Updated: Sep 23, 2020
What follows is a reality check. It's about the various kinds of products that claim to kill infectious microorganisms (bacteria, viruses, fungi, spores, etc.) when you add them to an HVAC system.
Some of these technologies – the good – really can work quite well – that's can work, and you'll see in a minute when to be wary. Some of them – the bad – do work but can harm people and pets. Some of them – the bogus – have no significant impact on infectious microorganisms but can impact a homeowner's bank balance.
For fairness, these are in alphabetical order.
Electrostatic or ionic cleaners
Electrostatic – sometimes called ionic – air cleaners insert into HVAC ventilation, usually between the return duct and the air handler. These are very effective for dealing with dust and small particulates, causing them to be attracted to a charged wire or plate, which removes them from the airflow. They also eliminate the need for replaceable air filters.
The EPA identifies ESP (electrostatic precipitation), ionizers and ion generators as similar technologies that target particulates and not microbes.
Note: There's also some opportunity for confusion because commercial disinfection electrostatic spray nozzles charge the spray droplets, which makes them cling to surfaces. These are not HVAC-attached tech; they are used by protectively dressed decontamination crews under special circumstances, and some of the various liquids that may be sprayed are harmful to humans and pets while airborne.
If you recognize the pedigree of an electrostatic or ionic system, or if they publish test specifications on the very low levels of ozone they generate, these are an interesting addition to indoor air treatment, even if they are not fully effective at microbial disinfection.
But there are some bogus to bad offerings out there, as the EPA warns:
Many electronic air cleaner devices — including portable and duct-mounted ESPs, ionizers or ion generators… can generate high amounts of ozone… a well-documented lung irritant… that should not be used in occupied spaces.
Filters (fibrous media) including HEPA
Every HVAC system needs some type of air filter, either electrostatic or fibrous.
Fibrous air filters with a MERV 13 rating may trap a lot of smaller particles without adding so much airflow resistance that they overload a system; an HVAC pro's advice is necessary when adding filters rated MERV 16 or higher or HEPA filters to a system.
While specific HEPA filters have a small-enough porosity to stop microorganisms, these add so much airflow resistance that they may require a total HVAC system replacement; also, smaller pores are more quickly blocked, requiring more frequent replacement of the filters.
The EPA identifies fibrous filter media as a technology that targets particulates and not as a technology that targets microbes.
Hydrogen peroxide, hydro-peroxide plasma, peroxygen
This can be very harmful to humans, especially since it can be made to sound beneficial and because many people may be confused or duped by surrounding statements. These attachments go by many names but all, at some level, involve hydrogen peroxide.
In sealed, enclosed chambers with no humans present, vaporous hydrogen peroxide can kill bacteria and viruses and other microorganisms if concentrated enough and if there's a long enough exposure to it.
Hydrogen peroxide is toxic. It can irritate skin, eyes and mucous membranes even at very low concentrations; at concentrations as little as 75 parts per million (per US NIOSH) it is immediately dangerous to life and health.
Inhaling airborne hydrogen peroxide can also narrow the airways and cause spasms of the vocal cords, making breathing difficult, possibly requiring a ventilator or breathing tube.
Also, most processes that create hydrogen peroxide (or one of its kissing-cousin aliases) also create ozone (H2O and O2 and O2 become H2O2 and O3).
PCO – photocatalytic oxidation
In PCO systems, the high energy of UV light on the broad surface of an in-airstream plate coated with titanium dioxide reacts with – and chemically transforms – gases in the airstream.
The EPA warns:
PCO air cleaners have been shown to generate formaldehyde, acetaldehyde, nitrogen dioxide, and carbon monoxide.
The EPA identifies PCO as a technology that targets gases and not as a technology that targets microbes.
UVC – the EPA calls it UVGI (for ultraviolet germicidal radiation)
Unlike the more familiar day-at-the-beach UVA and UVB "soft" ultraviolet rays that can eventually mean a sunburn, the hard and deep rays of shorter-wave UVC aren't something that reaches the earth from natural sources (because the atmosphere blocks them). But they can, so to speak, burn microorganisms to death. Here's what the EPA has to say:
Given sufficient exposure time and lamp power, UV light can penetrate the outer structure of a microorganism’s cells and alter its DNA, preventing replication and causing cell death. But lighting power must be high, and exposure time must be long
The EPA identifies UVGI as a technology that targets microbes. Here's more that the EPA has to say about UVGI:
· Can be effective at high intensity with sufficient contact time
· Can be used to inactivate microbes on cooling coils and other surfaces
· Effectiveness increases with lamp intensity
They have more to say about intensity, including a note that it "is typically low in residential UVGI air cleaners"
Beware of bogus credentials! Just because something says UV doesn't mean that it puts out hard, infection-lethal UVC rays. And even if it does emit UVC, it may not emit it with enough intensity to be effective. You can't ask a flashlight to do the work of a lighthouse.
Be suspicious of references to reflections of UVC, since almost no reflections retain any germicidal abilities. Be wary of claims involving LED emitters, since their state of the art does not permit germicidal illumination levels at market-acceptable pricing.
The best approaches to effective UVC disinfection appear to involve clusters of long, rod-like mercury vapor gas discharge tubes that can collectively provide sufficient intensity to deactivate microorganisms.
Links to credible info about HVAC-attached disinfection tech
CDC Hydrogen Peroxide Gas Plasma guideline specifies an enclosed chamber and length exposure
Limits on peroxygen germicidal agents
Effectiveness of ultraviolet radiation using mercury vapor lamps
Hydrogen peroxide chemical irritation and need for monitoring
Ionizing air cleaners and ozone hazards
EPA Residential air cleaners (an excellent 74-page overall guide)
COVID-19 and air filtration
EPA: COVID-19 and air cleaners/purifiers
WebMD: Coronavirus Puts UV in the Disinfectant Spotlight
UVC reduces CO and VOCs