Let’s Talk About Indoor Air Exchange Rates in 2022: Implications for COVID and Beyond

In grad school, I studied outdoor air pollution and how it can affect our health. Although I’m not an expert on indoor air pollution, I do think about indoor air quality a lot. I’ve always been the type of person to check outdoor air quality and enjoy a cross-breeze.

Indoor air quality has been on my mind a lot recently because of the COVID pandemic. We’re spending a lot of time indoors at home, away from public gathering spaces, but we’re also worried about going into indoor spaces like stores and classrooms because of COVID exposure risk.

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With so many people confused about guidelines, having trouble getting COVID tests, not getting vaccinated, and not being able to find the right kinds of masks to wear, I’ve been wondering if there could be any systemic, infrastructure-level ways that could minimize risk of COVID exposure: something that doesn’t depend on individuals’ compliance levels.

In the context of COVID, gathering indoors has always been seen as more risky than gathering outdoors. However, there are plenty of situations where being indoors is pretty necessary, such as classrooms, offices, and retail spaces. What is our understanding of how COVID spreads indoors, and how do our current guidelines for indoor air exchange rates deal with that? Can we rethink those established guidelines and update them to include viral transmission?

We’re Indoors…A Lot.

Did you know that according to the EPA, we spend 90% of our lives indoors?1 Now, with the pandemic, that percentage feels closer to 100%. Back in the “before times”, it was pretty normal to not really think about indoor air quality or indoor air pollution. We just assumed that if it smelled weird or felt musty, running the fan or opening the windows was enough.

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Air purifiers have exploded in popularity in recent years. Wildfires have been popping up all over the globe, really affecting air quality both indoors and outdoors, and making people realize that clean air isn’t guaranteed. In addition, idling trucks, neighbors that smoke, and allergies are other really legitimate reasons to want to invest in an air purifier. If you want to learn about 7 things to consider when you’re buying an air purifier, I have a blog post about it!

Now with COVID in the picture, we have really to reexamine how safe our indoor air is, not just for those of us who hate the smell of smoke or have allergies, but for everyone. In fact, the standards by which air exchange rates were established way back in the day were to meet basic air quality standards, rather than to minimize infections.2

So, it’s timely and essential that we update our air exchange standards to not only keep the air fresh from odors and all the stuff we have already been concerned about, but COVID too.

Health Effects of Indoor Air Pollution

Something that’s been bothering me a lot recently is hearing all these stories about folks having to go back to work and school with the Omicron variant going around and not feeling safe. Teaching in an unventilated room for hours at a time, working in offices with coworkers that have been doing who knows what, and working in restaurants and other retail spaces that don’t have windows or proper ventilation: it can really damage your health, and guess what, it’s been the case even before COVID.

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Indoor, or household air pollution contributes to close to 4 million deaths worldwide according to the World Health Organization.3 A lot of that is due to the way food is cooked indoors, directly using fuel, in countries that don’t have modernized kitchens and cooking setups.

Even if we don’t cook our dinners directly on fires while burning fuel, there are a ton of other indoor air pollutants that are very common, even in modern homes. They include things like cigarette smoke, gases and particulate matter from cooking and candles, aerosols, fumes from various household chemicals, radon, dust, dirt, etc. Specific molecules that are responsible for indoor air pollution include carbon monoxide, nitrogen dioxide, sulfur dioxide, and ozone.4

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Another source of indoor air quality concern for a lot of folks are allergens. They can really affect one’s quality of life! This can be in the form of animal dander, house dust/dust mites, pollen, and different types of mold. Most people who have severe allergies work with their doctors and allergists to figure out ways to minimize their exposures, through a combination of tactics such as cleaning surfaces, using specific bedsheets and linens, using air purifiers, and medications.

But COVID doesn’t care if you vacuum religiously, clean with the nicest surface cleaner, don’t use candles, or abstain from smoking. COVID seems to spread through the air, so it’s important to know what we can do to detect it and eliminate, it regardless of who’s around.

The General Confusion Around COVID Protocols

With COVID still running rampant, it’s now or never to reconsider the infrastructure around indoor air pollution. I’ve seen efforts of it throughout my city during the pandemic – shop doors propped open, a cheap plastic fan running in the corner, and maybe in the really hip orthodontist’s office, an air purifier outfitted with a HEPA filter.

It’s important that folks are trying, but the thing is, the general public doesn’t know how much air per hour is being exchanged by doing those things, and if it’s really changing the levels of COVID in the air. In addition, we really haven’t been taught how to protect ourselves from COVID indoors aside from masking and vaccinating, but even the messaging around masks has been inconsistent.

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First, it’s that masks don’t work, then it’s to wear whatever masks you can but not N95s because they’re only for certain professionals, and then it’s to wear N95s. It’s hard to know what we have to do to protect ourselves indoors when we’re still learning about how COVID spreads, too. With every passing month, it seems like there are new guidelines about types of masks to wear, how long to quarantine, if we have to test after quarantine or not, what symptoms look like, etc. Plus, we all know how controversial vaccines have become, and it’s so sad that that’s the case.

Science is constantly being updated and refined. Of course, changing policies to reflect what’s been discovered is very important. However, it’d be great to be able to rely on something long-term that’s going to influence everyone, regardless of personal choices and political beliefs. Improving ventilation and air exchange systems is sounding more and more like that solution.

Antiquated Air Exchange Systems

Outdoor air can enter buildings through natural ventilation, infiltration, and mechanical means. Natural ventilation refers to open windows, doors, and other ways that let in air. Infiltration is when outdoor air enters buildings through cracks, joints, spaces, and other unintentional ways. Mechanical includes heating, ventilation, and air conditioning (HVAC) units, as well as fans that blow air out in one direction, like those found in kitchens and bathrooms.

Try to find these setups in your workplace or in your own home. Those fans in your kitchen and bathroom are really useful for quickly letting out smoke, fumes and odors from various household activities!

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Actively exchanging air to avoid infections is already a thing in nursing homes, hospitals, and planes. Air exchange is crazy high in planes! The air is exchanged 10-30 times an hour, using a mix of outside air and HEPA filters.5,6 The risk of catching COVID on planes has always been pretty low, but concrete information about that is still being established and new variants which are contagious to different extents can influence those results.

The American Society of Heating, Air Conditioning, and Refrigerating Engineers (ASHRAE), which sets standards for things related to heating, ventilation, air conditioning, refrigeration and allied fields, recommends offices exchange air 2-3 times, schools 5-6 times, and hospitals 6-12 times per hour. It’s hard to know if schools and office buildings adhere to this, though. In homes, it’s supposed to be 0.35 times per hour. How’s the airflow in your home?

Also, in the context of COVID, instant in-air COVID abundance measurements aren’t a thing yet (although they are in development), so we really can’t be sure about COVID levels around us and how things like open windows, AC, air purifiers, and HVAC systems influence it.

Is Carbon Dioxide a Legitimate Proxy for COVID?

One way that researchers are estimating COVID levels is by using indoor CO2 levels, because CO2 is directly expelled from our mouths like the virus. CO2 levels outdoors are around 400 ppm,7 and indoors are generally 800-1000 ppm according to ASHRAE recommendations and estimates from the 90’s.8,9

Currently, studies are being done to evaluate how many times air is exchanged in classroom settings, those with 1 open window, and with a cross-breeze facilitated by 2 open windows. Because these studies are still preprints, I won’t cite them here, but the results are all pointing towards the fact that air exchange isn’t great even if we might think it is. We need to see more research on how air exchange occurs under various conditions and if COVID abundance is affected by it in the same way that CO2 levels are.

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The risk with using CO2 levels to estimate COVID levels is that if CO2 levels appear low, it might give a false sense of security that the COVID levels are low, too, when they might not be. Still, having quick and easy-to-read CO2 monitors that are accessible throughout schools and offices may give us insight into how well a space is ventilated.

Being aware of CO2 levels and improving indoor air quality can have health benefits that go beyond COVID. It’s been shown that CO2 and other indoor air pollutants in poorly ventilated school spaces lead to other health and cognitive issues!10,11

I also found some interesting news articles from 2021 about how schools in New York City had been issued air purifiers to use in classrooms to help minimize COVID transmission. $43 million was spent on the devices, which didn’t even have HEPA filters. Even with 2 air purifiers running, the air exchange in one of the classrooms tested would only be 1.8 times per hour, according to an Illinois Institute of Technology professor’s calculations.12 This news perplexed me, because clearly, the decision to spend so much money on air purifiers for schools was based on very little scientific information.

One Harvard study from 2021 found that indoor air had to be exchanged through any method, including open windows, HEPA filters, and MERV 13 filters (both of which I explain in my 7 things to consider when you’re buying an air purifier post) 4-6 times per hour to reduce far-field airborne transmission of COVID in smaller spaces like retail shops and classrooms.13

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More research is necessary to confirm the above results. Changing the entirety of the air in a classroom or retail space 4 to 6 times every hour sounds like it’ll take a decent amount of air movement and active circulation! I don’t know what that’ll take, or what percentage of classrooms and retail spaces actually achieve that level of air exchange.

I just hope it gets talked about more and becomes a priority for infrastructure-related improvements in 2022 and beyond.


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If the way air was exchanged in buildings was upgraded, we could really help people who work and learn indoors. Those beneficial effects will easily extend beyond political lines and differences in individual effort.

Air is air, and hopefully with updated HVAC systems, more window-open time, and other circulation methods, it’ll be exchanged at a high enough rate that it’ll reliably suppress COVID transmission.

There are some roadblocks though, such as the fact that it’ll cost money to update decades-old air exchange systems set in place, noise levels being too high, and energy efficiency.

As we move towards this “new normal”, it’s really important to question long-standing guidelines about indoor air quality and update them to reflect current conditions.

  • How do we increase air exchange in residential homes, classrooms, and office spaces?
  • How do we measure COVID levels in the air and quantify improvement in air quality?
  • Can we develop some sort of handheld or structurally installed device that measures COVID levels in the air?
  • How will those in power be convinced that putting in a lot of money to upgrade our air exchange systems is worth it?
  • Can we get better, updated indoor air quality standards based on modern research?

I can’t answer these questions, and I’m sure there’s a ton I didn’t cover, but I really hope they get addressed before this critical COVID wave passes, COVID gets even more “normal”, and more pressing issues take over.

What do you do to allow for air exchange in your space? Are you like me, and always have the windows open if the air quality outside is good? Do you like to use an air purifier too? I’ll keep an eye out for updates on this topic!

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