CO2 in the Home




CO2 monitors of useful accuracy are available at modest price, and the information they provide about air quality in the home can be used in different ways.


A first question in response to a home CO2 reading might be about its health implications. This issue is addressed by Usha Satish et al. in their 2012 paper

Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance [1]

The writers point out that it has been thought that adverse effects attributed to raised CO2 levels in the home are actually due to other pollutants commonly associated with CO2. They describe how their experiment was designed to eliminate this possibility and so isolate the effects of CO2.

Before presenting their own findings, they summarise previous research on levels of CO2 much higher than those likely to be encountered in the home:
“CO2 concentrations > 20,000 ppm cause deepened breathing; 40,000 ppm increases respiration markedly; 100,000 ppm causes visual disturbances and tremors and has been associated with loss of consciousness; and 250,000 ppm CO2 (a 25% concentration) can cause death.”

They measured the decision-making performance of participants in their study while they breathed air at three levels of CO2; 600, 1,000 and 2,500 ppm. (these are all levels below that at which any health risks have been noted). Moderate decreases in performance were found in most tests at the two lower levels, and large decreases at the highest level.

Reference was made in the previous post (Indoor Air Quality) to recommendations by the German Federal Environment Agency [2], in which levels of indoor CO2 “below 1000 ppm are regarded as harmless, those between 1000 and 2000 ppm as elevated and those above 2000 ppm as unacceptable.” The question of what level should be aimed at in any particular household then becomes a matter involving priorities; while many people would agree that 2000 ppm should not be exceeded, some might feel that to aim at a much lower level of CO2 is unrealistic, or that the necessarily high rate of air change would not justify the associated energy cost.

A second question about an indoor CO2 reading asks what it reveals about the rate of air change in the room. This can be estimated if the rate of CO2 produced by its occupants is known, and has reached equilibrium with that being extracted from the room through ventilation. An approximation in use for the CO2 output of a typical sedentary adult is 20 litres per hour, but a more accurate figure based on physical data for a particular individual can be obtained from a calculation of metabolic rate [3]. The delay in reaching equilibrium after a change in conditions can be considerable, but the time required for measurement can be reduced if curve fitting software is used to predict the final value from an initial set of readings [4].

Where data logging is available, a more elegant approach can be used, in which the decay of the CO2 level is recorded under normal conditions of ventilation, but starting when the room is vacated following a period of occupation. This method is discussed in Simple and Cheap Air Change Rate Measurement Using CO2 Concentration Decays by Roulet & Foradini (2002) [5].

Estimation of air change rate by CO2 measurement may be considered as a third method alongside pressurisation and energy-balance, which were  referred to in an earlier post (A Change of Air, 11 May 2019).

References

[2] Umweltbundesamt. Gesundheitliche Bewertung von Kohlendioxid in der Innenraumluft. Bundesgesundheitsblatt - Gesundh – Gesundh 2008:1358–69
(open access)
[5] Open access, available at Researchgate


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