Why At Air Support Project, We Strive for 6 to 12 ACH
(COVID-19 Series, Part 6 of 7)
Table of Contents
What Is Considered “Enhanced Filtration” for Infection Control?
In our previous post, we explored the relationship between time, viral density, and filtration, highlighting how these factors interact to influence the risk of infection in indoor environments. This understanding is crucial as we explore more effective ways to mitigate COVID-19 transmission.
Here, in Part 6 of 7, we focus on the importance of achieving a specific target for air changes per hour (ACH) to enhance indoor air quality (IAQ) effectively.
Outdoor air quality is regulated by the EPA using an Air Quality Index (AQI) [1] which informs individuals when air quality is poor, such as during wildfire smoke events, and recommends actions, like avoiding prolonged outdoor exposures and wearing a respirator mask. However, no such monitoring or regulation exists for indoor air quality.
Since COVID-19 is known to spread in the indoor air at higher rates than outdoor air and can cause significant short- and long-term health consequences, numerous agencies have started to create recommendations for indoor air quality. Along with reducing infection risks for COVID-19, enhancing ventilation and filtration can also reduce wildfire smoke, allergens, VOCs, and other airborne contaminants.
In this part of our series, we will take a closer look at what different agencies and research recommend, and explain why at Air Support Project, we strive for 6-12 ACH as the gold standard for reducing infection risk in settings like classrooms, offices, and homes. Join us as we discuss the benefits of enhanced ventilation and filtration and how it can be achieved affordably and efficiently.
6-12 ACH Offers A Quicker Purging Time
COVID-19 can transmit and infect in a matter of seconds to minutes in normally ventilated spaces [2], [3] – therefore, time is of the essence when removing it from indoor environments! One of the most effective ways to clear out these infectious particles is by increasing the rate at which old, contaminated air is purged and replaced with fresh or filtered air. This rate is known as air changes per hour (ACH).
One way to envision air changes per hour is by comparing filtering the air to filtering the water in a fish aquarium. If you go to the pet store to purchase a fish aquarium, there is typically a specialist who will help you pick out the appropriate sized and powered filter for the tank. If you put a small fishbowl-sized filter in a large 50 gallon tank, it will be insufficient, and your fish will suffer for it. You need filters that can move all the water through in the tank at an appropriate rate based on the tank size and number of fish in the aquarium.
In the same way, adding an air purifier that is not the correct size and power for a room will not work well, and when the result is the buildup of SARS-CoV-2 virus in the space, the occupants will suffer. However, research has been done to see how many air changes per hour is necessary for reducing infections.
The CDC’s Guidelines for Environmental Infection Control in Health-Care Facilities (2003) shows that the time it takes to remove airborne contaminants from the air depends on the ACH in the space, with higher ACH clearing airborne contaminants at much quicker rates [4].
As shown in image 1 below, having 6 ACH would take 46-69 minutes to remove the contaminants from the air, while having 12 ACH would take approximately 23-35 minutes to clear out the contaminants after the source of contamination has left.
These shorter purging times are ideal because during the time it takes the ventilation/filtration methods to remove the particles from the air, a person who is not masked could be infected.
Less than 6 ACH can increase the time it takes for airborne contaminants to be cleared from a space to over an hour. During that hour, infection risks to individuals can vary depending on who is masked in the space, and the type of mask worn.
Recall from Part 4 Does One-Way Masking Work? researchers show that in normally ventilated spaces when only the non-infected person is wearing a mask, there is approximately an infection risk of:
- 20% risk after an hour if they are wearing an FFP2 (N-95 equivalent)
- 90% risk after 30 minutes for a person in an unmodified surgical mask
- 90% risk after a few minutes for unmasked person [5]
Since an infected person, also known as “source”, may be present in the space unmasked and continuously breathing out viruses into the air, it is important to have a high enough ACH to keep up with purging the infectious aerosols. For this reason, we aim to reduce particle density and clear contaminated air from a space in under an hour, which, as can be seen from the above image, requires 6-12 ACH.
We recommend leaning toward the higher end of this 6-12 ACH range in any of the following situations:
- when occupancy in the space is higher (higher likelihood a person could be contagious in the space, including asymptomatic cases),
- when one cannot get 6 foot distance (viral density is stronger closer to the source),
- when the space has unmasked occupants,
- when many people are talking, singing, or exercising (more aerosols are shed during these activities than breathing alone),
- when a person is known or suspected to have an infection.
These situations can result in higher concentrations of virus in the air, so higher ACH rates (closer to the 12 ACH) are needed to keep up in order to reduce risks of spread.
A Closer Look at Recommendations
Over the past few years, several authorities have given different ACH recommendations. Let’s take a closer look at the details to see if we can reconcile these differences.
For instance, the CDC’s Ventilation in Buildings (2023) recommends enhancing ventilation to “5 ACH or more” [6]. We suggest drawing your attention to the “or more”, and the details within this text:
- large volume spaces with very few occupants (e.g., a warehouse) may not require 5 ACH and spaces with high occupancy or higher-risk occupants may need higher than 5 ACH;
- while ACH levels higher than 5 may reduce infectious aerosols further, the potential benefits of increased ventilation should be balanced with the additional upfront, periodic maintenance, and energy costs that may be incurred;
- a Lancet Commission Report that draws on available scientific evidence proposes ACH levels of 4 as “Good,” 6 as “Better,” and >6 as “Best,” underscoring that ACH represents a continuum.
- limited studies have demonstrated this protective benefit of increased ACH, although an optimum number remains uncertain.
One notable part of the CDC’s quoted information above is the statement that higher ACHs can reduce infection risks further, but must be weighed with costs. In reality, 6-12 ACH can be met at a feasible cost without compromising the health of building occupants, and should not be seen as a barrier to making recommendations based on the research available. Secondly, they state the difficulty in assessing ACH is due to the variability of the size and number of occupants in a space. This issue has been remedied in key standards created by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers)– an organization that creates standards for heating, ventilation and air conditioning (HVAC) professionals and serves as a liaison to the general public, offering key information throughout the COVID-19 pandemic with their infection control standards for indoor spaces.
ASHRAE’s Standards Support 6-12 ACH
ASHRAE has released infection control ventilation standards in both 2021 and 2023 that include higher ACH than the CDC’s 5 ACH minimum. They have also created an alternative to ACH using a metric called “equivalent clean air flow” that takes into account occupancy level and activity level of the people in the space.
To start, ASHRAE’s Ventilation for Industrial Settings During the COVID-19 Pandemic (2021) [7] states:
“Maintain between six and twelve air changes per hour (ACH), which will provide a greater than 99% purge in 30-60 minutes. This reduction applies when the building is unoccupied – the concept of purging does not apply if a source is still present (i.e., potentially infectious individuals are still in the workspace). It is important to maintain as much outside air and/or sufficiently filtered recirculated air as possible to achieve the goal of greater than 6 ACH.”
In their most recent update, the 2023 ASHRAE Standard 241 Control of Infectious Aerosols [8], which can be previewed here [9], the agency sets minimum equivalent clean air flow rates (instead of ACH) for infection risk mitigations for long range transmission according to the building’s occupancy category. Equivalent clean airflow rates are measured in cubic feet per minute (cfm) or liter per second (l/s) instead of air changes per hour (ACH) because it allows for better variation based on the number and activity level of people in the room.
For instance, for a classroom setting, it recommends the equivalent clean airflow rates of 40 cfm per person (20 l/s per persons), which– for an average sized classroom of 25 people– converts to approximately 1000 cfm (500 l/s), which equates to 6-12 ACH using a tool for converting cfm or l/s to ACH [10]. Recent research also supports the use of 6-12 ACH.
Supporting Studies for 6-12 ach
Studies using different combinations of purifiers and ventilation systems have allowed for insights on the proper determination of the number and type of portable purifiers needed in different spaces, and the range of ACH needed, to reduce infections.
For example, a 2022 study [11] found that using HEPA filters to achieve 7-12 air changes per hour (ACH) can consistently remove SARS-CoV-2 and reduce viral load to below the detection limit. Although this study shows amazing results of what purification can do, the researchers also note that because air does not pass through the air cleaner evenly, and because there are areas where the aerosols tend to linger or require time to remove, filtration should be combined with measures such as masking that offer short-range protection in that time and space.
In places where people may have to be unmasked, it is even more important to get effective ventilation and filtration. A 2021 study [12] on the effects of ventilation and filtration in a dental setting found that adding portable air cleaners along with ventilation was more effective in clearing out aerosol particles, versus using ventilation alone. A combination of ventilation and portable air cleaners was able to consistently remove more than 80 percent of aerosols in 10 or fewer minutes, whereas ventilation alone took up to 30 minutes. In a setting such as a dentist’s office, where patients cannot mask while being worked on, this time difference is especially important, as it can prevent the buildup of potentially infectious aerosols.
Air Quality Improvement Reduces Illnesses and Absences
One way to measure the effectiveness of adding air filtration and ventilation is by seeing its impacts on reducing illnesses and absences.
In a study assessing the effectiveness of air cleaning in reducing the risk of respiratory infection and resulting missed work days by parents in two day care centers using a calculation model, researchers concluded there was a 30% reduction in infections, and a reduction of approximately 32% in absences from work by parents when air cleaners were added [13].
They found adding portable air purifiers into classrooms in the daycares increased non-infectious air flow rates by 137% and 126% in both daycare centers, bringing most rooms into the range of Lancet COVID-19 recommended standards, and some to the more recent ASHRAE 241 standards, as can be seen in image 2.
The researchers conclude:
“Our study offers compelling evidence to support the implementation of air cleaners in daycare centers as an effective and cost-efficient strategy for mitigating the spread of respiratory infections among children.”
Another study published in 2022 found that increasing ventilation to six air changes per hour reduced COVID-19 transmission by 82.5% in schools [14].
The researchers state:
“To the best of our knowledge, this is the first and largest retrospective cohort study in schools aimed at assessing the impact of mechanical ventilation in mitigating the risk of COVID-19 infection. The results demonstrate the effectiveness of the mechanical ventilation and the possibility of applying these techniques in a similar way in all indoor environments that represent the natural habitat of humans and which require complex, targeted management, not only of the control of thermal comfort, odors, perceived air quality, and energy use, but also of respiratory infections.”
Image 2: Air cleaners increased the non-infectious air flow
In a 2021 study [15] that tested portable air cleaners in classrooms for the reduction of COVID-19 risks, the researchers found the aerosol concentration was reduced by more than 90% within less than 30 min when running the purifiers at a rate of 6 ACH. The researchers used four HEPA purifiers (avoiding air purifiers that rely on the use of ozone generators and ionizers) dispersed evenly around the classroom, each with a smoke CADR of approximately 200 cfm per unit giving a total ACH of 5.7 in the classroom. They concluded that the estimated inhaled dose of virus after 2 hours is reduced by a factor of six when using the purifiers when compared to not using them. The researchers state that when one infective person is present in the room, it is estimated that:
“A susceptible person in the room for 2 hours will take up a dose of 22 RNA-containing particles, representing a 4.7% risk of becoming infected. In a room with 25 persons, this leads to an overall risk of 70% that at least one of the other 24 persons becomes infected. For an air exchange rate of 5.7 h−1 (5.7 ACH) the risk per person reduces to 0.7% and 16.5% that at least one of the persons in the room becomes infected.”
The researchers also advise if you only have one purifier, put it in the middle of the classroom, if you have 2 put them in opposite corners with care not to obstruct their airflow, and if you have 3 or more, then disperse them evenly around the room. Having more than one purifier helps with mixing the air in the room evenly so particles do not build up in any areas. The units should also have high clean air delivery rates (CADR), with a combined total of approximately 600-800 cfm for an average classroom. Improving ventilation and filtration in schools and workspaces can significantly reduce illnesses and the resulting absences.
6-12 ACH: A Vital Metric
As you can see from the research above, achieving 6-12 air changes per hour (ACH) is vital for significantly reducing airborne contaminants and mitigating infection risks in indoor environments. The evidence supports that higher ACH rates lead to quicker purging times and reduced viral densities, which are crucial for preventing COVID-19 transmission, and result in less illness and absences.
While traditional HEPA purifiers may be costly and have practical limitations, there are affordable and efficient alternatives, such as fan-and-filter air cleaners, that can meet and exceed these standards. In our next and final post of this series (Part 7: Getting the Required ACH With More Affordable Fan-and-Filter Models) we will explore these innovative fan-and-filter models, discussing how they work and how DIY models can be assembled to achieve the desired ACH at an affordable price point. Stay tuned to learn more about these accessible and effective tools for maintaining clean air.
“When the water in the fishbowl turns toxic, you don’t treat the fish, but the water.”
[1] Air Support Project. (2023, July 31). Reading between the AQI lines. https://airsupportproject.com/reading-between-the-aqi-lines/
[2] Iqbal, S. S. (2023c, September 25). Here’s how much coronavirus people infected with COVID-19 may exhale. Science News. https://www.sciencenews.org/article/coronavirus-infected-covid-19-exhale
[3] Alsved, M., Nyström, K., Thuresson, S., Nygren, D., Patzi-Churqui, M., Hussein, T., Fraenkel, C., Medstrand, P., & Löndahl, J. (2023c). Infectivity of exhaled SARS-CoV-2 aerosols is sufficient to transmit covid-19 within minutes. Scientific Reports, 13(1). https://doi.org/10.1038/s41598-023-47829-8
[4] Appendix B. air. (2024, January 11). Infection Control. https://www.cdc.gov/infection-control/hcp/environmental-control/appendix-b-air.html
[5] Bagheri, G., Thiede, B., Hejazi, B., Schlenczek, O., & Bodenschatz, E. (2021b). An upper bound on one-to-one exposure to infectious human respiratory particles. Proceedings of the National Academy of Sciences of the United States of America, 118(49). https://doi.org/10.1073/pnas.2110117118
[6] Ventilation in Buildings: Community, work, and school. (2020, February 11). Centers for Disease Control and Prevention. https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation.html
[7] American Conference of Governmental Industrial Hygienists & American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2021). Ventilation for industrial settings during the COVID-19 pandemic. In ACGIH® White Paper Series [Report]. https://www.ashrae.org/file%20library/technical%20resources/covid-19/ashrae-acgih-covid-19-white-paper.pdf
[8] ASHRAE Standard 241, Control of Infectious Aerosols | Ashrae.org. (n.d.). Web Starter Kit. https://www.ashrae.org/technical-resources/bookstore/ashrae-standard-241-control-of-infectious-aerosols
[9] Standards and guidelines. (n.d.). Web Starter Kit. https://www.ashrae.org/technical-resources/standards-and-guidelines
[10] How to convert ACH value into CFM or L/s | cove.tool Help Center. (n.d.). https://help.covetool.com/en/articles/4147774-how-to-convert-ach-value-into-cfm-or-l-s
[11] Ueki, H., Ujie, M., Komori, Y., Kato, T., Imai, M., & Kawaoka, Y. (2022). Effectiveness of HEPA Filters at Removing Infectious SARS-CoV-2 from the Air. MSphere, 7(4). https://doi.org/10.1128/msphere.00086-22
[12] Ren, Y., Huang, Q., Marzouk, T., Richard, R., Pembroke, K., Martone, P., Venner, T., Malmstrom, H., & Eliav, E. (2021). Effects of mechanical ventilation and portable air cleaner on aerosol removal from dental treatment rooms. Journal of Dentistry, 105, 103576. https://doi.org/10.1016/j.jdent.2020.103576
[13] Vartiainen, V. A., Hela, J., Luoto, A., Nikuri, P., Sanmark, E., Taipale, A., Ehder-Gahm, I., Lastovets, N., Sormunen, P., Kulmala, I., & Säämänen, A. (2024). The effect of room air cleaners on infection control in day care centres. Indoor Environments, 1(1), 100007. https://doi.org/10.1016/j.indenv.2024.100007
[14] Buonanno, G., Ricolfi, L., Morawska, L., & Stabile, L. (2022). Increasing ventilation reduces SARS-CoV-2 airborne transmission in schools: A retrospective cohort study in Italy’s Marche region. Frontiers in Public Health, 10. https://doi.org/10.3389/fpubh.2022.1087087
[15] Curtius, J., Granzin, M., & Schrod, J. (2021). Testing mobile air purifiers in a school classroom: Reducing the airborne transmission risk for SARS-CoV-2. Aerosol Science and Technology, 55(5), 586–599. https://doi.org/10.1080/02786826.2021.1877257
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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“Airborne Contaminant Removal” graphic showing the time required to remove airborne contaminants at 99% and 99.9% efficiency over time (in minutes) at different ACH ranging from 2 to 50. Published under Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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