All heterotrophic organisms emit CO₂ as a waste product of their metabolism. Ventilation units, equipped with adequate monitoring systems, can reduce the concentration of CO₂ and, consequently, that of all bio-effluents, including viruses.

CO₂ and heterotrophic organisms

All heterotrophic organisms, i.e. those incapable of autonomously synthesising all their own organic molecules starting from inorganic compounds, emit CO₂ as a waste product of their metabolism. According to EN 16798-2, a person at rest breathes approximately 500 l/h of air and releases approximately 20 l/h of CO₂, while when doing physical exercise, oxygen consumption and carbon dioxide production increase. For example, people doing activities such as climbing breathe around 3100 l/h of air and emit 124 l/h of CO₂.

Amount of air breathed by an adult human at the level of activity performed (l/h)

Carbon dioxide has a density of 1.8 g/l at 25°C, therefore 500 l/day of CO₂ emitted at rest by a person corresponds to 900 g/day of CO₂.

Daily CO₂ emissions of a person at rest

The daily CO₂ emissions of humans is therefore equal to those of a small car that travels 8 km in mixed cycle conditions (considering unit CO₂ emissions of 114 g/km).

Comparison between the CO₂ emitted by a small car and a person at rest

The concentration of CO₂ in the atmosphere is equal to 0.04% of the volume of air, while leaving our lungs it is 4%.

Concentration of CO₂ in the atmosphere and on leaving our lungs

When humans breathe outdoors, the carbon dioxide they generate is quickly diluted into the atmosphere, without any significant increase in concentration. The same is not in indoor environments. For example, assuming a uniform distribution of CO₂ in a 30 m² studio flat occupied by two people and without air change, in just one hour the concentration increases from 450 ppm (i.e. 810 mg/m³) to approximately 990 ppm (1780 mg/m³).

Changes in CO₂ concentration in a 30 m² closed environment occupied by two adults, without air change

Even pets, being heterotrophs, emit CO₂ through respiration. The minimum energy requirement of a dog can be calculated using the following formula, derived from Kleiber’s law of 1930:

Where:
Q is the animal’s energy requirement [kcal/day]
P is the weight of the animal in kg.

Considering that one litre of oxygen is needed to consume 5 kcal of energy, and the respiratory ratio of CO₂/O₂ is 0.82, a medium-sized dog (25 kg) will emit 360 g of CO₂ per day, approximately 40% of the CO₂ emitted by an adult.

Daily CO₂ emissions of small, medium and large dogs

Returning to the previous example, if a medium-sized dog (25 kg) stays in the studio flat with the two adults, after one hour the total CO₂ concentration will be approximately 1100 ppm.

Change in CO₂ concentration in a 30 m² closed environment occupied by two adults and a medium-sized dog, without air change

Difference in CO₂ concentration between indoors and outdoors

One way to counteract the increase in CO₂ concentration is to change the air inside closed spaces. Opening windows is a viable strategy, especially in homes, however it can cause discomfort in both winter and summer, as well as wasting energy due to the operation of heating and cooling systems. In residential and commercial buildings, especially the most recent constructions that are generally well insulated, a ventilation system is recommended.

The ventilation system allows filtered and conditioned air from outside to be introduced into the building to ensure proper IAQ.

When the building is occupied, if the system features an intelligent controller, the air flow-rate can be adjusted based on the values measured inside the building, providing air changes when and where necessary in order to save energy.

Ventilation system with control based on CO₂ concentration

In Europe, EN 16798-1 can be used to size these systems. Based on the method nr.2 of this standard, considering a class IEQ1 building, in order to ensure a high percentage of satisfied occupants (> 85%), the difference in CO₂ concentration between inside and outside should not exceed 550 ppm.

According to the World Meteorological Organisation (WMO), the average atmospheric CO₂ concentration is currently 418 ppm (WMO, 2023).

Increase in atmospheric CO₂ concentration from the 1970s until today

In light of these considerations, the absolute CO₂ concentration inside a class IEQ1 building (EN 16798-1) should be approximately 970 ppm; beyond this value, the threshold of 85% satisfied occupants is no longer guaranteed.

Maximum CO₂ concentration for the different building categories described in EN 16798-1

Fresh air flow-rates in commercial buildings in accordance with EN 16798-1 method nr.2

The air flow-rate to be introduced into a commercial building based on the CO₂ difference between inside and outside can be calculated using the following formula:

Where:
qtot = outside air flow-rate [l/s]
n= number of occupants
qp = amount of CO₂ emitted by each occupant [l/h]
ΔCO₂ = difference between the desired indoor CO₂ concentration and the CO₂ in the outside air.

The formula does not take into account pollutants emitted by building materials or interior furnishings, so it is best used for high-density commercial buildings such as schools and cinemas. 

Considering n=1, qp=20 l/h and a minimum fresh air flow-rate per person of 4 l/s, the following air flow-rates per person can be obtained.

Fresh air flow-rates based on the CO₂ difference between indoors and outdoors

In summary, because every occupant, and even our four-legged friends, constantly exhales CO₂ as a metabolic by-product, indoor levels can climb rapidly in a well insulated space. Left unchecked, high CO₂ concentrations erode comfort, cognitive performance and occupant satisfaction. By deploying ventilation systems equipped with real-time CO₂ monitoring and smart controllers that modulates fresh-air supply to maintain appropriate ΔCO₂, it’s possible to optimise energy use and ensure a healthy, productive environment. 

Ultimately, smart CO₂-based control transforms ventilation from a fixed cost into a responsive asset, one that safeguards both wellbeing and efficiency.

The contents of this blog post regarding IAQ can be examined more in depth by reading the white paper
“Indoor air quality - Guaranteeing health and comfort in buildings”

Download the white paper

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