What Is The Composition Of Human Breath?

When we exhale, what gasses are present and in what percentages? I know the basic cycle is that we breathe in O2 and exhale CO2. But, we’re also breathing in nitrogen and a bunch of other gasses. So, what exactly are we breathing out?

Thanks for your help.

CO/CO2 = 400–800 ppb in non-smoker’s respiration
CH4 = ~3–8 ppm
NH3 = 100–250 ppb

Lot’s of stuff.


It’s pretty much the same as we breathe in, with a bit less oxygen and a bit more CO2.

CH4? Is it just what was already present in the air we’re breating, or do we somehow produce it?

Too busy to track down a good cite of exhalation concentrations, but I think this

mixed ppm and ppb. There are about 350-400 parts per million (by volume) of CO2 in the atmosphere, so 400-800 ppm in an exhalation sounds reasonable.

Though 250 ppm of ammonia (NH3) seems awful high, so I assume that really is parts per billion.

I don’t know whether 8 ppm or 8 ppb of methane makes more sense.

I had a lung capacity, and oxygen uptake/CO2 release etc. tests run recently. They used a trace gas as a marker. A full answer would require a physiology major to provide the fine details.

The air we breath in is about 21% oxygen and 79% nitrogen by volume (partial pressure) neglecting water vapour. The air we breathe out is about 16% oxygen, 75% nitrogen, 4% CO2 and 5% water vapour by volume.

Note that the 79% nitrogen you breathe in is the same quantity of nitrogen as the 75% you breathe out - it’s just that the volume you breathe out is greater due to added water vapour and increased temperature.
The traces of methane and ammonia reported by Walloon I have no idea about. The 400-800 ppb may be right for CO, it’s way too low for CO2.

Forgot the cite! It’s a pdf:


Due to the added water vapour, but not due to the increased temperature. Assuming ideal gas behaviour, the volume percentage of nitrogen will not change as you increase the temperature - the specific volume of all the gases in the mixture will increase in the same degree.

If you compare like with like (dry gas in, dry gas out), and assume that each molecule of CO[sub]2[/sub] replaces a molecule of O[sub]2[/sub], you should find the volume percentage of nitrogen in exhaled gas is the same as in air.

Well spotted.

I’m not usually a fan of smilies, but :smack: