How can carbon dioxide be toxic

The Federal College provides information - technology

November 2011
Danger from carbon dioxide

On the one hand, carbon dioxide is praised as a non-flammable and non-toxic substance and is classified as a refrigerant in safety group A1; on the other hand, one hears again and again about the dangerousness of this substance and that it can lead to fatal accidents with CO2 has come. What is right?

Whether a substance is toxic or not depends on how it is viewed. The famous quote from Paracelsus “The dose alone makes the poison” already says that the decisive factor is ultimately the concentration of the substance that you are exposed to.

First of all, it should be noted that carbon dioxide is classified as non-toxic. The occupational exposure limit [1] (AGW) for this substance is 0.5% by volume. In the natural atmosphere, CO comes2 to about 0.038% by volume (with an increasing tendency).

If one looks at the effect on the human body, the following picture emerges:

Carbon dioxide content in the air we breatheHazards and effects of increasing exposure to carbon dioxide
approx. 0.5 - 1% by volume corresponds to approx. 9 to 20 g / m³ If inhaled only briefly, there is generally no particular impairment of body functions.
approx. 2 - 3% by volume corresponds to approx. 35 to 60 g / m³ Increasing irritation of the respiratory center with activation of breathing and increase in pulse rate.
approx. 4 - 7% by volume corresponds to approx. 70 to 130 g / m³ Aggravation of the aforementioned complaints; in addition, circulation problems in the brain, dizziness, nausea and ringing in the ears.
approx. 8 - 10% by volume corresponds to approx. 140 to 180 g / m³ The above-mentioned symptoms worsen up to cramps and unconsciousness with short-term death.
> 10% by volume Death occurs briefly.

This means that the hazard posed by CO2 must be assessed differently than that of the HFC refrigerants in group A1. Carbon dioxide is dissolved in the blood, influences the respiratory center in the brain and the oxygen uptake of the red blood cells. Breathing is accelerated even with a carbon dioxide content in the air of just a few percent and concentrations of 8% or more are already life-threatening, even if there is still enough oxygen in the air we breathe.

The HFC refrigerants, on the other hand, have i. A. only a weak effect on the body. Concentrations from approx. 6% have a narcotic effect. It usually only becomes life-threatening at around 20% by volume due to the displacement of oxygen.

As a final assessment, one can say that carbon dioxide is not toxic, but CO2 is already dangerous at lower concentrations than “conventional” refrigerants. However, CO2 is an environmentally friendly refrigerant with a GWP of 1. It is not considered to be hazardous to water, is not flammable and will not decompose into toxic substances even in an open flame.


[1] The occupational exposure limit (OEL value) is the limit value for the time-weighted average concentration of a substance in the air at the workplace in relation to a given reference period. The occupational exposure limit indicates the concentration of a substance at which acute or chronic harmful effects on health in general are not to be expected.