What is a normal CO₂ level?
The level of CO₂ in outside air is usually between 415ppm and 500ppm (ppm means ‘parts per million’).
 
As we all know, the atmospheric level of CO₂ has been increasing rapidly in recent times and as recently as 50 years ago, it was only around 325ppm. You can depress yourself by looking at the graph from 2 Degrees at the bottom of this page to see how low it was when you were born.

Still, even though atmospheric CO₂ has gone up a lot in recent times, room CO₂ levels should ideally be below 600ppm. That level indicates a well-ventilated space.

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Below 800ppm is acceptable in some circumstances. CO₂ levels above 1000ppm should be strongly avoided. 

 

The concentration of CO₂ in each outbreath is 40,000ppm, so CO₂ in poorly ventilated rooms can build up into many thousands of ppm very quickly. In very small, unventilated spaces, like cars with the air conditioning set to ‘recirculate’, the rise in CO₂ to unhealthy levels is staggeringly quick.

A CO₂ level below 600ppm shows the air is fresh

Why you feel sleepy on long car journeys. Look how quickly the CO₂ level rises in this screenshot from the Aranet app:

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Tuberculosis (TB) is transmitted through the airborne route in a similar way to COVID-19. 

 

A study from China published in 2020, showed that transmission of TB in poorly ventilated university residences was reduced by 97% when the CO₂ level was reduced to below 1000ppm by better ventilation. 

 

The CO₂ level had reached as high as 3000ppm or more in some buildings. Click on the image below to open the study in a new window.

 

Remember:

The lower the CO₂ level the better!

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Make yourself miserable by seeing what the CO₂ level was when you were born: