOCs or other harmful gasses. They also need to be changed regularly.
The final disadvantage of fibrous filters is that the better they are at catching particles (i.e. the higher the MERV rating) the more power a fan needs to keep pushing air through them. Depending on your fan motor, this can raise your energy bill, especially in larger buildings with extensive HVAC systems.
Variation on this Technique: There is one other form of fibrous filter which is worth mentioning- the electret filter. This filter's fibers have an electrostatic charge (either naturally or because the fibers have been treated to give them such a charge), encouraging contaminants to stick; (think of how a balloon reacts to a sweater). This means the fibers don't need to be woven in an electret filter, increasing airflow and decreasing the required power. However, they still have many of the other advantages and disadvantages of HEPA filters.
Overall rating: HEPA is great at what it does, but it shouldn't be asked to do more.
Electrostatic Precipitation (ESP)
Not to be confused with the electret fibrous filter above, electrostatic precipitators run a high current through a series of electrodes (usually wires). A fan then draws air near these electrodes. When contaminants get close to the negatively charged electrodes, they also acquire a negative charge. The air is then drawn past a series of positively charged metal plates that attract the negatively changed contaminants out of the air.
Electrostatic precipitators are principally used to filter dust and exhaust from industrial processes, but home versions also exist.
Advantages: ESPs are excellent at removing particulate matter of almost all sizes. ESPs are also used to collect microbes for certain scientific studies, but this is not their primary application as a purification system.
Because the particulates catch on plates rather than gather on a filter, the fan doesn't have to work harder to pump the air through like it might for a highly-rated MERV filter. If an HVAC system is too old for a highly-rated MERV filter, ESP might be a cost-effective alternative to updating the entire HVAC system.
Disadvantages: The collecting plates need to be regularly cleaned, but this is a minor disadvantage.
The main disadvantage of ESPs is the ionization field requires a lot of power, which might negate any savings from the lower power draw of the fan. (One group of scientists recently developed an energy efficient version using a carbon brush ionizer, but it did not successfully collect many particulates. It was, however, quite effective at removing acetic acid, acetaldehyde, and ammonia under laboratory conditions.)
Some models of ESP can generate harmful gasses such as ozone and nitrous oxide. You may remember the Ionic Breeze ESP filters from Sharper Image in the 2000's and the performance and health concerns associated with those products.
As with all passive purifiers, ESPs require air in the room to pass through their mechanism in order to be treated.
One variation on this technique: Polarized media filters work on the same principle as electrostatic precipitators in that they run a constant electrical charge. However, instead of using metal plates to collect ionized contaminants, polarized media filters run an electrical charge through the filter screen itself, polarizing it to attract contaminants. This makes polarized media, mechanically speaking, similar to electret filters in the previous section. The main difference is that polarized media require live current.
Overall rating: Interesting and effective, but HEPA is simpler if you have the fan power.
When air is given a little electrical charge, it begins to ionize as electrons are pushed one way and protons another. When the air is given a LOT of electrical charge though, electrons and protons begin to flow freely in opposite directions, creating a plasma. (Fun fact, this is actually what lighting is-an electrical arc flowing freely through air that has been turned into a plasma.) Plasma purifiers draw air through this electrical arc, breaking chemical bonds.
Advantages: Plasma purifiers appear to be effective at removing existing gaseous pollutants. As they do not contain physical filters, it is safe to assume they do not clog easily. (Also, it's just kind of cool to think of contaminants being zapped into oblivion by miniature lighting bolts.)
Disadvantages: Plasma purifiers are relatively new. Per the EPA's guide, further study will be required to see how effective they are at removing particulates. They also can produce harmful byproducts such as ozone, carbon monoxide, and formaldehyde. Power efficiency is also something you should vet in reviewing plasma purifiers, due to the fact that they are quite literally lighting in a box.
Overall rating: A fascinating concept, but still passive and of limited application.
Note the category is not called "absorbent media." The difference between adsorbent and absorbent is rather technical, but essentially adsorbent materials allow molecules to stick to their surfaces without swelling overmuch. An air purifier with adsorbent media is principally used to remove gasses and odors.
The most common form of adsorbent media is activated carbon. You might see activated carbon (often called "activated charcoal") advertised as a cure-all for everything from yellow teeth to cholesterol. Activated charcoal does have its medical uses but not as many as are advertised. However, activated carbon's use as an air and water purifier is well established.
Activated carbon is created by treating a carbon source (such as coal, coconut, or bamboo) in such a way as to create many little pores throughout the medium. These pores increase the surface area of the carbon; since adsorbent media work by letting gasses stick to their surface, more surface area means more gas is adsorbed.
Advantages: Adsorbent media work well to purify the air of gas and odors, though their effectiveness varies by type and concentration of gas. ActivePure uses activated carbon in some of its devices as a supplement to its own anti-VOC technology, especially in units designed to address cigarette and wildfire smoke contamination.
Disadvantages: Adsorbent filters need to be changed regularly. Also, purifiers with only a small amount of carbon may be insufficient for the size of the room. Like all passive purifiers, adsorbent media requires air to pass through it in order to be treated.
The most unique disadvantage of adsorbent media is the captured gas can be rereleased into the air. This is caused by an effect called the concentration gradient driving force; if left to its own devices, any contaminant suspended in a fluid will move around until diffusion is equal. This means if there is a greater concentration of contaminants on the filter than in the air, natural forces could move these contaminants back into the air, especially during changes in temperature or humidity.
Variation on this technique: There is also a type of medium used known as a chemisorbant medium. In this case, the filter is treated with a compound to chemically (rather than mechanically) bind gasses to it. This removes the possibility of gases being released from the filter.
Overall rating: Great at what it does, and not great at what it doesn't do.
Ultraviolet Germicidal Irradiation (UVGI) (Passive Methods)
Ultraviolet light (aka ultraviolet radiation aka UV) can be used in both passive and active purification methods. Here, we will be referring to the passive method.
Ultraviolet light is the part of the electromagnetic spectrum just outside the range of human vision. UV light is classified into UVA, UVB, and UVC. UVA and UVB have longer wavelengths and are responsible for sunburn and skin aging. UVC has a shorter wavelength that is normally blocked by the ozone layer before it can reach earth's surface. The shorter the UV wavelength (i.e. the further it is from visible light) the better it is at damaging cells (including harmful pathogens) but the less deeply it penetrates. Most air purifiers emit UVC light, but some make use of UVA or broad-spectrum UV.
Passive UV purifiers work in one of two ways. The first uses a portable purifier or the existing HVAC system to pump air past a UV light. The second method is called Upper Room Germicidal UV, which shines partially-shielded UV bulbs towards the ceiling and waits for the natural air currents in the room to move air past the bulbs. Both these methods theoretically sterilize the air, freeing it from bacteria and viruses.
Advantages: With proper exposure time, UV units can be effective at neutralizing pathogens.
Disadvantages: UVGI, when built into an HVAC system, has many challenges that can negate its efficacy. First and foremost, air in an HVAC system is moving very quickly. Too quickly, in fact, to neutralize most pathogens. For instance, it takes a purifier's UV bulbs 3-14 seconds to eliminate the COVID-19 virus. Meanwhile, air in an HVAC system is moving at 400-1800 feet per minute. At the slowest speed, microbes would be exposed for around 0.15 seconds rather than the requisite three.
Upper room UV does eliminate the disadvantage of HVAC UV, but it's still a passive system. One has to wait for the pathogens to move upward towards the ceiling before they are neutralized, leaving plenty of time for people to breathe them in first.
If a UV bulb is not coated with the right catalyst, it can produce ozone. Ozone is wonderful in the upper atmosphere but not healthy close to the ground where humans can breathe it in, (which will be discussed further in the ozone section).
UVGI works quickly with more powerful bulbs, but more powerful bulbs increase energy costs. These bulbs also need to be regularly replaced.
If the UV purifier's shield is damaged, it could expose occupants of a room to eye injury or skin damage.
UVGI does nothing against dust, particulate matter, or harmful gasses.
Overall rating: Depending on the contaminant types you are concerned about, you can find a better passive or active solution.
Thermodynamic Sterilization (TSS)
This is the proprietary technology of a single competitor of ours, though similar purifiers are currently in development by others. We're mentioning it because we want you to be thoroughly informed of your options.
TSS heats air in a chamber to 200°C to literally sterilize it. According to the company this "destroys mold, dust mites, bacteria, viruses, pollens, pet dender [sic], tobacco and other organic allergens."
Advantages: TSS doesn't use fans but lets air pass through it via convection; this keeps it quiet, and keeps the power draw low. (Okay, even we'll admit that's a hot idea.) We also assume this slows down the speed at which air passes through it, allowing bacteria, viruses, and mold to be exposed for longer.
Disadvantages: Please note that as only one other company uses this technology, data on the method is limited. We can't see how this method would remove particulates (such as allergens and tobacco smoke) as the company claims. It seems to us that burning up particulates might create smaller particulates, which are even more dangerous than larger ones. However, we are open to seeing studies that show otherwise.
Even if we are wrong about the particulates, it's still a passive method, albeit a particularly fiery one.
Overall rating: Insufficient data for a meaningful answer.
Types of Active Purifiers
Active purifiers go head-to-head with contaminants on their home turf. Unlike passive purifiers, which require air to pass through their mechanism, active purifiers aim to treat the air before people breath in contaminants. For this reason, we believe some active purifiers are superior to most passive purifiers. Active purifiers fall into five general categories.
Ultraviolet Light (Active Methods)
We spoke about passive UV methods in the previous section. The active method of UV purification is called whole room UV, which is exactly what it sounds like; an entire (unoccupied room) is exposed to a UV light. This method is most frequently seen in hospitals.
Advantages: Active UV is excellent for treating hospital rooms between patients. It also allows the pathogens to be exposed long enough to be neutralized, unlike passive UV.
Disadvantages: Whole room UV cannot treat pathogens in shadows. It also cannot be used in occupied rooms, allowing for surfaces and air to be reinfected. As with passive UV methods, ozone can be produced by improperly coated UV bulbs.
Another variation on this method: Far-UVC is a type of purifier that uses a section of the electromagnetic spectrum with such a short wavelength that it theoretically can't penetrate living human cells (only the outermost layer of dead skin). This would allow far-UV to treat a room actively while said room is currently occupied. This technology shows great promise, but both its effectiveness and its safety are still being studied.
Overall rating: Whole room UV is effective at sterilizing surfaces, but other methods are better at air purification. Far-UV shows promise but is still in its infancy; this leaves concerning gaps in our knowledge of possible side effects.
Intentional Ozone Generation
We mentioned that some of the methods above could generate ozone as an unintentional byproduct. Well, this method used UV or electrical discharge to intentionally generate ozone in *unoccupied* rooms.
Most oxygen is O2, meaning two oxygen molecules bonded together. Ozone's chemical formula is O3, and it is very unstable. It desperately wants to slough off its extra oxygen atom onto something else and go back to being O2 again. When ozone gives its extra oxygen molecule away to another molecule, that molecule oxidizes, changing its chemical composition.
Ozone is most commonly used in buildings that have experienced fire, smoke, or flood damage.
Advantages: Ozone is very good at binding to odor molecules, and thus at eliminating the odors. It also can neutralize bacteria, mold and viruses, including the virus that causes COVID-19.
Disadvantages: Ozone can kill houseplants; it also irritates the lungs of pets and humans. It also may cause more serious problems such as low birth weight or damage to the nervous system. For this reason, ozone generation cannot be used in occupied rooms and thus cannot provide ongoing protection. It is also difficult for consumers to manage the exact amount of ozone their device generates.
Ozone does nothing to eliminate pathogens or particulates.
Overall rating: We recommend staying away from this one unless you are a professional trained in using it under the proper risk-management protocols.
Active ionizers use a carbon filter brush or electrode to charge air molecules, generating ions. You'll notice this is almost identical to the electrostatic precipitators mentioned above. The difference with active ionizers is that these ions are sent out into the room (either with the natural air currents or via a fan). Rather than waiting for contaminants to come to them, these ions cling to particulates in the air, charging them. These charged particulates become attracted to other particulates, clumping together to form larger particulates. These new super-particulates are too massive to remain airborne, and they fall to surfaces in the room. Alternatively, the charged particulates pass near walls or surfaces and cling to them; (once again, the way a balloon interacts with a sweater).
Advantages: The main advantage of ionization is that it is active. It treats the air in the room, rather than waiting for the air to come to the purifier. It works on allergens and particulates, and even has some ability to react to pathogens. Fanless ionizers also have the advantage of being quiet.
Disadvantages: The main disadvantage of ionization is that it does nothing for surfaces; in fact, it makes surfaces worse, since surfaces become the reservoir onto which ionizers deposit contaminants. These contaminants can be stirred back into the air easily. Ionization also does nothing to cleanse the air of VOCs or other gasses. Ionizers may produce ozone as an unintentional byproduct.
One final note: when particulates are charged, they'll stick to anything, perhaps including your lungs. In fact, ionized particulates may stick to the lungs at a much higher rate than non-ionized particulates, which rather defeats the purpose of an air purifier.
Two variations on this method: You will also see something called bipolar ionization or needlepoint bipolar ionization on the market. Instead of generating ions that are either positive or negative, bipolar ionization uses ultrathin electrodes (the needlepoints of their namesake) to produce negatively charged hydroxyl ions (OH-) and positively charged Hydrogen Ions (H+). Instead of clumping particulates together, hydroxyl ions react with viruses, bacteria, and VOCs, neutralizing them directly.
Needlepoint bipolar ionizers, however, need regular cleaning due to the fact that the ultrafine electrodes burn the carbon brush in their mechanism. (Some do have wipers that automatically unclog the electrodes.) As always, exposed electrodes have the chance to produce ozone.
A variation on this variation is Dielectric Barrier Discharge Bi-Polar Ionization, which encloses the electrode inside a tube. The electric charge tries to jump out of the tube but can't, thus ionizing the air around it.
The EPA seems to consider bipolar ionization an insufficiently vetted technology.
Note: Some companies also use the term bipolar ionization to refer to machines which produce a dipolar hydroxyl radical (HO•). This is not technically ionization but polarization. In other words, the hydroxyl radical does not gain charge from an extra electron but instead is polarized by the loss of an entire hydrogen atom. The hydroxyl radical will be discussed further in the upcoming section.
Overall rating: Traditional ionizers have more disadvantages and risks than advantages. Bipolar ionizers are pretty cool.
Photocatalytic Oxidation (PCO)
This technology is ActivePure's granddaddy...i.e. it is the technology that led to our technology. Because of this, there will be a lot of overlap in how both techs work. Before we can explain either though, we need to talk about the great-grandaddy of both our own technology and PCO-the sun.
When the sun hits the earth's atmosphere, it forms what are known as reactive oxygen species. These are exactly what they sound like-unstable, oxygen-based molecules that easily react with other molecules. The most important of these is the hydroxyl radical (not to be confused with the aforementioned hydroxyl ion); hydroxyl radicals form when sunlight knocks away one of water's hydrogen atoms, leaving a reactive molecule with the formula HO•. (Hydroxyl can also occur when ozone grabs an oxygen atom away from water.) Hydroxyl desperately wants to stabilize itself by grabbing another atom. It can grab its own atom back again, it can grab another hydroxyl and form hydrogen peroxide, or it can grab carbon or hydrogen from contaminants in the air. This later reaction is the one we are interested in. Hydroxyls pull apart methane, benzene, and other carbon-based gasses. They also grab atoms from the outer shells of viruses, bacteria, and fungi, rendering them inert.
PCO is an early version of technology designed to mimic this outdoor process indoors. A shielded UV bulb shines on a catalyst (usually titanium dioxide), generating hydroxyls and other reactive oxygen species. These molecules are blown through the room by a fan (or allowed to diffuse through the room via natural air currents), deactivating viruses, bacteria, and mold, and removing odors and VOCs.
Advantages: PCO is fairly successful at quickly eliminating viruses, bacteria, mold spores, and VOCs. As it is an active technology, this purification occurs throughout the room, rather than just inside the purifier.
Disadvantages: Early PCO tends to produce undesirable byproducts such as ozone (from the UV bulb) or formaldehyde (possibly from the incomplete breakdown of VOCs or methane). PCO does nothing against dust, allergens, or particulates. UV bulbs also need to be occasionally changed, and the catalyst has a finite lifespan.
Three variations on this technique:
One very interesting and energy-efficient idea for room purification uses certain types of titanium dioxide paint designed to encourage PCO. Hydroxyls then form when sunlight hits the paint. However, this can produce the same undesirable byproducts.
A second variation of PCO involves using a different catalyst to generate hydrogen peroxide rather than hydroxyl and superoxide. This technique is called dry hydrogen peroxide. However, hydroxyl is far more effective at breaking down viruses in the air than hydrogen peroxide.
Some of our competitors use something called Photo Electrochemical Oxidation (PECO), which confines the reactive oxygen species to a treatment chamber rather than sending them throughout the room. This reduces the active method of PCO to a passive method, negating many of PCOs advantages.
Overall rating: PCO is old news. Advanced Photocatalysis is where it's at.
At last, we come to it-the grand finale! Advanced photocatalysis works on the same principle as PCO, but developments in the composition of the catalyst, the coating of the bulb, and the cell shape drastically reduced the byproducts generated. Thus a star was born, and that star is ActivePure.
Advantages: Advanced photocatalysis purifiers are extraordinary at reducing pathogens in the air. Study after study show these purifiers reduce 99.99% of viruses, bacteria and mold in minutes. Under laboratory conditions, it reduced 99.9% of the virus that causes COVID-19 in three minutes. This is one of the reasons the ActivePure Medical Guardian is a Class II Medical Device. The ActivePure Medical Guardian is proven to reduce the following by 99.9% in 30 minutes or fewer:
MS2 bacteriophage RNA virus, which is a non-enveloped virus and thus much harder to inactivate than SARS-CoV-2, whose envelope is more vulnerable.
PHI-X174 bacteriophage DNA virus, a surrogate for Hepatitis C and HIV.
Staphylococcus epidermidis gram-positive bacteria, surrogate for Methicillin-Resistant
Staphylococcus Aureus (MRSA), a major cause of hospital-acquired infections.
Erwinia herbicola gram-negative bacteria, surrogate for the bacteria which causes black plague.
Aspergillus niger fungal mold, the cause of Aspergillosis.
Bacillus globigii bacterial endospore, which is generally considered among the most difficult pathogens to neutralize. ActivePure Technology reduced concentrations by 99.98% in 30 minutes.
Though the Medical Guardian's advanced photocatalysis is supplemented with a HEPA filter, it is essential to remember that this reduction isn't merely occurring when the air passes through a filter. It's cleaning the air while it's in the room.
To quote the late, great Billy Mays, "But wait, there's more!" Advanced photocatalysis may reduce bacteria, fungi, and viruses on surfaces as well. The ActivePure Medical Guardian, for instance, reduced COVID-19 on surfaces by 93.27% in three hours under laboratory conditions.
Disadvantages: Alas, nothing in life is forever. Eventually, UV bulbs do need to be replaced, while the cell reagent coating degrades through use. Considering the peace of mind provided by knowing you have the very best though, this is more of a molehill than a mountain.
Advanced Photocatalysis doesn't reduce particulates or pet dander. For this reason, HEPA plus Advanced Photocatalysis is a truly unbeatable combination, covering all the bases with almost none of the disadvantages. For this reason, we pair many of our active machines with passive HEPA filters.
Overall rating: Ooooh yaaaah!
Some Final Words
When it comes to pathogens, passive purifiers alone just can't cut it. Advanced photocatalysis allows for 24/7 protection of the air we all breathe, making it far superior in a post-pandemic world.
That doesn't mean passive technology doesn't have its uses though. We recommend all consumers educate themselves on the contaminants and pathogens they are most concerned with and then apply the best technology (or combination of technologies) to address those concerns.
We hope this summary of purification technology was informative and engaging. We also hope it will assist you in choosing the best purifier for your home or business. Once you've made a decision, contact your local ActivePure reseller. You may also contact us directly at www.ActivePure.com, and our team will help connect you.
Note on Sources
We are indebted to the EPA's guide "Residential Air Cleaners: A Technical Summary" as an extremely helpful resource for refreshing our memories regarding the details of many technologies.
We are also indebted to Medstart Direct's two industry white papers. "Overview of Air Purification and Disinfection Technologies to Create Cleaner Air in Indoor Occupied Spaces, with a Focus on Pathogen Neutralization in Light of the COVID-19 Pandemic" and "Fighting Viral Spread with Air Purification Technology."
If a specific fact has no other citation, it is from one of these three sources.