Air Purifiers With UV Lights: Why UV-C Filter Alone Is Not Enough?

During the epidemic, air purifiers with UV lights have become wildly popular. It is well-known that UV-C lights are used to sterilize viruses and are used as disinfection agents. But does that necessarily mean that UV air purifiers eliminate viruses as well? It’s not all that simple.

The use of ultraviolet germicidal irradiation (UVGI) in air purifiers is a matter of some debate. Namely, short-wavelength ultraviolet light (UV-C) ranging from 100 nm and 280 nm is often used to inactivate microorganisms by disrupting DNA. However, the amount of UV-C needed and the time of contact needed for germicidal action is something air purifiers cannot seem to provide; and with a very good reason (health considerations).

ultraviolet light spectrum
UV-C light is a short-wave ultraviolet light with wavelength ranging from 100 nanometers to 280 nanometers.

To explain this, let’s look at this statement by Environmental Protection Agency (EPA) on UV-C filters:

“UVGI cleaners use ultraviolet radiation from UV lamps that may destroy biological pollutants such as viruses, bacteria, allergens, and molds that are airborne or growing on HVAC surfaces (e.g., found on cooling coils, drain pans, or ductwork). If used, they should be applied with, but not as a replacement for, filtration systems.” (EPA’s Guide to Air Cleaners, July 2014)

As we can see, UV-C filters alone are not sufficient to adequately deal with bacteria and viruses. Air purifiers with UV-C filters have to also use so-called filtration systems; this translates to HEPA filters in most cases.

Another tidbit of information on combinations of UVC filters and HEPA filters was published in PLoS ONE Journal by Ashley Hammond et al.:

“The experimentally contaminated air was filtered using a portable device which combined HEPA filtration and UV light. After a 45-minute run, viable levels of tested bacteria in the air were reduced by >99%, and surface contamination was reduced by at least 87%. The combination of UV light alongside HEPA filters ensures the captured organisms are killed.”

As we can see from these scientific sources, the best air purifiers will combine a UV light filter with a HEPA filter.

Now, in the air purifier industry, we have a lot of air purifiers with UV-C light filters. However, as we see, a UV-C filter is not enough; a good HEPA filter is essential in order for a UV air purifier to work. With this insight, we can pretty much categorize UV air purifier produces into two big categories:

Good And Bad UV Air Purifiers: Essential Differences

During the evolution of air purifiers, some brands vent directly with UV-C filters. Others, however, started off by designing air purifiers with good HEPA filters and added UV-C filtration only later on.

Given that studies show UV-C filter is a good addition to a HEPA air purifier, and not the other way around, we can divide air purifiers into two groups:

  • Bad Air Purifiers With UV Filters. These are brands that produce UV-first air purifiers. The UV-C filter is the primary way how to capture dust, pollen, and smoke, and eliminate bacteria and viruses. HEPA filters only play a secondary role here; these are usually your standard HEPA filters. Example: GermGuardian air purifiers.
  • Good Air Purifiers With UV Filters. These brands made an effort to produce air purifiers with the best HEPA filters first (HEPA-first air purifiers). Only after the HEPA filtration was taken care of, they decided to add a UV-C filter for additional microorganism removal. Example: Medify air purifiers.

Here’s the main difference between bad and good UV air purifiers:

Bad UV air purifiers count on UV filters to do most of the work. Good UV air purifiers count on HEPA filters to do most of the work.

The reason why HEPA-first air purifiers with UV filters are much better than UV-first air purifiers with HEPA filters is simple:

  • Capacity of UV-C light filters is very limited. Ultraviolet filters can eliminate a very small amount of microorganisms that penetrate HEPA filters.
  • Capacity of HEPA filters is almost limitless. All HEPA-classified filters with an ASHRAE certification will remove at least 99.97% of 0.3-micrometer air particles. That means that a UV filter positioned behind a true HEPA filter only has to trap or inactivate 30 out of 1,000,000 incoming air contaminants. In other words, good HEPA filters have a penetration rate of 0.03%.

HEPA filter penetration rate – percentage of air pollutants like dust, smoke, bacteria, viruses, mold, and so on, that HEPA filters cannot capture – is essential here.

The more air particles a HEPA filter can capture, the fewer will be left for the UV-C filter to handle. It is also harmless to increase the quality of HEPA filters. On the other hand, using higher-voltage UV-C light can create too much ozone which may be harmful.

That’s why the best air purifier with UV filters are the ones that have:

  • Low-voltage UV-C filters.
  • Best HEPA filters. HEPA filters can include H10, H11, H12, H13, and, added until quite recently by Medify, H14 HEPA filters.

Most air purifier brands on the market will either have high-voltage UV-C filters EPA advises against or low-voltage UV-C filters with standard H10, H11, or H12 HEPA filters (H13 HEPA are already quite rare).

The only air purifier brand that uses the best combination – low-voltage UV-C filters with H14 HEPA filters – is Medify. The best current example of what a perfect UV air purifier should look like is the Medify MA-112 air purifier with optional UV light + H14 HEPA filters.

best air purifier with uv filter
Medify MA-112 study for air particulate reduction.

The key here is the quality of HEPA filters:

Specifically, the H14 HEPA filters that Medify recently start employing can remove more than 99.99% of 0.3-micrometer most penetrating particle size (MPPS) air pollutants. That means that the penetration level is not 0.03% as with most HEPA filters; it is only 0.01%.

hepa filter and uv-c filter
Currently the best UV + HEPA filter combination.

0.01% penetration rate means that if an air purifier sucks in 1,000,000 air particles, the H14 HEPA filter will capture 999,999,990 of them. What is left for the UV-C filter (with limited capacity as it should have) is only 10 particles. That’s how you can get the purest possible air and the highest indoor air quality (IAQ).

As we have seen, you need to be a bit careful when selecting the air purifiers with UVC filters. If these filters are too strong (high-voltage), they can create an excess of ozone that may be harmful.

That’s why it is a lot smarter to use a low-voltage UV filter in combination with the best HEPA filters. It is up to the HEPA filter (with almost limitless capacity) to remove almost all air pollutants from indoor air. UV-C filter is there only to finish the job completely. This is how you can ensure that you will have a UV air purifier that produces the highest quality air for breathing.

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