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Carbon dioxide sounds like environmental pollution to us, since it is an odorless, colorless and tasteless gas present in the atmosphere in a proportion of around 400 ppm (parts per million), and rising, as the latest reports indicate. It is rising mainly due to emissions from industry and transport and is positioning itself as a potent greenhouse gas which, paradoxically, is vital for regulating surface temperature and life on earth.
However, when we talk about CO₂ concentration indoors, we are referring to CO₂ as a by-product of our own metabolic activity. It can also be found as a result of the combustion of any carbon-containing substance, so poorly sealed cookers or other indoor combustion processes help to increase CO₂ levels.
We could say that CO₂ is not a pollutant as such, since it is a gas that living beings exhale during respiration. But it can be toxic by displacement of oxygen. The "fresh" air we inhale should be approximately 21% oxygen, compared to 0.04% CO₂. Exhaled air, on the other hand, contains approximately 16% oxygen and 4% CO₂.
In indoor spaces such as schools, offices or our own homes, from a concentration above 800 ppm of CO₂ (0.08%), symptoms of fatigue, reduced performance, headaches and increased respiratory rate begin to appear. The comfort limit is at 1500 ppm, because if we stay above this value for a prolonged period of time , the effects of fatigue increase considerably and its effect on discomfort and loss of performance is direct.
So, to maintain the healthy ratio of oxygen to CO₂, we need to renew the air in enclosed spaces. And, therefore, this is one of the reasons why ventilation is, and always has been, key to proper indoor air. Ventilation also helps us to dilute other indoor pollutants (such as formaldehyde or volatile organic compounds), as well as to remove the excess humidity we produce indoors (we also exhale water vapour when breathing, showering, cooking, etc.).
We have always ventilated a little by eye. Following popular wisdom, but inefficiently and poorly adapted to real needs.
Where manual ventilation is used, it is often insufficient, especially in winter, when we do not usually open the windows for more than a few minutes in the morning. And in cases where a controlled mechanical ventilation system is available , it is usually operated entirely manually. So, again, it is up to the olfactory sensor that each of us has as standard to decide to change the fan's mode of operation. At most, as is the case in office buildings and in general tertiary use (training centres, hotels, shops, etc.), it will be programmed according to a pre-set schedule, which is not necessarily in line with actual use either.
The CO₂ concentration is therefore the key indicator that the minimum air renewal required for a healthy indoor environment is ensured. By measuring and monitoring the CO₂ evolution we can interpret, in real time, when and how ventilation is being carried out, and whether it is being carried out efficiently.
As we have a "mania" for breathing, adjusting ventilation to peak production will help to ensure fresh and healthy indoor air, in the right proportion of oxygen and CO₂. For example, in a space with no ventilation other than open windows, sleeping with the bedroom door closed, ajar or open will not be the same. In fact, CO₂ levels in a bedroom after a long night can easily reach 2000 ppm.
The pandemic situation in which we continue to find ourselves has made us aware of the need to ventilate indoor spaces. Schools have an important task ahead of them to ensure the required air renewal, without their energy (and therefore financial) bills going through the roof as soon as the heating season starts. This is no easy task, and healthy indoor air requires monitoring, diagnosing and acting on other parameters that condition it, although starting by ventilating according to CO₂ concentration is certainly a big step.
