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IAQ Advocacy Basics 3/4: Eliminating Recommendation Consternation

One barrier to advocacy has been that parents and advocates don't know how much ventilation is enough to ask for. In the absence of quantified ventilation targets, schools often rely heavily on the "to the extent possible" phrasing of many recommendations, resulting in ventilation increases that fail to produce results, leading to erroneous disenfranchisement about the effectiveness of engineering controls.

The recommendations available do not all quantify target ventilation rates, making it difficult for advocates to identify the right recommendations to focus on. Furthermore, one source of confusion for advocates has been the diversity of recommendations themselves and where they originate. In this post, we'll detail the foremost quantified recommendations we know of and provide hyperlinks to them for reference.

As a quick aside, if airflow dimensions and CO2 concentrations are unfamiliar to you, you can always stop by our article about Dispelling Dimension Disorientation to understand their meaning. And if you're unsure how these units relate to one another, then you can also read Clearing Up Conversion Confusion to get details.

Recommendation Roundup

Below are the foremost quantified recommendations known to us:


Originated / Supported / Substantiated By

​6-12 Air Changes per Hour (ACH)



Lancet COVID-19 Commission

Chair – ASHRAE Epidemic Task Force, Bill Bahnfleth

Relevant Standard: ASHRAE Standard 170

Harvard T. H. Chan School of Public Health

Rhode Island Department of Public Health

California Department of Public Health

Scientific Literature

30 cfm per person

Lancet COVID-19 Commission

Dr. Joseph Allen of the Harvard T. H. Chan School of Public Health

10 liters per second per person (LPS/person)

World Health Organization (WHO)

Under 800 ppm of CO2

Centers for Disease Control and Prevention (CDC)

UK Scientific Advisory Group for Emergencies (SAGE)

The above table is a quick overview of recommendations. For more detail, here is a survey of ventilation and filtration recommendations, along with scientific literature about them.

At least 6-12 Air Changes per Hour (6-12 ACH) – Various

There is strong overlap in expert recommendations, relevant standards, and scientific literature in the range of at least 6-12 air changes per hour (ACH) for significantly reducing the far-field risk of infection. Below are the organizations whose recommendations support or overlap with this. We'll start on the low end of the overlap and work our way up, ending with scientific literature.

Overlap: Rhode Island Department of Public Health

On May 27th, 2022, the Rhode Island Department of Public Health published Guidance for Airflow, Ventilation, and Air Filtration indicating that a minimum of 4-6 ACH and no less than 15 cfm per person should be employed to acceptably limit airborne transmission.

Overlap: Harvard T. H. Chan School of Public Health: 6 ACH “Ideal”

As of at least November of 2020, the Harvard T.H. Chan School of Public Health's updated Risk Reduction Strategies for Reopening Schools recommended implementing ventilation and supplemental air cleaning, with 6 ACH being the “ideal” target, to reduce SARS-CoV-2 transmission. The Harvard Healthy Buildings guidance issued in 2020 has been succeeded by increased airborne transmission that spurred expert recommendations, specific studies, and calls by ASHRAE members and industrial hygienists to adopt 6-12 ACH, as codified in ASHRAE’s standards for healthcare facilities.

ASHRAE Epidemic Task Force Chair, Bill Bahnfleth: 6+ ACH

Bill Bahnfleth is the former president of ASHRAE and chaired the ASHRAE Epidemic Task Force, so his is a highly qualified expert opinion. In Mr. Bahnfleth's tweet pictured below, he recommended stepping up our minimum air change rate targets from 4 ACH to 6 ACH or better in light of the Delta variant's increased transmisibility:

Furthermore, Mr. Bahnfleth reminded readers not to neglect non-HVAC avenues to reduce risk. Examples of this is include in-room air cleaners such as portable HEPA filters or DIY units like Corsi-Rosenthal boxes.

Mr. Bahnfleth made the same recommendation in late June of 2022, on the basis that it is attainable and reduces relative risk, as shown here:

Bill Bahnfleth likewise put forth 6 air changes per hour of ventilation and/or HEPA filtration in professional correspondence that he has publicly shared, as shown here:

Air changes introduced by outdoor air, HVAC filtration, and in-room air cleaners can combine additively (see slide 28) to provide increased risk reduction, and may be necessary to go beyond what can be achieved by MERV 13 filters and minimum outdoor air requirements, as Mr. Bahnfleth stated above.

Relevant Standard, ASHRAE Standard 170: 6-12 ACH

In Mr. Bahnfleth’s letter above, he references healthcare ventilation standards in the abstract. He has repeated the same in webinars as well as in other public venues such as this post on October 4th of 2021, such as the tweet:

The relevant standard we can find is ASHRAE Standard 170, “Ventilation of Health Care Facilities”.

ANSI/ASHRAE/ASHE Standard 170-2021 - Ventilation of Health Care Facilities
Cover of ASHRAE Standard 170

The latest version as of this writing, ASHRAE Standard 170-2021, is freely available as a preview-only standard and indicates that virtually all inpatient spaces shall be ventilated to a total of at least 6 air changes per hour of combined outdoor air and HEPA filtered return air, consistent with Mr. Bahnfleth’s recommendations for schools as Chair of the ASHRAE Epidemic Task Force.

Lancet COVID-19 Commission: Best - >6 ACH

In November 2022, the Lancet COVID-19 Commission Task Force on Safe Work, Safe Schools, and Safe Travel indicated greater than 6 ACH as their "Best" recommendation in their Proposed Non-infectious Air Delivery Rates (NADR) for Reducing Exposure to Airborne Respiratory Infectious Diseases.


In June 2021, the American Conference of Governmental Industrial Hygienists (ACGIH) co-authored a technical paper with ASHRAE titled Ventilation for Industrial Settings During the COVID-19 Pandemic, recommending to “Maintain between six and twelve air changes per hour (ACH)”.

AIHA: 6-12 ACH

On September 9 2020, the American Industrial Hygiene Association (AIHA) released a guidance document titled, Reducing the Risk of COVID-19 Using Engineering Controls. In their guidance document, AIHA noted the significant infection reductions achievable by implementing 6-12 ACH and indicated that air change rates in the higher end of that range may be necessary for non-healthcare facilities where occupant density cannot be limited to fewer than 1 person per 30 square feet / six feet apart.

California Department of Public Health: 6-12 ACH

In July 2021, the California Department of Public Health (CDPH) published Ventilation and Filtration to Reduce Long-Range Airborne Transmission of COVID-19 and Other Respiratory Infections: Considerations for Reopened Schools. Among the recommendations and strategies shared in this document, CDPH indicated that ventilation rates of at least 6-12 ACH in healthcare settings provide a good reference for workplaces and public spaces where airborne illnesses like SARS-CoV-2 may be present.

Scientific Literature

6 ACH Within Range Necessary to Significantly Reduce Epidemic Attack Rate

Dr. Shelley Miller at CU Boulder presented results from a simulated influenza epidemic in a heavily urban area, finding that the attack rate of the disease remained at or near 100% even up through approximately four air changes per hour. Dr. Miller noted that, "as we increase our ventilation rates up to 9 air changes per hour, the attack rate decreases dramatically."

YouTube frame grab from Shelley Miller presenting. Title: Increase (clean) outdoor air supply. Clean virus-free air supplied by ventilation dilutes indoor shared air, reducing virus levels; contaminated air is exhausted by ventilation removing the infectious agent. Studies show that building ventilation can be as effective as public health interventions. Existing ventilation rates are too low to prevent or control airborne infectious diseases indoors--and might need to be increased by 10x. Inset photo of Dr. Shelley Miller speaking. Two graphs visible, with the bottom graph depicting a fall from 100% attack rate around 3-4 air changes per hour down to 0 around 9 air changes per hour.
Screen grab from Dr. Shelley Miller's presentation.

Dr. Miller's findings suggest that 4 ACH may be an inadequate minimum target for infection control, and provide further support for the 6-12 ACH found in several of the above recommendations.

Real-World Studies Indicate 6 Air Changes per Hour Can Yield 82.5% Reduction of Infection Risks

The expert recommendations and standards above have coincided with studies finding a significant risk reduction being achieved with 6 ACH, as shown subsequently.

Goethe University (6 ACH)

In March 2021, Goethe University in Frankfurt published their study Testing mobile air purifiers in a school classroom: Reducing the airborne transmission risk for SARS-CoV-2. The study found that achieving about 6 air changes per hour with HEPA filters yielded a factor-of-six (82.5%) reduction in inhaled dose of SARS-CoV-2.

Based on the occupant count of 29, and the room area and height of 550 sqft with 12 ft ceilings, this provided about 22 cfm/person of equivalent clean air from portable HEPA filters.

The test was conducted in a classroom where CO2 concentration was measured to be 1,000 ppm, which amounts to about 12 cfm/person based on applying equations and CO2 generation estimates disseminated by the Harvard TH Chan School of Public Health -- just a little bit more than the 10 cfm/person required for classroom spaces by International Mechanical Code. So, supplementing with 6 ACH of portable HEPA filtration reduced risk by 82.5% over and above that baseline level of ventilation.

Hume Foundation (6 ACH)

In March 2022, Hume Foundation in Italy published their pre-print findings from a ventilation study, titled Controlled Mechanical Ventilation (CMV) works. Table 2 of the pre-print findings associated 6 ACH with an 82.5% reduction in transmission. Hume Foundation indicated, "It’s clear that the CMV, especially if adequately sized (6 or more air changes-hour), has the ability to reduce the Sars-CoV-2 infection risk by over 80%."

In July 2022, Hume Foundation published their study with the title, Increasing ventilation reduces SARS-CoV-2 airborne transmission in schools: a retrospective cohort study in Italy's Marche region. The background statement of their study indicated that ventilation ranging from 10-14 L/s/person (21-30 cfm) reduced the likelihood of infection for students by 80% compared with a classroom with only natural ventilation.

Lesser Air Change Rates Yield Lesser Risk Reductions

As might be expected, lesser air change rates such as 5 ACH have yielded lesser risk reduction factors in other scientific literature.

In May 2020, CERN published their study, SARS-CoV-2 aerosol transmission in schools: the effectiveness of different interventions, in which they found up to a 75% dose reduction from using portable HEPA filters to target 5 ACH. The CDC found 5 ACH to yield a 65% reduction in the inhaled dose in their study, Efficacy of Portable Air Cleaners and Masking for Reducing Indoor Exposure to Simulated Exhaled SARS-CoV-2 Aerosols.

30 cfm per Person

Lancet COVID-19 Commission Task Force on Safe Work, Safe Schools, and Safe Travel

The Lancet Commission recommended greater than 30 cfm/person as their "best" airflow per person recommendation in their Proposed Non-infectious Air Delivery Rates (NADR) for Reducing Exposure to Airborne Respiratory Infectious Diseases. This recommendation is triple the 10 cfm/person specified in the per-occupant component of breathing zone ventilation for classroom spaces in International Mechanical Code (IMC) and in ASHRAE Standard 62.1.

The Lancet COVID-19 Commission made this recommendation alongside their recommendation for 6 ACH or better. One reason for this might be the common objection by building owners that air change rates do not scale with occupancy. Notably, air change rates and airflow per person are apples and oranges, but as will be shown in a subsequent blog post, for various classroom sizes and occupancies, they may provide comparable levels of ventilation.

Dr. Joseph Allen of the Harvard T. H. Chan School of Public Health

Previous to the Lancet recommendations, Dr. Joseph Allen of the Harvard T. H. Chan School of Public Health had already recommended that building ventilation should target 30 cfm/person both in his book and at the 2022 White House IAQ Summit.

10 L/s per Person – World Health Organization (WHO)

In 2021, the World Health Organization (WHO) indicated a minimum recommended ventilation rate for non-residential settings of 10 L/s/person in their Roadmap to improve and ensure good indoor ventilation in the context of COVID-19. In imperial units, this is approximately 21 cfm.

Under 800 ppm of CO2

US Centers for Disease Control and Prevention (CDC)

In 2020, CDC indicated in FAQ #9 of their page on Ventilation in Buildings that CO2 concentrations of less than 800 ppm may be a benchmark for adequate ventilation, as shown here:

One potential target benchmark for good ventilation is CO2 readings below 800 parts per million (ppm). If the benchmark readings are above this level, reevaluate the ability to increase outdoor air delivery. If unable to get below 800 ppm, increased reliance on enhanced air filtration (including portable HEPA air cleaners) will be necessary...
Excerpt from CDC's "Ventilation in Buildings", FAQ #9

UK Scientific Advisory Group for Emergencies (SAGE)

In July 2021, UK SAGE published Ventilation and Filtration to Reduce Long-Range Airborne Transmission of COVID-19 and Other Respiratory Infections: Considerations for Reopened Schools. The document indicated that CO2 concentrations should be maintained below 800 ppm and associated this with approximately 10 liters per second per person.

Excerpt from UK SAGE ventilation recommendations

So, Which Recommendations to Advocate For?

In practice, ventilation and HEPA filtration can yield significant positive impacts when applied in schools. A study in Georgia schools titled Reducing SARS-CoV-2 in Shared Indoor Air found those that implemented both improved ventilation and filtration achieved a 48% reduction in Covid incidence. Indoor Air Care Advocates anticipates that schools may be able to outperform this metric by improving ventilation and adding HEPA filtration to target the specific ventilation rate targets cited by experts and validated in the above scientific literature. But which ones?

These recommendations reside in several different positions along the continuum of risk reduction, and they are expressed in several different units, making it difficult to ascertain which to advocate for. So, what should advocates be asking for? That's what we'll explore in our next post. Stay tuned!

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