This post explains the quantitative evaluation by Indoor Air Care Advocates to sift through existing recommendations to arrive at a coherent and effective advocacy position regarding ventilation targets for schools.
In their Position Document on Infectious Aerosols, the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) indicates that "What seems indisputable is that existing minimum outdoor air ventilation rates are significantly lower than levels recommended for infection control."
If the 10 cfm/person typical of ASHRAE Standard 62.1 and International Mechanical Code are inadequate for classrooms, then what is enough to assert control over the school staffing shortages, increased chronic absenteeism, learning loss, and academic performance we currently see?
School air quality advocates need to lead communities with a unified, consistent, and actionable messaging. This means making sense of a diversity of quantified recommendations and science to settle on a coherent expectation to communicate to schools. Those recommendations bucket as follows:
6-12 air changes per hour (ACH)
800 ppm of CO2
Advocates have to balance two primary factors in choosing advocacy positions. First, advocates will be best supported in advocating for the recommendations that are issued by the greatest number of authoritative experts. Second, advocates must choose the advocacy position that will achieve the intended result: advocating for too little can result in weak outcomes that damage the very credibility and effectiveness of future advocacy.
An Exercise in Comparing Apples to Oranges
Experts and authoritative organizations have issued recommendations and guidance in diverse units. As explained in our previous article, Dispelling Dimension Disorientation, the following units are most commonly used:
Air change rate in air changes per hour (ACH)
Airflow per person e.g. in cfm/person
Carbon dioxide concentration, expressed in parts per million (ppm) of CO2
These units do not all directly relate to one another without making some assumptions about classroom size and occupancy. But there is no standard classroom size or occupancy for US K-12 schools. How, then, can one quantitatively compare dimensionally disparate recommendations?
Here, we define an ensemble of five hypothetical classrooms representing the smallest, largest, and average occupancy and room sizes we've come across. We then apply the equations and models from our post entitled Clearing Up Conversion Confusion to each room, comparing each recommendation against the others in turn.
Hypothetical Classroom Sizes and Occupancies
The K-12 classroom sizes we've heard of from facilities managers and parents across the nation range between 400 square feet to about 900 square feet. A common moderate class size in this range is 750 square feet. Classroom ceilings range from 8-12 feet; we've selected ten foot ceilings for all our hypothetical classroom sizes.
Meanwhile, we've selected 14, 20, and 30 students as the minimum, average, and maximum for our analyses. Adding in at least one teacher, the occupant counts we therefore selected were 15, 21, and 31.
We combined these into the following five hypothetical classroom sizes and occupancies:
V [cu. ft]
Carbon Dioxide Assumptions
In order to convert between ventilation rates and carbon dioxide (CO2) concentrations, it is necessary to make assumptions about ambient CO2 concentrations as well as the CO2 generation rate attributable to students and faculty.
For ambient CO2 concentration, we selected 415 ppm based on the 2022 observation from the Mauna Loa Observatory. For student and faculty CO2 generation rates, we have selected the rates disseminated by the Harvard TH Chan School of Public Health in their 5-step guide to checking ventilation rates in classrooms:
Indoor Air Care Advocates found the CO2 generation rates in the T.H. Chan School of Public Health to provide accurate estimates of airflow from outdoor air in a Berkley Michigan classroom based on CO2 concentration within a few percent of the information available from an open records request regarding the univent system outdoor airflow settings used there. This real-life validation suggests that the figures disseminated by the public health experts at the T.H. Chan School appear to be applicable for our purpose.
The results of the comparison can be found in the following spreadsheet:
The results are described subsequently.
Results in Ranked Order
Various: At Least 6-12 ACH
The bottom end of 6-12 ACH (namely 6 ACH) met or exceeded every other recommendation within a tolerance of 10% except in the case of cramming the highest student occupancy into the smallest room--a scenario that we already know undermines both educational and public health goals. In particular the only recommendation that 6 ACH fell slightly below in the other scenarios was the 30 cfm/person recommendation, providing only 27 cfm/person in the small, low-occupancy hypothetical scenario.
The high end of 6-12 ACH (namely 12 ACH) exceeded all other recommendations except one recommendation in one scenario: the recommendation for 30 cfm/person as applied to the worst-case classroom.
Splitting the difference of 6 and 12 ACH (i.e. 9 ACH) likewise exceeds all other recommendations except 30 cfm/person.
Lancet: At Least 30 cfm per person
The Lancet's Best recommendation on an airflow per person basis is to provide 30 cfm per occupant in classrooms. This recommendation meets or exceeds all the recommendations in all classroom scenarios studied here, except in the Low-Occupancy Large scenario and the moderate-occupancy / average-size classroom scenario where it fails to provide at least 6 ACH.
WHO: At Least 10 liters per second per person
The WHO's recommendation to provide 10 L/s/person amounts to about 21 cfm/person, which is less than the Lancet's airflow per person recommendation (30 cfm/person). In addition to falling short of the Lancet airflow per person recommendation, 10 L/s/person fails to meet the bottom of the widely-issued recommendations for at least 6-12 ACH in all but the worst case classroom, where 31 people are packed into a hypothetical 400 square foot space.
CDC: At Most 800 ppm of CO2
The CDC's benchmark for good ventilation is 800 ppm of CO2. In all five hypothetical classroom scenarios, this falls slightly behind the CO2 level that would be achieved by adhering to WHO recommendations (~750 ppm), far behind Lancet recommendations (~650 ppm), and in almost all cases fails to make at least 6 ACH. The only case where 800 ppm met within 10% of another recommendation was in requiring 8 ACH in the worst-case classroom, where 31 people occupy a 400 square foot space.
Recommendations in terms of airflow per person and carbon dioxide concentration are resilient to high occupancy, but thinking critically about where this comes into play, we realize these scenarios are to be avoided to begin with: cramped and over-occupied classrooms. Recall from our previous post that the American Industrial Hygiene Association acknowledges that in these scenarios, higher air change rates are called for. Therefore, recommendations on a basis of air change rate are not uniformly devoid of accounting for occupancy as claimed by some.
Even if an advocacy position had to be chosen to accommodate the undesirable educational condition of over-crowding, a mere 9 ACH is within the recommended 6-12 ACH and exceeds all but one of the other salient quantified recommendations for such a space. In fact, the only recommendation that exceeds 12 ACH in the worst-case classroom is 30 cfm/person, by the Lancet.
In conclusion, the advocacy position from Indoor Air Care Advocates as of this writing with regard to quantifiable ventilation is twofold:
Target at least 6-12 ACH; OR,
Target at least 30 cfm/person.
6-12 ACH represents the overlap of recommendations made by the greatest number of authoritative experts. Nevertheless, air change rates are widely argued against by building owners and professionals as a valid metric for a ventilation target because they fail to directly take occupancy into account. The fact is that 6 ACH exceeds the existing occupancy-based recommendations in all but the absolute worst case, a cramped classroom. Nevertheless, 30 cfm/person as recommended by the Lancet is somewhat comparable with 6 ACH in many cases, and meets or exceeds the airflow per person of ventilation air provided in the Goethe University study that found an 82.5% reduction in the inhaled dose.
Our conclusion is that if advocates receive pushback for recommending the most widely-recommended and scientifically supported ventilation rate target for classrooms of 6-12 ACH, then the Lancet's 30 cfm/person recommendation serves as the strongest recommendation to fall back upon. Incidentally, 30 cfm/person coincides with approximately 650 ppm of CO2 in the hypothetical classroom spaces we included in our analysis.
Conclusion of Series
We hope this series helps advocates and implementers understand:
How airflow is quantified (via Dispelling Dimension Disorientation);
How to convert between different ventilation rates (via Clearing Up Conversion Confusion);
What the salient quantified standards are (via Eliminating Recommendation Consternation); and,
Why Indoor Air Care Advocates holds the advocacy positions it does (via this post).
Ultimately, Indoor Air Care Advocates' advocacy position is that schools should provide at least 6-12 ACH or at least 30 cfm/person, as these are the most widely made, most scientifically backed, and strongest recommendations we have encountered. In practice, we tend to ideally target around 9 ACH when specifying protections for specific classroom environments, conferences, or other indoor areas where large numbers of people will congregate.
The bottom line is that ventilation that (1) is unquantified in one of the three main units described here, or (2) fails to meet any of the recommendations made by experts, is categorically indefensible.
We hope this blog series serves to communicate the strong evidence basis for advocacy positions that align with the most widely-overlapping expert recommendations for schools.