Executive Summary
The current portable air filter setup at CS Viamonde’s La Mosaïque (LAMO) [Viamonde is the French public school board in Toronto; LAMO is its largest elementary school] does not meet the minimum set of recommendations based on the latest scientific and engineering results related to the mainly aerosol nature of COVID19.
Children in our elementary schools cannot be vaccinated against COVID. Because of this, we need to rely on ventilation to mitigate the risk due to COVID. Our children are in danger.
In classrooms with no mechanical ventilation – as is the case with LAMO – this means relying on portable air filters. And there aren’t enough of them! We must act now to increase the number of portable filter units so that we can achieve between 4 and 6 air changes per hour.
I recommend that additional portable air filter units be purchased or reallocated from schools which have sufficient mechanical ventilation to achieve 4 to 6 ACH already.
Discussion
To combat COVID we need to replenish the air in classrooms at target rate of between 4 and 6 “air changes per hour” (ACH). It means that the entire air volume in the classroom needs to be filtered or cleansed every 10 to 15 minutes. This value comes from several sources, including:
- The University of Toronto: https://www.fs.utoronto.ca/services/hvac-mechanical-utilities/covid-hvac-strategy/
- JAMA article (April 2021): https://jamanetwork.com/journals/jama/fullarticle/2779062
- Nature article (Nov 2020): https://www.nature.com/articles/s41598-020-76442-2
- ENVERID Webinar Slide Deck (Nov 2020): https://enverid.com/wp-content/uploads/2020/11/Marwa-Zaatari-enVerid-Webinar-New-Models-for-COVID-Mitigation-11-10-20.pdf
- Rhode Island Department of Health (https://covid.ri.gov/covid-19-prevention/indoor-air-circulation)
The JAMA article directly addresses the issue of classrooms and makes it crystal clear that between four and six air changes per hour is needed:
An ACH of between four and six is entirely achievable using portable HEPA filters in the classroom, but only if we augment the number of portable HEPA units.
The issue of inadequate ventilation has been brought up, recently in the August 13th Toronto Star article found here:
As pointed out by Mr. David Elfstrom, Professional Engineer and expert in ventilation, the portable HEPA units used in our classrooms, the Austin Air HM400, are not sufficient to achieve either the 4 ACH or 6 ACH targets. He recommends “3 units of the [HM400 portable air filter] pictured running on Medium speed.” [1] In this document I will independently confirm his recommendation by running the filter specifications on a range of classroom sizes that correspond to the LAMO context.
The Austin Air HM400 units that are used in LAMO classrooms are rated [2] as follows:
| Fan speed | Air Volume Rate (cubic feet per minute; CFM) | Air Volume Rate (cubic meters per minute; m3/min) | Air Volume Rate (cubic meters per hour; m3/hr) |
| High | 400 CFM | 11.33 m3/min | 680 m3/hr |
| Medium | 200 CFM | 5.66 m3/min | 340 m3/hr |
| Low | 75 CFM | 2.12 m3/min | 127 m3/hr |
Because I don’t have exact measurements for each classroom at LAMO, I am assuming a range of values from 700 square feet to 1575 square feet, with 10-foot-high ceilings. This gives us a range of room volume ranging from, in metric, 198 to 446 cubic meters.
| Classroom size | Surface Area (Square Feet) | Volume (Cubic Feet) | Surface Area (Square Meters) | Volume (Cubic Meters) |
| Small (25’ x 28’) | 700 ft2 | 7000 ft3 | 65 m2 | 198.2 m3 |
| Medium (30’ x 30’) | 900 ft2 | 9000 ft3 | 83.6 m2 | 254.9 m3 |
| Large (35’ x 45’) | 1575 ft2 | 15750 ft3 | 146.3 m2 | 446 m3 |
To determine the number of air changes per hour (ACH) possible in certain sized room at a specific fan speed setting on the Austin Air HM400, we use the following equation:
Even in the very best-case scenario, with the smallest classroom and the (noisy) highest fan speed, we only produce enough air flow to change / filter / cleanse the air at just over half (57%) the best recommended rate: 17 minutes per change, rather than 10 minutes per change, as you can see in the table below.
| Classroom size | Fan Speed | ACH possible with one HM400 Unit | Percentage of 4 ACH Target | Percentage of 6 ACH Target | Minimum Number of Additional HM400 portable filters required |
| Small | High | 3.43 ACH | 86% | 57% | 1 more unit |
| Medium | High | 2.67 ACH | 67% | 44% | 1 more unit |
| Large | High | 1.52 ACH | 38% | 25% | 2 more units |
But it’s likely that the classrooms are larger than the smallest value used and the fan noise at highest speed is unsustainable in a classroom. So, more realistically, at medium speed for a medium-sized classroom, we only manage to achieve a change rate of 22% compared to the target of 6 ACH. That means that the air is changed once every 45 minutes, rather than the desired safe target of once every 10 – 15 minutes, as shown in the table below.
| Classroom size | Fan Speed | ACH possible with one HM400 Unit [3] | Percentage of 4 ACH Target | Percentage of 6 ACH Target | Minimum Number of Additional HM400 portable filters required |
| Small | Medium | 1.71 ACH | 43% | 29 % | 2 more units |
| Medium | Medium | 1.33 ACH | 33% | 22% | 3 more units |
| Large | Medium | 0.76 ACH | 19% | 13% | 5 more units |
As we can see from the table above, for small and medium sized classrooms and at a realistic fan speed (with respect to noise), we need between two and three more portable HEPA units to achieve between 4 and 6 ACH. This value of two to three more portable HEPA units corresponds to the recommendations made by Mr. David Elfstrom in the context of the recent Toronto Star article on the status of ventilation in Ontario classrooms. It also corresponds to the recommendations for a 900 sq foot classroom with 12 students in the ENVERID Webinar by Dr. Marwa Zaatari.
We need more portable air filters in the classrooms!
Update (Aug 24 10am)
The values I based the flow rates on are inflated. The specs on the Austin Air website are for unfiltered air. The “true airflow rate” is much lower (~60% of posted value): 250, 125 and 40 CFM instead of 400, 200 and 75. (source & personal correspondence with Mr. Josh Pasco @ Austin Air). What this means is that we need to increase the number of portable air filter units even more. So, instead of two to three units, we need between three and five for a medium classroom.
As of 5:30pm: note, as per [3], that ACH values in the medium speed table are half those originally posted. Now updated to 1.71, 1.33 and 0.76 ACH.
Conclusion
The current portable air filter setup at CS Viamonde’s La Mosaique does not meet the minimum set of recommendations based on the latest scientific and engineering results related to the mainly aerosol nature of COVID19.
Children in our elementary schools cannot be vaccinated against COVID. Because of this, we need to rely on ventilation to mitigate the risk due to COVID. Our children are in danger.
In classrooms with no mechanical ventilation – as is the case with LAMO – this means relying on portable air filters. And there aren’t enough of them! We must act now to increase the number of portable filter units so that we can achieve between 4 and 6 air changes per hour.
From a non-expert perspective, I recommend that additional portable air filter units be purchased or reallocated from schools which have sufficient mechanical ventilation to achieve 4 to 6 ACH already.
James Andrew Smith, PhD,
Associate Professor, Lassonde School of Engineering
York University
[1] Discussion on the Toronto Star article: https://twitter.com/DavidElfstrom/status/1426885261267451910
[2] Air flow rates for the HM400 are published on the Austin Air website: https://www.austinair.ca/products/healthmate-hm400-standard-air-purifier
[3] Aug. 24 5:30pm. Fixed typo in ACH values at medium speed. ACH values were originally for high fan speed (3.42, 2.66 and 1.52). Actual values are medium speed are 1.71, 1.33 and 0.76 based on CFM values stated on manufacturer’s website. See Aug 24, 10AM update: the values should be reduced to about 60%.
James Andrew Smith is Professional Engineer and an associate professor in Electrical Engineering and Computer Science Department in York University’s Lassonde School. Originally from Québec, James has degrees in Electrical Engineering from the University of Alberta and a PhD in Mechanical Engineering from McGill. He did a post-doc from 2006-2008 at the Institute for Sports Science in Jena, Germany. His engineering research background includes galloping robots, human birth and clothing. James believes that we can improve the way we teach. He 2018-19, he lived in Strasbourg and taught at the INSA Strasbourg (France) and Hochschule Karlsruhe (Germany) while on sabbatical with his wife and kids. Some of his other blog posts discuss the family’s sabbatical year, from both personal and professional perspectives.
It’s important to point out that I have an interest in this, generally, but do not have a technical background in ventilation or PPE, in spite of my mechanical engineering degree or any work I’ve done in biomedical engineering — so any opinion I express on this topic is not officially within my area of technical competence and should not be taken as a “professional engineering recommendation”. If you’re looking for proper professional advice on PPE, HVAC, etc., hire a registered Professional Engineer who is trained and who practices in those domains.