Powerful Ventilation and Filtration: Key to Reducing Viral Density
(COVID-19 Series: Part 3 of 7)
Table of Contents
Enhancing Indoor Ventilation and Filtration Creates Safer Indoor Environments
Welcome to Part 3 of our COVID-19 Series. In Part 2 we learned how CO2 monitoring can serve as an indicator of ventilation quality and, by extension, infection risks in indoor spaces. Elevated CO2 levels signal poor air exchange, which can lead to a higher concentration of infectious aerosols – including those carrying the SARS-CoV-2 virus – while also prolonging the time the virus can remain infectious in the air.
In this part, we advance this discussion by examining how specific ventilation and filtration strategies can effectively decrease the presence of virus-laden aerosols in indoor environments. We will dissect the science behind viral dose, particle size, air exchange rates and filtration efficiency, highlighting how these measures can be optimized to safeguard public health significantly.
As we navigate these preventive strategies, we aim to provide actionable insights that empower individuals and communities to improve the air quality in their immediate surroundings, ultimately fostering safer indoor environments during the ongoing pandemic and beyond. Join us as we continue to explore robust solutions to one of the most pressing health challenges of our time.
Viral Dose: What Is It?
Just like how the more alcohol a person drinks, the drunker they get, the more virus a person breathes in (aka their viral dose), the greater the chance they have in getting an infection, having higher viral loads in their body, and having worsened health outcomes [1]. Thus, reducing the amount of virus one breathes in is an important intervention. You can think of viral dose as the following formula:
Viral dose (amount of virus breathed in)= duration (time spent in a setting) x viral density (viral concentration in the breathing zone)
Let’s keep this equation in mind as we look at the role ventilation and filtration play, especially when it comes to reducing viral concentration in the air we breathe.
What Is the Difference Between Ventilation and Filtration?
If you recall from part 2 in this series, ventilation refers to the process of moving air into and out of a building or room, and distributing it for healthy breathing, using natural processes (such as opening windows and doors), and/or mechanical processes (such as using fans or mechanical ventilation systems). In our last blog, we visualize ventilation by imagining a dirty fish aquarium. We related ventilation to scooping cups of dirty water out and adding cups of fresh water back in to remove and dilute contaminants.
In sticking with this metaphor, air filtration would be like adding a water filter to your aquarium, capturing contaminants in its filter. As dirtied water runs through the filter, it is cleaned, and fresh water is cycled back into the aquarium. Bigger aquariums require bigger filters, and aquariums with more fish in them will need more filtration to keep up with the higher volume of contaminants entering the space.
The same types of variables are key factors when filtering the air. One of the biggest differences between a dirty fish bowl running through a water filter and a room with viruses in the air running through an air filter is that you can see the fish bowl’s water get cleaner and clearer over time. “Seeing” the effects of air filters requires having some understanding of types of air filters you are using, the types of infectious particles you are trying to clear out, and how to measure the outcomes. Let’s start by taking a closer look at air filters.
Types of Air Filters and Devices
Air filtration devices that use HEPA (High Efficiency Particulate Air) filters are commonly referred to as HEPA purifiers, and are portable devices made in a variety of shapes, sizes, and powers. HEPA filters can remove at least 99.97% of airborne particles (such as dust, pollen, mold, bacteria, pet dander, smoke, and the small particles viruses travel on) that are run through the filter. Ensuring that the device is pulling in enough air for the size of its room, and at a speed that makes a difference in transmission is part of the challenge.
Air filtration devices can use other types of air filters as well, such as the Do-It-Yourself DIY air cleaning “boxes” that have become popularized recently, that use MERV 13 or higher filters. MERV stands for Minimum Efficiency Reporting Values and reports a filter’s ability to capture different sized particles. These filters are often used in air conditioning (HVAC) systems in homes and buildings, where they filter air from the return vent, and can be bought at different levels of MERV ratings from 1 to 16. Filters with MERV 13 or higher ratings can trap very small particles, like the kind that spread COVID-19, with MERV 13 being the most popular for use since it can fit in most air conditioning systems without overstressing the system. Higher MERV filters can cause greater resistance and strain, and should be checked prior to use for compatibility with an HVAC system. However, simply installing MERV 13 or higher filters in HVACs is not enough filtration to mitigate COVID-19 risks, (although it helps, especially when the system is run constantly in the “ON” position as opposed to on “AUTO”) [2]. Air conditioning systems alone cannot move enough air through the filters to mitigate COVID-19 risks.
In order to really see a risk-reducing effect, most spaces will need supplemental air cleaners/purifiers that use either HEPA filters, or a combination of MERV 13 or higher filters and fans (often called fan-and-filter models), at a number that is right for the space based on its size and occupancy. Any number of either type of unit will help to reduce the density of infectious particles in a space, and thus reduce the amount of infectious respiratory particles that enter our breathing zone over time. But in order to truly gauge how much protection is offered for different spaces, we need to look at how much filtration is needed to reduce the viral dose enough to prevent infections.
It is also worth noting that both ventilation and filtration take time to clear viral particles out of the air, and can leave spaces of time where a person is vulnerable to breathing in enough viral particles to cause an infection (an infectious dose). For example, if a person next to you is contagious, and a purifier is on the other side of the room, you could easily breathe in an infectious dose of viral aerosols before the air has a chance of reaching the unit to be filtered. Yet, there is something you can do to protect yourself and others from this type of transmission (short-range): Wear a mask [3].
Many people do not realize that masks are a form of air filtration– and, an extremely efficient form at that, since they directly filter the air entering and exiting one’s breathing zone. There are three common types of masks used during the pandemic: cloth, surgical, and respirator (ie N95). We will go into this more in later parts of the series. But first we need to know more about what exactly it is that we are filtering out.
What are we diluting and/or filtering out?
Inhalable virus laden aerosol particles (also called bioaerosols) are particles in the 100 nm–5 μm size which have been demonstrated to carry viable SARS-CoV-2 (the name for the virus that causes COVID-19). These particle sizes are generated by common activities like breathing, speaking, singing, coughing, and sneezing, and readily move beyond the 6 foot physical distancing guidelines (although they will be denser closer to the source, so physical distance is still a good idea). However, due to their ability to remain airborne for extended periods of time and to be carried by air currents and through HVAC systems in an indoor environment, they can move much further from the source and build up in the air quickly, infecting people not just in the same room, but anyone with shared air in the building. These bioaerosols are the types of particles we need to dilute and/or filter out to mitigate airborne transmission.
Air filtering with HEPA and/or MERV 13 or higher rated filters is a recommended COVID-19 mitigation strategy by many authorities, including ASHRAE (the American Society of Heating, Refrigerating and Air Conditioning Engineers), a nonprofit organization that develops and publishes standards for the heating, ventilating and air conditioning industry [4], as well as the US Environmental Protection Agency (EPA), a federal agency that provides technical assistance to support recovery planning of public health and infrastructure [5].
According to the EPA, MERV 13 filters are measured to capture particle sizes in three categories:
- 30 nm-1.0 μm: It captures these at greater than or equal to 50%
- 1 μm -3.0 μm: It captures these at greater than or equal to 85%
- 3.0 μm -10.0 μm: It captures these at greater than or equal to 90%
HEPA filters can capture all of these particle sizes at a rate of 99.97% or better. Notice that 2 and 3 are bolded. That is because ASHRAE states most viral aerosols are in the particles in the 1 µm to 5 µm size range, which MERV 13 can filter out to 85-90% on each pass through the filter. Although HEPA filters have a higher rated filtration efficacy than MERV 13 filters, especially for the smallest of the particle ranges, and their filters capture a higher percent of particles on each pass through the filter (99.97%), portable air cleaners made with MERV 13 filters typically can pull in a lot more air, and at much faster rates.
With a higher flow rate than HEPAs, along with their ability to capture 85-90% of virus-laden particles on each pass, air cleaners made with MERV 13 filters can end up having a higher efficiency of air cleaning power over time than HEPA purifiers, which gives them an advantage when it comes to cleaning a room.
However, a HEPA purifier is better for uses involving directional air flow– in which a person is using the purifier to direct a column of highly purified air directly into their breathing zone– since it removes more particles on a single pass through the filter [6]. In other words, if you are trying to protect yourself with a stream of highly purified air blowing on you, HEPA would be a preferable option. But, if you are trying to filter viruses out of the air of a large space to prevent build-up over time, air cleaners made with MERV 13 filters are the preferable option. You can learn more about how MERV 13 filters outperform HEPA as room particulate filters here [7].
Using either type or combination of units will help prevent virus-laden particles from building up indoors over time, therefore reducing the amount that ends up in your breathing space. However, in order to achieve meaningful reduction, we have to be able to measure how much ventilation and filtration a place currently has, and find out how much more is needed to reduce the bioaerosols enough to reduce infection risks and significantly lower transmission. How can we measure ventilation and filtration?
Measuring Using Air Changes Per Hour (ACH)
We often measure ventilation and filtration rates using Air Changes per Hour (ACH) – how many times the air in a defined space is replaced by fresh outdoor or filtered air every hour.
One way to visualize this is that if a space has 6 ACH, then every 10 minutes all the air in that space will be replaced by fresh or filtered air.
In a space where a contagious person is present and continuously breathing out the virus, one can imagine the density of infectious aerosols in a space increasing over time, similarly to how smoke builds up in a room while a person is smoking a cigarette.
In order to prevent these aerosols from building up and lingering in the building, we need to change the contaminated air out with fresh or filtered air faster than it fills up.
In image 1, you can see the density of particles in a normally ventilated classroom (with 1.34 ACH), and how increasing ventilation and filtration to 5 ACH significantly reduces the density of these particles over time [8].
Image 1: Aerosol concentration in the room when switching from 1 to 5 ACH
![Image 1 Description: Total density of aerosols is reduced from 5,500 [1/cm^3] to 2,100 [1/cm^3] when increasing air changes per hour from 1.34 to 5. Credit: From Figure 6 of Strategies to minimize SARS-CoV-2 transmission in classroom settings: combined impacts of ventilation and mask effective filtration efficiency (2021)](https://airsupportproject.com/wp-content/uploads/2024/07/uhvc_a_1944665_f0006_c-1024x859.jpg)
Image 2: Procedural (Surgical) Mask Without and With Mask Fitter
Although 5 ACH significantly reduces aerosol density in just 30 minutes time, the researchers in this study conclude that 5 ACH ventilation/filtration alone is not able to achieve probabilities needed to significantly reduce infection from COVID-19, and thus should be used in conjunction with moderate to high EFE (effective filtration efficiency) masks, such as surgical masks modified with mask fitters, or respirator masks like N95s.
In image 2, you can see procedural masks, also called surgical masks, can have significant leakage of aerosols when unmodified. Adding a mask fitter will reduce leakage, giving more protection.
The researchers conclude: “Reductions provided by ventilation and masks are synergistic and multiplicative.” That is, they work together, and in doing so, are more efficient than the sum of either alone.
Impactful, But Not Sufficient on Its Own
Having understood the importance and effectiveness of ventilation and filtration in reducing viral density, it is essential to recognize that these measures, while impactful, are not entirely sufficient on their own to prevent COVID-19 transmission. Wearing a mask that has a high effective filtration efficiency is an important part of protecting oneself and others from COVID-19 infection. One may ask, what if we just wore a mask, and didn’t enhance ventilation or filtration at all?
The next part of our series (Part 4: Does One-Way Masking Work?) will focus on one-way masking—where one person wears a mask and is around other unmasked people—and evaluate if this strategy can offer adequate protection against COVID-19 on its own in normally ventilated spaces. By examining the role of one-way masking in isolation, we aim to provide a comprehensive understanding of how different preventive measures work individually and synergistically to safeguard our health.
At Air Support Project, we believe it is important for people to understand the concepts behind IAQ and mitigations against viruses and contaminants. We’re here to provide education that will assist you on your journey towards cleaner air and safer spaces. Stay tuned as we pivot into the specifics of one-way masking and its implications for reducing viral transmission as a strategy on its own.
“High-consequence risks have a distinctive quality. The more calamitous the hazards they involve, the less we have any real experience of what we risk: for if things 'go wrong, it is already too late.”
[1] Hooman Parhizkar, Leslie Dietz, Andreas Olsen-Martinez, Patrick F Horve, Liliana Barnatan, Dale Northcutt, Kevin G Van Den Wymelenberg, Quantifying Environmental Mitigation of Aerosol Viral Load in a Controlled Chamber With Participants Diagnosed With Coronavirus Disease 2019, Clinical Infectious Diseases, Volume 75, Issue 1, 1 July 2022, Pages e174–e184, https://doi.org/10.1093/cid/ciac006
[2] What kind of filter should I use in my home HVAC system to help protect my family from COVID-19? | US EPA. (2024, May 7). US EPA. https://www.epa.gov/indoor-air-quality-iaq/what-kind-filter-should-i-use-my-home-hvac-system-help-protect-my-family
[3] Public Health Agency of Canada. (2022). Use of portable air cleaners and transmission of COVID-19. https://www.publichealthontario.ca/-/media/documents/ncov/ipac/2021/01/faq-covid-19-portable-air-cleaners.pdf
[4] Debunking myths about MERV, air filtration | Ashrae.org. (n.d.). Web Starter Kit. https://www.ashrae.org/news/ashraejournal/debunking-myths-about-merv-air-filtration
[5] What is a MERV rating? | US EPA. (2024, March 5). US EPA. https://www.epa.gov/indoor-air-quality-iaq/what-merv-rating
[6] Benisek, A. (2023, January 29). What Is a Corsi-Rosenthal Box? WebMD. https://www.webmd.com/allergies/corsi-rosenthal-box
[7] Westerman, W. (2023, October 26). How can MERV13 filters outperform HEPA as room particulate purifiers? CleanAirKits. https://www.cleanairkits.com/blogs/news/how-can-merv13-filters-outperform-hepa-as-room-particulate-purifiers
[8] Rothamer, D. A., Sanders, S., Reindl, D., & Bertram, T. H. (2021). Strategies to minimize SARS-CoV-2 transmission in classroom settings: combined impacts of ventilation and mask effective filtration efficiency. Science & Technology for the Built Environment/Science and Technology for the Built Environment, 27(9), 1181–1203. https://doi.org/10.1080/23744731.2021.1944665
Sarah Masih, MD, MBA
Sarah Masih is the Executive Director of Air Support Project and has extensive experience in project management, fundraising, and communications. Her deep commitment to public health and her collaborative approach with community organizations and health experts make her a vital force in the fight for cleaner air and healthier communities.
SHEA O'NEIL
Volunteer
Creator of COVID-conscious advocacy works: writings, art, and social networking. BA degree in psychology, with over 11 years of continued education studies in allergy and autoimmune research, and continued research the past 3 years into airborne irritants and COVID-19. Parent, disabled rights advocate.
S. Riley
S. Riley is a mutlidisciplinary content creator and technology enthusiast with a background in crowdfunding and project management. He holds a BA in English, and is grateful to be able to utilize his unique skill set in service of Air Support Project's humanitarian mission.
Melissa Smallwood
Melissa Smallwood is Science and Technology Policy researcher with a background in neuroscience, psychology, and social science, and the author of The Future of Long COVID: A Threatcasting Approach. They are interested in how disability and healthcare justice intersect with emerging technologies, and are an advocate for COVID mitigation strategies to help end the pandemic.
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what's in the air we breathe?
On average, we breathe 6 liters every minute, mainly indoors. Indoor air quality is often worse than outdoor air which underscores our need for cleaner air and safer spaces. Our air is made up of microparticles such as dust, pollen, mold, viruses, pet dander, and VOCs. These particles can enter our bloodstream through our lungs after we inhale them. The air we breathe impacts our daily and long-term well-being and health. Follow us at Air Support Project. We want you to know better so you can breathe better.