Introduction

Carbon dioxide is a chemical compound of carbon and oxygen with the empirical formula CO₂, a non-combustible, acidic and colourless gas; it dissolves well in water: here it is often colloquially called “carbonic acid” – especially in connection with carbon dioxide-containing beverages. With basic metal oxides or hydroxides it forms two types of salts called carbonates and hydrogen carbonates.

CO₂ is an important component of the global carbon cycle and, as a natural component of air, an important greenhouse gas in the Earth’s atmosphere.

With sufficient oxygen supply, CO₂ is produced both during the combustion of carbon-containing substances and in the organism of living beings as a product of cellular respiration. Plants, algae and some bacteria and archaea convert CO₂ into biomass by fixation. Photosynthesis produces glucose from inorganic CO₂ and water.

CO₂ can have a toxic effect; however, the concentrations and quantities in the air or through the absorption of lemonade, for example, are far from sufficient for this. It has a broad spectrum of technical applications: In the chemical industry, for example, it is used to produce urea. In solid form as dry ice it is used as a coolant, supercritical carbon dioxide as a solvent and extraction agent.

Presence in the atmosphere

Carbon dioxide is a natural constituent of atmospheric air. All living matter is carbon based and the element is an essential part of life on Earth. The major reservoir of carbon in the atmosphere is CO₂. Measurements of levels of CO₂ over the past 50 years show a dramatic rise from around 315 ppmv in the late 1950s to present day values of around 375 ppmv.

Analyses of ice drill cores have shown that up until 1800, levels of CO₂ were pretty constant at around 280 ppmv, but have risen drastically since the beginnning of the industrial revolution. The cause of this rapid rise is a combination of the burning of fossil fuel, deforestation and changes in land use.

Carbon dioxide is a potent greenhouse gas, tapping outgoing terrestrial radiation in the infrared region of the spectrum, and an increase in its atmospheric mixing ratio is expected to lead to warming of the Earth’s surface. Estimates based on future global population and land use suggests that levels of CO₂ may double from pre-industrial levels to around 700 ppmv, and that global surface temperature may increase by around 2-4.5 °C over the next 100 years.

Indoor pollution

In indoor air, owing to exchange of air due to ventilation, the same concentrations as in ambient air are to be expected. However, this only applies if there are no sinks or sources in the room. A sink is, for example, alkaline masonry.

What are the sources of CO₂?

The most important source in the room is normally humans. The concentrations occurring depend on the number of people in a room and on the ventilation intensity. For example, different measurements in a closed bedroom overnight containing two people, maximum CO₂ concentrations of between 1200 ppm and 4300 ppm have been determined. With a closed window and the door half-open, the maximum concentration was only 1700 ppm.

Another example: Hundreds of measurements in offices revealed concentrations between 350 ppm and 2350 ppm (median: 555 ppm). Significant differences were found between naturally ventilated buildings (median: 750 ppm @ 300 measurements) and air-conditioned buildings (median: 465 ppm CO₂ @ 330 measurements).

In a classroom having an interior volume of just 200 m³, when occupied with 45 people, with closed windows and an air change rate of around 1/h, after 1h a CO₂ concentration of about 3000 ppm resulted. From such findings, there resulted the recommendation to ensure sufficient ventilation.

In certain cases, sources other than humans can also play an important role. Moreover, combustion processes frequently take place indoors. The carbon dioxide formed in this way passes, together with other combustion products (e.g. particulate matter PM) into the indoor air. Most sources of combustion gases indoors are visible, e.g. tobacco smoke, open flames of cooking and heating appliances or burning candles. The emission formation caused by them can therefore be predicted and the emissions can be removed as a precaution by ventilation. An invisible source is, for example, a leaking chimney, but this situation rarely occurs.

Of subsidiary importance for the CO₂ concentration in indoor air is CO₂ emissions by plants. During darkness, plants also release small amounts of carbon dioxide. From literature data, it is possible to estimate the carbon dioxide release per leaf area and hour by plants in the dark. For a leaf area of 1 m², this roughly corresponds to 1% of the amount of CO₂ released by an adult per hour. This small amount stands in comparison to the consumption of CO₂ by the photosynthesis processes proceeding in the plant in light.

What are the health effects of CO₂?

The above mentioned concentrations of CO₂ in the atmosphere do not have any disadvantageous direct effect on human health. Objectively measureable effects are not observed until about 5000 ppm to 10000 ppm. These effects consist of an increase in respiration frequency, changes in the blood pH and a reduction in physical capability. At concentrations greater than 15000 ppm, breathing becomes more difficult, and concentrations above 30000 ppm can cause geadaches and dizziness. Above 60000 ppm to 80000 ppm, unconsciousness and death may be expected.

Possible health effects can be:

· headache

· dizziness

· drowsiness

· sweating

· dimmed sight

· reduced hearing

· increased hart rate and blood pressure

· shortness of breath

· muscular tremor

High levels of CO₂ are directly correlated to low productivity, higher sick leave and higher risk in infectious disease transmission, making carbon dioxide a crucial concern in home, office and school environments.

Evaluation, guideline and limit values

EN 13779, developed by the European standardisation bodies in CEN, contains a differentiated assessment of the carbon dioxide concentration in indoor air. This standard introduces the four quality categories IdA 1 to IdA 4 to classify indoor air quality. of “recreation rooms in which smoking is not permitted and in which pollution is mainly caused by human metabolism“.

Quality categories	Description	Absolute CO2 concentration in indoor air [ppm].
IdA 1	High indoor air quality	≤ 800
IdA 2	Medium indoor air quality	800 – 1000
IdA 3	Moderate indoor air quality	1000 – 1400
IdA 4	Low indoor air quality	> 1400

As a result, a hygienic evaluation of carbon dioxide in indoor air leads to a plan of action to reduce CO₂ concentrations and improve the indoor quality. If a value of 1000 ppm CO₂ is exceeded, ventilation should be provided. It also makes sense to ensure an appropriate air exchange before a CO₂ concentration of 1000 ppm is reached. If a value of 2000 ppm CO₂ is exceeded, ventilation must be provided. A lower value of 1000 ppm CO₂ should be aimed for!

CO2 concentration [ppm]	Hygienic evaluation	Recommendations
< 1000	Hygienically harmless	No further action
1000 – 2000	Hygienically conspicuous	Ventilation measure (increase fresh air volume flow or air change) Check and improve ventilation behaviour
> 2000	Hygienically unacceptable	Check ventilation of the room. Check further measures if necessary

How can CO₂ be reduced?

CO₂ levels vary regularly indoors. Factors that affect CO₂ levels are in general the amount of people and length of time in a room, amount and time of fresh air ventilation etc.

The main action reducing CO₂ in indoor air is sufficient ventilation.

How do we measure CO₂?

At Aristoteles Consulting, we measure CO₂ with continuously registering measuring devices. This devices are equipped with an NDIR sensor (non-dispersiv). The measured data are saved by a data logger, which can be read out after the measurement to get information about the concentration and to create data tables and graphs showing time dependent variations.

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