The problem with indoor air quality is not about oxygen!

Recently, the Dutch prime-time television show Een Vandaag covered our study about the impact of classroom air quality in primary schools on the performance on standardized tests by children. While we are happy and feel honoured that our study achieves so much public attention, we also felt the need to correct one of the conclusions in this article – that the results are due to a lack of oxygen in the classroom.

One sentence in particular is misleading: “Kinderen zitten gemiddeld driekwart van de tijd in lokalen met te veel CO2 en dus te weinig zuurstof.” (English: Children spend on average three quarters of the time in classrooms with too much CO2 and thus too little oxygen). This sentence implies that higher carbon dioxide (CO2) levels indoors are crowding out oxygen and thus the lower cognitive performance is caused by a lack of oxygen. However, this statement is neither supported by existing literature nor did the study covered in this article measures oxygen concentration in classrooms to support such a conclusion.

Outdoor air consists of 21% oxygen and 78% nitrogen. CO2 makes up about 0.04%, in addition to other gases in the air (McArdle et. al. 2015, Chapter 13). This is equivalent to 400 parts per million (ppm). The covered study found CO2 concentrations in classrooms of up to 1500 ppm, which is the equivalent of 0.15%. It seems unreasonable to believe that an increase of around 0.11% in CO2 can lead to a substantial enough reduction in oxygen levels to impair cognition.

However, one might ask what are levels of oxygen at which a meaningful decrease in cognition can occur? For this question, it is useful to understand how oxygen uptake in the lungs works and what the effect of high altitude is. If you inhale normal outside air, the air reaches small bubbles in your lungs, called alveoli, through which oxygen is diffused into the blood. How well this exchange of oxygen between the blood and the lungs works strongly depends on the concentration of oxygen in both the inhaled air and in the blood.

This is explained by Henry’s Law, which in simple terms states that gas moves into a liquid (=blood) until the partial pressure of the gas in the liquid is equal to the partial pressure outside of it. Partial pressure can be seen as a measure of concentration. Therefore, if we inhale, air rich in oxygen (= high oxygen partial pressure) reaches the venous blood in the lungs where there is less oxygen because venous blood is the blood coming back from the muscles, which consumed oxygen to generate energy. Because of this difference in concentration, oxygen moves from the inhaled air into the blood, which then flows to the heart and is pumped through the arteries to the muscles and organs (McArdle et. al. 2015, Chapter 13). From research in high altitudes, we know that oxygen partial pressure decreases with higher altitudes. However, symptoms such as light headaches due to reduced oxygen concentration start to occur at altitudes of 1500 meters only (McArdle et. al. 2015, Chapter 24), at which the oxygen concentration drops to around 17% (rather than the regular 21%).

How can the human body withstand oxygen levels lower than 21% without any symptoms? That has something to do with the transporting of oxygen in the body. After oxygen is entering the blood, it binds to haemoglobin, the protein which transports oxygen in the body. Because oxygen is crucial for the functioning of the human body, haemoglobin always carries more oxygen than it releases to the muscles and organs. It has a safety buffer in case less oxygen is delivered from the outside air (McArdle et. al. 2015, Chapter 13). Thus, it is unlikely that an increase in the indoor CO2 concentration by about 0.11% can lead to a decrease in oxygen concentration that is substantial enough to lead to cognitive impairments.

Surprisingly, past studies hint towards other particles which cause the impairment in cognition, while the effect of carbon dioxide might be negligible. To read more about what current research assumes might be the true reason for impaired cognition due to bad indoor air quality, we invite you to read one of our previous blogs here.