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25 February 2018
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Why monitoring oxygen doesn’t protect from carbon dioxide

Carbon dioxide (CO2) is gas used or produced in many industries, if not directly in the products, in cooling and refrigeration systems. Possibly because of its association with breathing (we breathe in oxygen and breathe out CO2), the toxic nature of CO2 is not always appreciated. As a result, some believe that the level of oxygen (O2) in the air is a suitable indicator of safe CO2 levels. However, while monitoring O2 concentrations protects you from asphyxiation, it can’t be relied upon to protect against CO2 poisoning. Making a link between safe levels of CO2 and safe levels of O2 can be a fatal error.

Proven – no link
Studies show that even in the presence of normal concentrations of oxygen, exposures to 7% CO2 can cause death in only 5 minutes. Other work has demonstrated that breathing concentrations of 30% CO2, even with 70% O2, leads to unconsciousness in 30 seconds.

While these results prove that no direct link exists between safety from CO2 and concentration of O2, they maybe don’t reflect circumstances that you are very likely to be encountered in the work environment.

Proportionate displacement
So, what happens to gas levels if a leak raises CO2 concentrations to 3%. The majority of normal air (nearly 80%) is nitrogen (N2) gas. This means that, if a CO2 release occurs, most of the gas that it displaces will be N2 (fig 1). Therefore, the percentage increase of CO2 is not matched by a similar decrease in the O2 concentration. Carbon dioxide can reach exposure limit levels, but O2 levels could still be comparatively unaffected.

CO2 leak

Also, CO2 is heavier than air but O2 isn’t. So, while CO2 tends to hug the ground, O2 is evenly distributed. This means, in any space, the level of CO2 relative to O2 could be significantly different, depending on where you measured it. Therefore, use of an O2 monitor could give you a different result, depending on where it was fixed. An additional danger is that, in high concentrations of CO2, some oxygen sensors give a small upscale signal, i.e. a falsely high reading, which could result in undue complacency.

Eradicate unsafe practice
Some are still using levels of O2 in the air to protect against CO2. This is probably due to a lack of understanding about the toxic properties of CO2 or the relative displacement of the different gases in air as a result of a CO2 leak. Regardless, reliance on monitoring levels of O2 to protect against CO2 has led to fatalities and is a practice which must be eradicated.

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