Why is this interesting? - The CO2 Edition

Noah here. Back in August, Abe Burmeister wrote a WITI about ventilation. It was before the CDC had acknowledged airborne transmission in any significant way, and is something I’ve returned to often over the last six months. It opened like this:

This is the time of ventilation, the forgotten V in your HVAC system (Heating, Ventilation and Air Conditioning). There is growing consensus that COVID-19 spreads most virulently via prolonged airborne exposure indoors. Outdoor transmission definitely happens, but is relatively rare—contract tracing of super spreader events almost always points towards indoor spaces where individuals linger around. Ventilation is the term for bringing outdoor air indoors, and most Americans spend 90% of their lives inside buildings. Ventilation can bring the safety of outdoor air indoors, or be the mechanism that shuts it off. As we try to reopen schools and offices, ventilation is life and death, and it is almost entirely invisible.

Since Abe’s edition, the conversations around the role of ventilation has grown substantially. In one example after another, we see that the primary cause of spread is prolonged indoor exposure, particularly without masks. Part of Japan’s success is attributed to its message of avoiding the “three C’s”: “closed spaces (with poor ventilation), crowded places (with many people nearby), and close-contact settings (such as close-range conversations).” Ventilation solves so many of these issues because it ensures air, including the stuff that contains tiny viral droplets, is being frequently replaced.

The power of ventilation was well-articulated in a November CNN article about a Crossfit gym that took safety precautions that ultimately paid off when one of the coaches contracted the virus, and no one else got sick. At the center of those precautions were decisions about spacing equipment and, critically, keeping large doors open. As Linsey Marr, a professor of civil and environmental engineering and member of the gym, explained, “I did the calculations on how big the space was, what the typical wind speeds were in the area and if the doors were open what would the resulting ventilation be?” With the doors fully opened, she found the airflow more than sufficient.

Why is this interesting?

One of Marr’s tools to measure the effectiveness of her ventilation interventions was a carbon dioxide detector. As Marr explains in the piece, “Carbon dioxide is exhaled breath and is a good indicator of how much viruses might be building up in the air.” In other words, CO2 is a good proxy for understanding the relative safety of a space. High CO2 levels mean there is a lot of built-up breath, which might contain the aerosolized virus, while lower levels (in a space occupied by people) means air is being circulated in and out and represents a much safer environment. It also explains why the outdoors is so safe: with air always moving around, there’s little chance of breathing in someone else’s exhales for an extended period.

Here’s an explanation from a Smart Buildings article published in November: 

If we are in a room with several people, the measurement of the CO2 concentration provides a measure of what percentage of the air we inhale which consists of air that has already been exhaled by other people. The mass balance shows that a measured CO2 concentration of approx. 1200 ppm means that almost 2% of the air in the room has already had lung contact at least once. At this level, every 50th breath that a person takes in this room consists of air that has already been exhaled. The resulting specific corona infection risk is more complex to quantify, as it depends on various factors that are currently still being intensively researched. Notwithstanding these caveats, it is clear that CO2 measurement offers a cost-effective solution for classifying the current risk from potentially infectious aerosols.

Since reading about CO2 meters a few months ago, I bought one (the Aranet4, which is portable and Bluetooth) to try out. I carry it anytime I need to be inside for any reasonable amount of time. It updates measurements every two minutes and can help as an input for the safety of the environment. In a few months of using it, I’ve found that small interventions like an open window, or propped-open door can have huge effects on the CO2 levels. In my small home office, for instance, I can cut the levels in half by just opening a window part way for a few minutes. I now carry it around far less, as I’ve become more confident I can spot the signs of a well-ventilated space.

Thankfully, I’m also starting to see these ventilation ideas spread out into the world. Just last week I was looking to sign my daughter up for a sports class and the website had a full safety PDF that included information about the air changes per hour (ACH). The data essentially showed that the space was nearly as good as being outdoors. When combined with mandated masks for all children, I felt comfortable enough to sign her up. (NRB)

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Partner post: Ghia x WITI

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Quick Links:

• On grand entrances in New York (CJN)

• In case you want to learn more, here are a few of my favorite academics on ventilation Twitter: @linseymarr (Virginia Tech), @j_g_allen (Harvard), @ShellyMBoulder (University of Colorado), @kprather88 (UC San Diego), and @jljcolorado (University of Colorado)

• The delights of Sea Chantey Tik Tok (CJN)

Thanks for reading,

Noah (NRB) & Colin (CJN)

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