Reducing COVID Risk With A CO2 Detector

    Good ventilation is sometimes mentioned, without specifics, by public health officials as a way of reducing the risk of contracting COVID, as it prevents the concentration of aerosols in indoor spaces. This post lays out how we use a CO2 detector to get a handle on how well ventilated the Music Room is during rehearsals and classes.

   The Music Room is a non-profit community learning center based in the 100 year old Gill Hardware building on East Main Street in Orange, VA. Before the pandemic we hosted a community orchestra, a string program for school age children, MusikGarten for babies and pre-schoolers, and several small music groups. When COVID hit in the spring of 2020, we shut down completely. In late October we began a soft reopening, limiting activities to about a dozen people at a time, with masking and social distancing. We also allowed only one person at a time in the general rest room area, since in that smaller space, distancing isn’t always possible.

   I kept seeing where outdoors was better than indoors because 1) sunshine degrades the virus and 2) lots of fresh air disperses aerosols containing COVID, which prevents high concentrations of those aerosols in the air people are breathing. While we couldn’t bring the sun inside, we could bring in fresh air.

  The setup that works best for bringing in fresh air has two components. The first is to have two industrial fans upstairs. One at the top of the stairs blowing air from the downstairs into the upstairs, 

and another blowing air out one of the windows on one or the other side of the building, depending on the wind direction outside. 

The second component is to prop open the front door, which is at the opposite end of the main room from the stairs. 

The result is a constant flow of fresh air down the length of the room. This photo was taken just inside the front door, where the air enters, looking towards the stairs, where the air exits.

   At first I let in as much air as possible while keeping the room warm enough to make music. Then in early November, this article popped up. A trainer at a gym tested positive after he had worked with over fifty athletes over the course of a week – and none of them got the virus – and good ventilation at the gym was posited as the reason. The article recommended getting a CO2 detector to gauge the freshness of the air. The less CO2 in the air from exhalations, the less the chance of inhaling viral aerosols.

  I immediately ordered this CO2 detector and have used it at the Music Room for every gathering since then.

The photo below shows the detector on the newel post of the stairs and the chair where I usually sit to keep an eye on it during gatherings. 

   Back in November, when I first entered the room after it being unused for several days, the CO2 level was usually around 440 parts per million. During gatherings it would rise, and by adjusting how open the front door was, I could usually keep the level around 465 ppm to 480 ppm. Every so often, probably due to wind shifts outside, it would rise to 500, and I'd just open the door more to bring the level back down.

    In January and February, the baseline level I'd find on entry went as low as 390 ppm, and more usually in the 400 ppm to 415 ppm range. There was a correlation between strong winds from any direction and lower CO2 readings. When the baseline reading was lower, it was possible to keep the occupied readings lower as well - in the 445 ppm to 465 ppm range. 

    Having figured out what the best I could do was, I went online to see what others might be doing. Interestingly, searches on ventilation returned no hits from public health authorities like the CDC, FDA, or WHO. All these quotes below come from academics and businesses:

"Carbon dioxide CO₂ levels outdoors near ground level are typically 300 ppm to 400 ppm or 0.03% to 0.040% in concentration.

Carbon dioxide CO₂ levels indoors in occupied buildings are typically around 600 ppm to 800 ppm or 0.06% to 0.08% in concentration. You'll find this data in many indoor air quality articles and books and it's consistent with what we find typically in our own field measurements.

Carbon dioxide CO₂ levels indoors in an inadequately vented space with heavy occupation is often measured around 1000 ppm or 0.10% in concentration. I have measured levels around 1200 ppm in occupied basement offices in a hospital where the staff worked in an area which had no decent fresh air intake into their ventilation system."

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"Miller cites a 2019 study on a tuberculosis outbreak in Taipei University, Taiwan, where many rooms were poorly ventilated and reached CO2 levels above 3,000 parts per million (ppm). When engineers brought levels down to under 600 ppm the outbreak stopped. “According to the research, the increase in ventilation was responsible for 97% of the decrease in transmission,” said Miller, before going on to recommend a CO2 target of 600 ppm. . . . 

. . . . Prof John Wenger, director of the Centre for Research into Atmospheric Chemistry in UCC suggests a target of 1,000 ppm if CO2 is being used as a proxy for Covid in classrooms, and argues that room level transmission is “the key. It’s in the air, and it can fill a room. The amount of the virus in the air can accumulate, and we get an increased exposure. If you’re indoors, in a poorly ventilated room for a long time, then you’re at quite a high risk even if you’re distanced, because the air moves around.”

* * * * * 

"Given clean background air (no motor vehicles) CO₂ is about 400 ppm and exhaled breath is 4% (40,000 ppm). The rest is mass balance algebra. . . .

. . . . An indoor CO₂ level of 5000 ppm (the OSHA standard for workplaces) would mean that 11% of what you inhale has passed through someone’s lungs. An indoor CO₂ level of 700 ppm (a non mandatory design standard) means that 0.7% of your inhalation is recycled exhalation."

Based on these bits of context, we seem to be doing as well as we can at the Music Room. As I write this, the latest edict from the state of Virginia increases the limit on indoor group sizes from 10 to 50. Between that and the number of people vaccinated, the next step will be to see how to go about having small audiences for open rehearsals or recitals. My hope is the the warmer weather will allow letting in more fresh air to keep the CO2 levels down with the higher number of people. 

Followup post here.