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School COVID-19 safety Q&A with air quality expert Prof. Jeffrey Siegel (July 26)

Last Updated Jul 29, 2021 at 11:24 pm EDT

The Ontario government and science advisors are already releasing plans and recommendations for keeping children safe when they return to school in September. But how should parents, teachers and students gauge how effective measures like air filters and distancing will be at protecting child and community health? Here is your chance to ask.

Air quality and building design expert University of Toronto Professor Jeffrey Siegel answered your school-related COVID-19 questions in a LIVE interview on Monday, July 26, on our YouTube channel as well as here on our website.

Siegel is part of the Dalla Lana School of Public Health and the Department of Physical & Environmental Sciences. He is also a member of U of T’s Building Engineering Research Group.

Here are a few questions Professor Siegel addressed:

(Questions were moderated and have been edited for grammar, punctuation and clarity)

Q: Why is air quality and ventilation so important to talk about right now, when it comes to COVID-19 in schools?

A:Ventilation has always been really important to talk about, and one of the big reasons why we haven’t talked about it, is because a lot of the health effects we see from ventilation, are things that happen a long time after exposure. We get exposed to all kinds of things in buildings, and that increases our risk of cardiovascular disease, of lung cancer, of a whole variety of things, but it happens much later in time. And so, with COVID-19, obviously there’s not that delay, so that’s why we care about it now, but I would argue that we always should have cared about it.


Q: When it comes to testing air quality. Do we know how many particles of COVID-19 have to be in the air for it to be infectious?

A: We don’t know, but even if we did know, that’s actually a really hard thing to measure. Not only does it matter whether there’s COVID in the air, but it matters whether it’s able to convey infection, and then it matters what sized particle. If it’s in big particle, it’s going to behave differently from a small particle, and end up in a different place in our respiratory system. Not to mention, there’s wide individual variation in how susceptible we are to the disease. Then we’re seeing some of the COVID variants certainly spread more easily.

So let’s do everything we can to treat the air, and worry less about measuring it. I would point out that the worst that happens is we have good indoor air quality and all the good things that come from it. In school we see better cognitive performance, reduced absenteeism, better performance on standardized testing, better learning.


Q: Are there any enforceable standards for indoor air quality including ventilation and filtration in schools?

A: The short answer is the way most ventilation codes are applied are at the time of construction, so if a school was built in 1940, the ventilation code that it’s built to was whatever code was in place in 1940. The reality is our buildings don’t keep up with standards.

Unfortunately we have no indoor air quality regulations and that’s true, almost everywhere in the world. The challenge is that our indoor environments are generally private indoor environments, and so it’s really hard. It’s not like outdoor air equality, where we the government can make a regulation and people have to meet that regulation; you can’t tell people you can’t cook, you can’t smoke, you can’t do all the things that generate air pollution in your building.


Q: When it comes to portable air filters, are there particular brands or features parents or educators should be looking at for their classrooms, and are there any features which aren’t as useful?

A: I am a big fan of putting HEPA filters in every classroom, They work, they don’t depend on [a school’s] central system that maybe has to reduce the airflow because it gets really cold outside, or maybe there are distribution issues and not every classroom gets the right amount of fresh air, or maybe that filter is at the end of its life and not performing very well. [In comparison,] HEPA filters, they’re simple, they’re easy.

There is a lot of what I’m going to call unproven technologies. So these are things like ionization, UV light in some forms (in some forms it’s very effective but in portable units, there’s much less evidence of effectiveness) plasma photocatalytic oxidation, hydroxyl radical air cleaners, all those things have not been proven. I understand that the manufacturer will show you all kinds of customer reports and so on. Let me just say you should have a lot of caution because a lot of times those don’t actually apply to real buildings.

The second part of things is that in some cases they can generate by-products as they operate and those by-products are going to be harmful. I think that potential for harm is pretty small, but we also know that a lot of them don’t work very well. So stick with what works.


Q: Is a single portable air filter machine per classroom going to provide protection for staff and students? Are these not designed for home use? Classrooms have 25 people in them. And as well, how often should those filters be cleaned, and if they’re not does that contribute to poor air quality?

A: I wouldn’t necessarily advocate taking a unit that’s designed for a small room, small be and bring it into the classroom, You have to do some sizing calculations to make sure it’s sized appropriately. And things in schools get used quite heavily so you want to make sure it’s something relatively robust that can take the stresses that are put on it.

Most reputable air cleaners are sold with something called the clean air delivery rate.  CADR is a measure of how much clean air the unit delivers. Someone like me does a sizing calculation says, ‘Okay, that’s a pretty big unit, we can get away with one of them, or, we’re going have two smaller units in this classroom. Ask the school board, ‘What is the CADR of the units?’ If you do a little bit of googling you’ll see some online calculators, and with the room volume you can calculate how many air changes are going through the device.

You can get to any or change equivalents you want, it’s just a question of how many units you need in the space, and how much noise you’re willing to tolerate.

On the question of when you should change the filters, change it when the manufacturer tells you to change it, and that’s going to be different for every unit because filters are designed to have different amounts of kind of holding capacity for material before they need to be changed. Also filters as they load up and get dirtier, they have a bigger pressure drop in some units. That means they move less air, so you’re getting diminished performance as it ages.

The economics of a portable filter, it’s not the cost of the filer. You go to the store and you look at the price tag and some of them can be a few hundred dollars. It can be expensive, but actually the real cost is in those replacement filters. When I talk to schools I say buy a year’s worth of replacement filters for all the machines you’re buying. So at least you value the economics correctly. Next, part of that engagement piece is saying, ‘Oh yeah I’ve got this filter, I need to change it.’ It’s one more thing for the list of the facility staff in that school.

Watch the full interview with web writer Dilshad Burman in conversation with Prof. Jeffrey Siegel in the video above.

Scroll through some of the questions submitted to this session below.

Note: questions were moderated before appearing in the chat window