So last week I ordered and received a carbon dioxide meter. It allows me to measure the CO2 concentration in parts per million for any space. On Friday, I went about a not particularly scientific but quite interesting analysis of our indoor spaces in the AMP Building.
The idea behind this is that humans exhale CO2, so measuring the concentration in a given room gives a bit of a window into how much ventilation is helping to keep rooms relatively clear and clean. This is not a scientific examination by any means, and scientists in the field of indoor air quality warn against taking the results from these tests too seriously. For one thing, CO2 is naturally occurring in the air. In fact, my reading from outside in the fresh air was 385 ppm, which is less than double my highest indoor reading. Measuring CO2 is also not equivalent to measuring how clean the air gets of particulate matter. Aerosolized exhaled particles won’t register on a CO2 meter, and those particles can stick around even when exhaled CO2 is diminished in a room via mitigation (open windows and such). So some of the figuring that we have to do in this case is by logical deduction (not necessarily terribly scientific, but useful nonetheless). If the air is being cleared of a portion of the CO2 that folks are exhaling, it is almost certainly being cleared of other aerosolized particulate matter that is also being exhaled. This doesn’t make the rooms ‘safe’ in any absolute sense, but it does contribute to a safer environment.
It is also worth noting that studies suggest that the severity of COVID-19 is related to the concentration of the virus in the inhaled dose. So in the worst-case scenario that we have someone in a classroom who is sick with COVID-19 but who is still unaware of this, increasing circulation can help to decrease the severity if anyone else in the classroom were to contract the disease from that person. Bear in mind that our other mitigations—mask-wearing, social distancing, and cleaning protocols—would have a pretty good chance of keeping folks safe even if an infected person were in the room. That’s what those mitigations are for—to prevent the spread of the disease if it does come into our environment. Air exchange is just another way for us to make transmission as unlikely as possible and to minimize the severity if the disease were transmitted.
I measured the CO2 concentration of different classrooms under different conditions, and I also took a baseline measure outside. The findings were quite revealing. Even a classroom that had been empty all day registered rather more CO2 than outside. Outside varied between 350 and 490. At climate.gov, they report that the average outside CO2 concentration is 409 ppm, so we come in a bit lower than that, at least at this time of year. At any rate, moving inside, the ppm was definitely higher. I measured 483 ppm in an empty classroom and 523 ppm in the gathering space. Bear in mind that only the 7th grade was on campus on Friday, and they were all in the AMP Building. In a classroom with kids talking (masked and socially-distanced, mind you) and the outside windows closed, the meter gave a reading of just over 600 ppm, while in a classroom of kids reading quietly, it registered a touch lower—at 584 ppm. What was interesting then, now that something of an indoor baseline had been established, was to engage in mitigation by opening windows. With the class continuing to read silently, I opened 5 windows and propped the back door open. Within 5 minutes, the CO2 ppm had dropped by over 100. And when I measured after the kids had gone home and the building was empty, the room was back to the equivalent of the outside air—380 ppm—after 5 minutes with the windows and door still open.
What is evident from this is that the effect of open windows and doors is dramatic. The CO2 ppm goes down quickly with the windows open, and it stands to reason that particulate matter is similarly dispersed, and potentially expelled—at least for some of it, as a result. Today was pretty cold, and we had doors opened and closed at different times to try to balance warmth in the classroom with fresh air from outside. But as we move towards and into spring, we will be leaving windows and doors open more and more as a way of keeping our environments, and our kids and teachers, as safe as possible during the pandemic.
As far as the metrics go, we had a clean slate this week—zero cases of COVID out of 172 people who were tested. The community metrics continue to be quite low. The 7-day average of positive tests per 100K remains below 12—as a reminder, 15 is considered elevated by Montgomery County, so we’re comfortably below that number. The positivity rate also continues to be quite low—below 3% as a 7-day average. The percentage growth has ticked into the region of positive growth over the past three days, which is something to keep an eye on. But COVID has been shrinking in the area for over a month—which is why we’ve reached such low levels—so to have it grow back a little bit still needs to be put into an overall context of a substantial easing in terms of the virus’s presence in our community since mid-January.