Carbon dioxide (CO2) and indoor air quality

Carbon dioxide (CO₂), also known as carbonic acid gas, is an odorless and colorless gas. It is among the most abundant gases in the Earth’s atmosphere, while occupying only less than one percent of the atmosphere volume. It also can be found in a solid form, called dry ice. Carbon dioxide has a considerable warming effect, which makes it “famous”, as one of the most important greenhouse gases. We may however ask ourselves: Is carbon dioxide dangerous for humans? How can it affect us? What can we do to limit these effects?   

Importance of Carbon dioxide

Put aside all the bad news about carbon dioxide… for now! Yes, it is a useful gas. Carbon dioxide is made of one carbon atom and two oxygen atoms, forming a very small molecule. This molecule is used by plants, in combination with water and with some energy, to produce carbohydrates while releasing oxygen. This process is called photosynthesis and is essential for plants. Since humans and animals depend on plants for the food and other benefits, they can’t survive without carbon dioxide. 

Carbon dioxide is a non-toxic gas, despite its “famous” aspect. It is classified as an asphyxiant gas rather than an harmful or toxic gas in accordance with the Globally Harmonized System of Classification and Labelling of Chemicals standards.

 It is a natural gas and is linked to human life: people exhale carbon dioxide while breathing, and inhale oxygen. In built environments, the amount of CO₂ can be easily used as an indicator of the adequacy of fresh air ventilation. Therefore, CO₂ is an indicator of good air quality.

However, carbon dioxide is a strong greenhouse gas: when released in the atmosphere, it builds up and can store great amounts of heat, contributing to climate change by increasing the temperature. 

Carbon dioxide indoors 

Carbon dioxide can be produced either naturally (human respiration, volcanic eruptions) or through human activities such as burning fossil fuels, deforestation, and land use change. Indoors, it is mostly generated by human respiration. Other sources may include smoking, fire activities (such as a fireplace), or open flame in the house, and cooking activities, whether with gas or with a wood stove. 

The concentration levels of CO₂ indoors depend on many factors including the size of the room, the number of occupants, the length of occupation in the room, the presence and type of ventilation (mechanical ventilation or open windows), the types of activities indoors (smoking), but also the outdoor concentration.   

Carbon dioxide concentrations are measured in parts per million (ppm), which is an equivalent of one milligram in a cubic-meter of air volume. Typical outdoor concentrations range between 350 and 450 ppm. At these levels, carbon dioxide has no harmful effect on humans since it is diluted in the atmosphere and fresh air is available. with carbon dioxide, this is likely to happen indoors rather than outdoors. 

Concentration levels of carbon dioxide indoors are usually recommended between 600 and 800 ppm, and must be kept at or below 1000 ppm. The acceptable CO₂ concentration levels can be different between countries or within a country. For instance, the recommended levels are 600-800 ppm in France, 1000 ppm in Hong-Kong, Quebec and in Switzerland. The USA National Institute for Occupational Safety and Health (NIOSH) considers adequate ventilation indoors at 1000 ppm as well. The UK standards for school requires a daily (9 am-3:30pm) average of less than 1500 ppm. 

Effects on human health 

The CO₂ asphyxiating effect is more prominent indoors. Human organisms need oxygen to fully function and perform at their best. When there is less oxygen, the body starts seeking it, which leads to exhaustion. While considering the elements influencing the concentration levels of carbon dioxide, one can wonder what might be the effects of these different concentrations on health. In general, after a prolonged exposure indoor, the effects of these concentrations can be summarized as follow:

• 500 ppm and higher: Eye irritation, sore and dry throat, stuffy or runny nose, sneezing
• 800 ppm and higher: Headache and fatigue
• 1000 ppm and higher: Cough and rhinitis, difficulty focusing, decreased performance

A study conducted in Switzerland schools in 2017 revealed that CO₂ concentrations could reach 4000 ppm in classrooms, within 10 minutes to 2 hours. The consequences of these elevated CO₂ concentrations were drowsiness: students were feeling sleepy and tired, which logically resulted in reduced performance. Another research  showed an increase in performance and cognition levels of up to 61% (from 1400 ppm to 950 ppm), and 101% (550 ppm) with the decrease of the indoor CO₂ concentration levels. Negative effects of CO₂ can be observed at concentrations as low as 1000 ppm, contrary to what was previously thought (5000 ppm). Furthermore, tentative scientific evidence suggests that prolonged exposure at higher levels (2000-3000 ppm) is linked to effects including stress, kidney calcification and bone demineralisation.   

What to do

As shown, indoors air quality can be affected by high levels of carbon dioxide concentrations. In absence of good ventilation and introduction of fresh air, these concentrations can lead to poor cognitive performance and fatigue, especially in schools and offices. A responsible way to tackle this issue is to properly monitor the gas concentration indoor. When mechanical ventilation is missing, it is a good idea to often open the windows to dilute CO₂ concentrations with more oxygen brought by fresh air.  

- Confiance Mfuka