We recently got this question from “Jack”. Here is our reply.
Jack, you’re wise to ask this question. It’s surprisingly tricky. There are three main components in air affecting “air quality”: carbon dioxide (CO2), other gaseous air pollution including organic chemicals and ozone, and small particle air pollution as from wild fires.
Carbon dioxide (CO2) is the most important factor for covid-19 and other airborne contagious diseases. (We will report on other types of meters in the future.) The CO2 impact is two-fold: CO2 itself begins to be deleterious to humans at only double its concentration in fresh air. At the levels that have been the norm for well-designed and operated buildings (900-1000 ppm), recent studies have shown a small but measurable amount of reduced cognitive performance compared to that with 100% fresh air (now around 425 ppm minimum).
The more important factor with covid-19 and other airborne viruses is that CO2 levels provide a means of measuring the amount of fresh air PER PERSON in the room. This ability is because people are continuously breathing out high levels of CO2 (around 40,000 ppm!). These high levels mix in with the room air and the incoming fresh air and come into equilibrium. There is a mathematical formula for translating the resulting CO2 level into fresh air cfm (cubic feet per minute) per person. And since air into the room or building must equal air out, the more air coming in leads to more airborne virus being flushed out.
As the covid-19 pandemic developed, we in the ventilation industry generally doubled the amount of fresh air/exhaust air in buildings, from 15 to 30 cfm per person. (We thus returned to the healthy fresh air levels being provided in the U.S. just after Spanish Flu pandemic in 1918!) Allowing for some variation in meter accuracy, this ventilation level corresponds to a CO2 reading of 800 ppm.
The technical problem is that 99% of the meters sold today are worthless. (See above picture for examples that failed our tests.) Either they start out wildly inaccurate, or they become miscalibrated within a few weeks. Decades ago, when the natural CO2 level was around 400 ppm, and buildings let a lot of air leak in and out, the industry started “automatic” CO2 meter calibration by assuming that buildings would regularly be exposed to indoor CO2 levels approaching the level of outside air, or 400 ppm. They devised meters that would keep track of the lowest CO2 reading over the course of a few weeks, and then readjust the calibration assuming that this lowest reading was 400 ppm! This is an obviously very wrong thing to do these days, with outdoor air CO2 at 425 ppm and rising, and more importantly, with better-built buildings that NEVER have the indoor air CO2 get anywhere close to such a low reading. (I know, I have measured this in multiple instances.)
Nevertheless, most meters sold today still do the auto-calibration, often cleverly called Automatic Background Calibration (ABC). Some meters allow you to turn off the ABC, but, amazingly to me, many meters do not allow you to turn it off.
Therefore, to get a functional CO2 meter, you need one that is accurate to start with, and does NOT have ABC. And to get a user-friendly meter, you need one that gives you a convenient color-coded GREEN signal when the measured ppm is less than 800. I have tested dozens of meters since 2020, and we now have only two that we recommend. We sell verified and upgraded versions of these meters, or you can buy them directly and take responsibility for proper set-up and labeling.
The Big Meter. This is a meter, typically for wall-mounting, that gives your customers, visitors, and building occupants a clear signal that you have healthy fresh air. Here is a meter at the entrance to a high school.
See the Apr. 29th, 2022 blog post for more details and how to order. Both meters can be ordered by emailing safetulator@kohlerandlewis.com or call Suzanne at 603-352-4828.
The Small Meter with a Graphic History: This is a small desktop meter, explained in full detail in this blog post of Dec. 15th, 2023. In addition to the current ppm reading and color code, it shows a colored bar graph with 24 hours of history. Both meters can be ordered by emailing safetulator@kohlerandlewis.com or call Suzanne at 603-352-4828.
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