A Layered Approach To Clean Air

In the pursuit of improved indoor air quality and infection mitigation, combining UVC and MERV 8 filters exceeds MERV 13 pathogen removal performance.

By Joe Kalman


Many recommended HVAC-oriented infection mitigation strategies include increased ventilation, upgraded filter efficiency, and ultraviolet energy (UV-C), often in a layered approach. The underlying engineering control strategy herein is to dilute (ventilation), remove (filter) or inactivate (UVC) pathogen concentrations in buildings. To help prevent the airborne transmission of the SARSCOV2 virus, ASHRAE and the Centers for Disease Control and Prevention (CDC) recommend upgrading HVAC air filters to at least a MERV 13 rating.1

Not all air handlers or packaged rooftop HVAC systems, however, can handle the static pressure commonly associated with high-efficiency MERV 13 or HEPA filters.

The CDC noted this practical reality in its ventilation guidance, calling for facilities to “increase air filtration [levels] to as high as possible without significantly reducing design airflow.”

And we know that matching filters to individual air handlers is paramount for efficient HVAC function. For example, after examining the HVAC systems at 17 federal buildings, the General Services Administration’s inspector general found that nearly half could not accommodate high-efficiency air filters, reporting that trying to use them “would reduce airflow and potentially cause the systems to fail.”2

Fortunately, there is an easy-to-implement alternative for the millions of aging HVAC systems that fall into this category.

Through a layered approach, facility engineers can combine medium-efficiency filters (MERV 8) with UVC or shortwave energy in the ultraviolet-C spectrum to exceed MERV 13-level efficiency rates without sacrificing airflow or compromising cooling capacity.

Complementing Filter Performance

Besides source control and increased ventilation, UVC has been shown to complement filter operation. It’s important to note that UVC destroys the infectious material within the aerosol-based particles3,4; although they can remain in the air, they are no longer infectious.

Blending UVC lamps with filtration can increase the overall system’s pathogen removal rate since the MERV 8 filter captures the larger containments, and the UVC inactivates the smaller, more susceptible pathogens (See Figure 1).


  • TOPIC: Most HVAC-oriented infection mitigation strategies for COVID 19 have recommended upgrading HVAC filters to the equivalent of a MERV 13.
  • ISSUE: Some legacy HVAC systems are not mechanically capable of operating with the pressure drop caused by high-efficiency filters.
  • IMPLICATIONS: Third-party research that shows facility engineers can exceed a 99% single-pass removal/inactivation rate by marrying germicidal UVC with a MERV 8 filter.
    In other words, facility engineers can combine medium-efficiency HVAC air filters with UVC fixtures to exceed CDC and ASHRAE recommended pathogen removal efficiency without sacrificing airflow or compromising cooling capacity.

For example, fungal and bacterial spores, which are less susceptible to UVC, are typically large (1-10 microns or more) and are, therefore, more easily removed by a MERV 8 or higher filter.5 On the other hand, viruses and vegetative bacteria, which are smaller (0.1 – 1 microns), can penetrate filters and are often highly susceptible to UVC. Therefore, combining filtration and UVC systems offers an ideal solution to address the full complement of microorganisms found in indoor environments.

In fact, the ASHRAE Epidemic Task Force (ETF) endorses flexibility in meeting its standard “air-cleaning technologies can be combined to produce the desired MERV 13-equivalent level of air cleaning.”6

UVC doses are calculated by determining the amount of short-wavelength ultraviolet energy a microorganism receives or the germicidal “residence time.” The dose or UVC fluence is a function of UVC intensity and the length of exposure time typically expressed as Joules per square meter (J/m2).

HVAC UVC air disinfection systems are commonly designed for at least 0.25 seconds of UVC exposure (assuming airflow is 500 fpm with 24 inches of in-line depth). The higher the air velocity or shorter the in-line depth, the higher the UVC output (dosage) must be to achieve the desired inactivation rate.

The effectiveness of UVC disinfection on various microorganisms has been well documented in the literature.7 It is reported as a microorganism-specific dose at which 90% of the microorganism is inactivated (expressed as D90). The average D90 UV dose for airstreams with normal relative humidity is approximately 6 J/m2 for both viruses and vegetative bacteria. You would typically apply this dose for a coil cleaning/ maintenance application. Note the D90 rate for SARSCOV2 virus is 6.11 J/m2.8

Pathogen Removal Performance
FIGURE 1: Microorganisms absorb UVC energy at different rates. Viruses, for example, are more susceptible to UVC inactivation, including Influenza, Measles, SARS, and Smallpox. Data Source: ASHRAE 2019 Handbook- HVAC Applications Ch. 62.

Combining Air Cleaning Options

According to ASHRAE, the MERV 8 and UVC pairing is not only less disruptive to existing HVAC equipment, but this combined approach can equal or surpass the performance of a MERV 13-filter-only configuration.

Research has shown “that the particle size of the SARSCOV2 virus is around 0.1 µm (micrometer). However, the virus does not travel through the air by itself. Since COVID-19 is human-generated, the virus is trapped in respiratory droplets and droplet nuclei (dried respiratory droplets) that are predominantly 1 µm in size and larger.”9 ASHRAE notes a MERV 13 and a MERV 8 filter are at least 85% and 20% effective, respectively, at capturing particles in the 1 µm to 3 µm size range, as reflected in Figure 2.

FIGURE 2: Models of filter performance for MERV 8 – 16 filters based on manufacturers’ data extended below 0.1 microns. Data Source: Kowalski, W., 2009, Ultraviolet Germicidal Irradiation Handbook (10), Figure 8.5.


Thus, the ASHRAE-recommended MERV 13 filters are significantly more efficient at capturing 1 µm to 3 µm particles than a typical MERV 8 filter.

The overall performance of air filters can be characterized by using the filter performance curves in Figure 2. The performance of UVC systems can be characterized by the average UV dose imparted to the airstream. In this case, we evaluate the performance of the UVC system against the SARSCOV2 coronavirus, which has a D90 dose of 6.11J/m2.11 D90 is the amount of UV-C light needed to inactivate 90% of the selected virus. Again, this dose is what you would typically apply for a UV-C coil cleaning/maintenance application.

One indicator of airstream disinfection performance is a decrease in the concentration of airborne microbes. The reduction rate using a UV system could, for example, be compared to that achieved by ventilation with clean air alone, as opposed to recirculated air cleaning.

A computer model was conducted Wladyslaw Kowalski12 on behalf of UV Resources to measure the average removal rates among three air cleaning scenarios:

    1. a MERV 8-filter-only,
    2. a MERV 8 filter combined with a typical UVC cooling coil system producing a UV dose of 3.3 J/m2 (roughly 7-10 watts sq./ft. and represented by Figure 5) and a
    3. MERV 13-filter-only.

The typical UVC cooling coil system is designed to operate at 55 F° and 500 fpm, and these conditions impose a 60% reduction of UVC output due to windchill effects (cold air reduces the output of UVC lamps) and a 15% lamp lifetime depreciation calculation, which makes this a conservative calculation. The UVC inactivation rate is based on the SARSCOV2 virus published diameter of 0.113 µm and UV rate constant per Walker.13

The computer modeling results shown in Figure 3 suggest that a MERV 13 filter alone can capture 85% of the aerosolized SARSCOV2 virus. In comparison, the model found that a MERV 8 filter coupled with a 3.3 J/m2 dose of UVC could yield a 78% combined removal/inactivation rate.

Pathogen Removal Performance, MERV Filters
Figure 3: Computer modeling comparison of SARSCOV2 removal rates by a MERV 8 and MERV 13 filter, as well as a MERV 8 filter combined with UVC producing 3.3 J/m2.


Calculations show that the layering of UVC disinfection and a medium efficiency MERV 8 filter can remove nearly the same level of pathogens as the MERV 13 benchmark. Said differently, the 78% removal rate modeled for 1 μm to 3 μm particles using UVC combined with a MERV 8 filter can attain 92% of the desired ASHRAE-recommended MERV 13 filter performance (78% ÷ 85% = 92%).

To validate this initial computer modeling, we commissioned an independent third-party efficacy test on an aerosolized SARSCOV2 virus in a BSL-3 laboratory using an ANSI/ASHRAE 185.12020 sealed bioaerosol test chamber.

Laboratory results demonstrate that the layering of air cleaning technologies exceeded the original computer modeling forecast which approximated pathogen removal near the MERV 13 objective.

As seen in the test result, Figure 4, the UVC together with the MERV 8 prefilter exceeded a 99% average single-pass reduction. In other words, the independent test shows that combining UVC and a MERV 8 filter can meet or exceed the pathogen removal performance of a MERV 13 filter.

Based on the test results, building engineers now have the option to layer technologies to obtain the desired pathogen removal rates while avoiding the associated static pressure, and additional expense, that comes with the use of a MERV 13 filter.

An increase in the germicidal dose (increase in lamps) can offer an even stronger performance. If installing UVC fixtures upstream of the cooling coil, it is important to specify UV-rated or UV-resistant air filters. As noted above, this combination of air cleaning technologies can also provide energy savings due to the lower pressure losses associated with the MERV 8 filter.

Pathogen Removal Performance
Figure 4: Independent testing has proved that combining the UVC together with the MERV 8 prefilter exceeded a 99% average single-pass reduction of the aerosolized SARSCOV2 virus. This exceeds the removal rate of the MERV 13 filter.

Pathogen Removal Performance, MERV filters
Figure 5: Facility engineers can combine medium-efficiency (MERV 8) with UVC to meet or exceed MERV 13-level efficiency rates without sacrificing airflow or compromising cooling capacity.


In the end, per ASHRAE and the CDC, facility managers are encouraged to upgrade all AHUs to high-efficiency MERV 13 air filters or higher.

Indoor Air Quality, IAQClean Air Survey Focuses On IAQ In The Workplace

Only 29% of workers describe their workplace air as very clean, according to a survey for the International Day Of Clean Air. Read more…

If faced with an AHU unable to accommodate these more efficient air filters, building engineers can combine UV-C with medium-efficiency MERV 8 air filters and still achieve federal indoor air quality recommendations.

Limiting HVAC airflow—as can some high efficiency filters—can cause HVAC engineers to increase the fan speed to overcome the static pressure to compensate, which raises energy consumption.

Societies such as ASTM, ASHRAE, and IUVA have begun addressing solutions for the problem of airborne disease transmission, and much of this focus revolves around the use of filtration and UVC. New guidelines and standards under development by these organizations will ensure proper design, testing, and installation of UVC systems and pave the way for widespread use. This will reduce the risk of airborne disease transmission in indoor environments.

Joe Kalman, UV ResourcesJoe Kalman is the Director of Sales for UV Resources and has worked in executive sales leadership and corporate strategy for more than 35 years. He works with leading facilities management companies, HVAC engineers, and has written and presented on such topics as indoor air quality, how to leverage ROI from germicidal UV-C and HVAC sustainability issues. He may be contacted at Joe.Kalman@UVResources.com.


* All references to “disinfection” are referring generally to the UVC germicidal inactivation of pathogenic biomass through the process of photodimerization and are not intended to refer to any specific definition by the U.S. Food and Drug Administration or the U.S. Environmental Protection Agency. In general, the effectiveness of UVC air disinfection is a function of time and intensity, (e.g., how much time/duration a pathogen is exposed to the UVC energy), as well as the amount of airflow (volume and velocity); the air temperature and RH; and the duct material reflectivity.

1 ASHRAE. ASHRAE Technical Resources; Filtration and Disinfection FAQ. Retrieved from https://www.ashrae.org/technical-resources/filtration-and-disinfection-faq

2 FEDWeek. IG Voices Unfiltered Concerns about Federal Building Air Quality. October 11, 2022. Retrieved from https://www.fedweek.com/fedweek/ig-voices-unfiltered-concerns-about-federal-building-air-quality/

3 Luo, H., L. Zhong. 2021. “Ultraviolet germicidal irradiation (UV C) for in-duct airborne bioaerosol disinfection: review and analysis of design factors.” Building and Environment 2021 197:107852. Retrieved from https://pubmed.ncbi.nlm.nih.gov/33846664/

4 Raeiszadeh, M., B. Adeli. 2020. “A critical review on ultraviolet disinfection systems against COVID-19 outbreak: applicability, validation, and safety considerations.” ACS Photonics 7(11):2941 – 2951. 16. ASHRAE. 2021. Retrieved from https://pubs.acs.org/doi/10.1021/acsphotonics.0c01245

5 Kowalski, Wladyslaw. (2009). Ultraviolet Germicidal Irradiation Handbook. Retrieved from https://download.e-bookshelf.de/download/0000/0138/90/L-G-0000013890-0002346490.pdf

6 ASHRAE Epidemic Task Force Available Resources (Online 2021) Filtration/Disinfection. Retrieved from https://www.ashrae.org/technical-resources/filtration-disinfection

7 IBID., 5.

8 IBID., 6.

9 IBID., 6.

10 IBID., 5.

11 IBID., 6.

12 Kowalski, Wladyslaw, Saputa, Dean, Jones, Daniel. Achieving MERV-13: UV C Can Help Less Efficient HVAC Filters Get There. HPAC Engineering. November 2021. Retrieved from www.hpac.com/industry-perspectives/article/21181503/achieving-merv13-uvc-can-help-less-efficient-hvac-filters-get-there

13 Walker CM, Ko G. 2007. Effect of ultraviolet germicidal irradiation on viral aerosols. Environmental Science & Technology 41(15):5460–5465 DOI 10.1021/es070056u. Retrieved from https://pubmed.ncbi.nlm.nih.gov/17822117

Click here for more information about Indoor Air Quality. 


Please enter your comment!
Please enter your name here