Uncertainty in using CO2 for ventilation clues in Classrooms.

 A LONG Thread... 1/ 

Short version:

High CO2: REAL BAD.

Low CO2: Meh...

In Between: Who knows... better get filtering.

2/ 

First lets talk about short versus long range transmission.

1) The highest COVID transmission risk is short range where the aerosols are highest in concentration. Well fitting masks and distancing are the best defense. 3/ 

2) But COVID laden aerosols can build up in poorly ventilated spaces, creating long range transmission risk throughout a room. Poorly ventilated spaces are where superspreading events occur. 4/ 

What we are trying to due with CO2 monitoring is find the poorly ventilated classrooms, before a person who is highly emitting viruses enters. Then we can fix these classrooms before they become superspreading events. 5/ 

In addition, classrooms with higher ventilation (lower CO2) have demonstrated decreased absenteeism and increased student performance. So we get ancillary benefits from fixing poorly ventilated classrooms! 6/ 

So, where does classroom CO2 come from? Your lungs exchange oxygen in air with carbon dioxide. Importantly, the amount of carbon dioxide that we emit depends on our sex, age, activity level and size. The air you breathe in is about 0.04 % carbon dioxide. 7/

The air you breathe out is about 4 % carbon dioxide.

Given its relatively high concentration and ease of measurement compared to other chemicals and emissions from people, carbon dioxide is a good proxy of potential emissions from humans (e.g. viruses). 8/ 

Let's stop here and say it again. CO2 does not equal IAQ (Indoor Air Quality). It is important to note the CO2 is only a proxy for human emissions and many other things impact our indoor air quality. 9/ 

Building materials, furniture, cleaning supplies, indoor chemical reactions and outdoor contaminants all impact indoor air quality. Hence, low indoor CO2 concentrations do not necessarily indicate good indoor air quality. 10/ 

Lets start by talking about CO2 monitor uncertainty. The most accurate consumer grade sensors are Non-Dispersive Infrared (NDIR). The infrared light passes through the air and carbon dioxide molecules absorb a specific frequency. 11/
poppendieck.com/IAQ/Consumer%2…

A detector determines the fraction of that specific frequency of infrared light that has passed through the sample. The fraction that passes through the sample is proportional to carbon dioxide concentration. 12/ 

Monitors using sensors other than NDIR can have errors depending on the calibration chemical. Although NDIR sensors report values to three or four digits they are only accurate to roughly +/- 50 ppmv. 13/ 

Finally, these sensors can drift over time. However, the sensors are typically linear so the the difference between the indoor and outdoor CO2 concentrations is typically fairly consistent for sensors. 14/ 

So the bottom line is that a CO2 reading of 900 ppmv could be 850 ppmv or 950 ppmv. In the real world a room hitting 4000 ppmv is bad, it doesn't matter if it is 3950 ppmv or 4050 ppmv. 15/ 

When we use CO2 as a ventilation clue, we need to make sure non-human sources of CO2 are not present, such as indoor combustion or vehicles idling outside air intakes. 16/

We also need to make sure we don’t have significant CO2 removal mechanisms like specialty filters or a significant number of plants (greenhouse level). 17/ 

So how do we use CO2 to identify poorly ventilated classrooms?

There are three potential approaches. The first deals with how much air we rebreathe that has been previously been in other people’s lungs. 18/

We can use CO2 to determine the fraction of air you are breathing that you are rebreathing. For instance, if the average indoor carbon dioxide concentration is 820 ppmv (820 carbon dioxide molecules per 1 million total air molecules) and the outdoor concentration is 420 ppmv 19/

and the exhaled concentration is 40,000 ppmv, then 1 % of every breath a person takes in the room will consist of air that has been in someone’s lungs. If the indoor concentration is 3620 ppmv, then 8 % of every breath a person takes in the room will consist of air that has. 20/ 

in someone’s lungs (or 8 times higher than if the average indoor concentration was 820 ppmv). 21/ 

For airborne viruses we want to keep that rebreathed fraction, as indicated by the CO2 concentration, as low as possible. The lower the rebreathe fraction, the lower the overall risk that IF there is a COVID19 emitter present, that you will inhale COVID19 laden particles. 22/ 

A second approach to use CO2 readings from a classroom is by examining the maximum daily CO2 concentration when the space is used at normal OCCUPANCY. 23/

https://threadreaderapp.com/thread/1456461078041333760.html