On CO2 at Shaftsbury Elementary School

Please hear my reality check on CO2. There is no cause for alarm at Shaftsbury Elementary. CO2 is not "toxic." It is an asphyxiant, which means that if there's an incredibly high amount (Fifty times higher than the school measured), then it displaces oxygen, and a lack of oxygen will be harmful without doubt. But to say that CO2 is "toxic" is the same as saying water is "toxic," because too much water deprives you of oxygen and you drown. In modern high-quality buildings, CO2 is used to measure and control the amount of fresh ventilation air. The amount of fresh air is dictated by code, but is very difficult to measure. CO2 provides a simple way to measure fresh air, because people give off CO2 as they breathe, and fresh air lowers it, by mixing in lower-level outside air. Normal outdoor air now contains 400 parts per million (ppm), rising by two every year. If indoor CO2 levels off at about 1,000 to 1,200 ppm, it means that fresh air is just about right, and code ventilation level has been achieved. It has nothing to do with the danger of CO2. It is simply a way to measure fresh air. To say that 1,000 ppm is a danger is to totally miss the point.


Low ventilation rate is a concern because it can lead to faster disease transmission among students, it can result in more build-up of harmful chemicals such as cleaners, pesticides, and construction glues and paints if present, and it can fail to mask poor hygiene. But CO2 in itself is of no concern whatsoever. The low ventilation rate should be corrected (preferably with a heat exchanger, to avoid high winter heating costs). But do not panic. This is not a very serious problem, certainly not a toxic exposure event.

— Bill Christian North Bennington