The coronavirus disease 2019 (COVID-19) crisis has certainly impacted most of our lives and the AEC industry in which we operate. As I write this article, our understanding of the coronavirus that causes COVID-19 continues to evolve in a very fluid manner.

In consideration of personal protective equipment (PPE) in social spaces, people have gravitated to variations of face masks. The U.S Food & Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) have both issued advice for proper respirator types and usage for respiratory protection preparedness. One of the primary challenges in filtering the coronavirus is that such virions are extremely small. According to emerging research, such as which is exhibited by the National Center for Biotechnology Information (NCBI), coronavirus virions are spherical with diameters of approximately 0.13 micrometers.


Just How Small is a Virus?

Let us put the size of a micrometer into perspective. Figure 1 exhibits a graphic representation of the relative diameter of a variety of particles. While bacteria exists in a wide variety of shapes and sizes, the figure categorically depicts bacteria as having a representative diameter of 1µm (micrometer). By comparison, the diameter of a strand of human hair can range between 17 to 181 µm (60 µm is depicted in the figure).

At the other end of the spectrum are viruses, which can range between 0.02 - 0.40 µm. Again, coronavirus virions tend to exist on the smaller end of the range. In simple and generalized terms, a coronavirus can be 1/10 of the size of a bacteria cell, which is 1/10 of the size of a dust particle, which is around 1/10 of the diameter of a strand of human hair.

Figure 1: Relative Particle Sizes.
Illustration by Daniel Overbey.



Can HVAC filters stop viruses?

Among other issues, this crisis has renewed my personal interest in air filtration as a critical strategy for indoor air quality.

Can HVAC filters effectively capture viruses? It depends. Let us consider the potential effectiveness of filters through the lens of their equivalent MERV ratings.


MERV Filter Ratings

The Minimum Efficiency Reporting Value (MERV) is a standard that rates the overall effectiveness of air filters. MERV ratings range from 1-20, with higher values indicating increased filtration across increasingly smaller particles sizes.

So, why not always use filters with high MERV ratings? The higher the MERV rating, the more expensive the filter; but also the smaller the pores in the filter that allow air penetration. Higher MERV filters can create more resistance in airflow than an HVAC system may be designed to handle; which could make it perform less efficiently. By reducing the air flow in an HVAC system, higher MERV filters can actually worsen the indoor air quality and put a damaging amount of pressure on the furnace/air-handler fan. Filtration levels require research and a balanced approach with energy and other performance considerations.

To put MERV ratings into perspective, most residential/light commercial HVAC systems will use a filter between MERV 8-13. On the high-end of the spectrum, MERV 14-20 filters are typically used in hospitals and similar settings.

Both LEED v4.1 and WELL v2 advocates for higher MERV ratings for air filters in mechanically ventilated spaces on new commercial projects.


Do Higher MERV Ratings Mean Better Virus Filtration?

When it comes to fractional efficiency (i.e., the proportional effectiveness) of air filters across a range of particle sizes, the MERV rating matters a great deal.

In response to concerns about bioterrorism and existing concerns about indoor air quality, Dr. Wladyslaw J. Kowalski, PE, and Dr. William P. Bahnfleth, PE, modified a classical model to assess the effectiveness in air filters across different MERV ratings. Figure 2 offers a sample of their published findings from their white paper, MERV Filter Models for Aerobiological Applications (2002).

Figure 2: Modeled Fractional Efficiency per Generalized MERV Filter Rating.

Adapted from Kowalski and Bahnfleth (2002).


As evident by Kowalski and Bahnfleth's data, the MERV rating has a significant effect across the full spectrum of particle mean diameter. Of particular interest here is for airborne microorganisms smaller than 0.3 micrometers, which includes the majority of viruses and smaller bacteria. Generally speaking, MERV 13-16 will filter over 80 percent of airborne microorganisms smaller than 0.3 micrometers.

There are many qualifiers to this data, however; as noted by Kowalski and Bahnfleth:

  • "It should be noted that the variations in filter efficiency for any given MERV ratings is probably on the order of at least +/- 20 percent."
  • "Most filters today use a range of filter fiber diameters, which provides considerable flexibility in marching the filter model to MERV or vendor data."
  • The actual curvature for specific filters with MERV ratings exhibited in the model "can vary considerably."
  • "The models presented are only generally representative of the entire array of MERV filters since considerable variation is possible by different filters with the same MERV rating."
  • The curvatures, which are depicted for 500 fpm, will shift as operating velocities vary. Yet, "this filter model should provide reasonably accurate estimates of filter performance at other operating velocities."
  • "Airborne microbes can be removed by filters at rates that depend on the filter performance curve and the mean diameter of the microbes."


Kowalski and Bahnfleth conclude by noting that the filter model should be taken in generality, simply illustrating "the kind of performance that might be expected." Moreover, the information presented "should not be considered to represent performance curves for each MERV rating in anything but a general fashion since considerable variations can occur" from one product to another with the same MERV rating.


Bottom Line

Taken as a general representation for the MERV ratings presented, the Kowalski and Bahnfleth data illustrates that better HVAC air filters make a huge difference when it comes to filtering virions and other small airborne microorganisms. There is good reason why LEED and WELL push for higher MERV ratings. As we reassess filtration in various aspects of our lives in the aftermath of the COVID-19 pandemic, we would be wise to scrutinize products for how they specifically perform against extremely small particles such as bacteria and viruses.