Your air in my lungs
It’s about 1%.
About 1/100th of all the air that I’m breathing in was recently in someone else’s lungs.
How I know
I’ve recently been measuring the CO₂ levels in the Recurse Center’s space. There are monitors on both floors I am able to read over Bluetooth, and I made a little bot that messages out when the levels go above 1,000ppm.
There are broadly two reasons why you might be interested in doing this: COVID/airborne disease precautions, and trying to minimize the cognitive effects of high CO₂ levels. Both are important to me, so this has been a fun project!
Anyways, this all got me wondering: how much of the air that I’m breathing was recently in someone else’s lungs?
I really wanted a simple formula, so I looked up a few facts. As of writing, the outside air is right around 420ppm of CO₂. According to Wikipedia, human exhaust is around 40,000ppm CO₂.
So, at 420ppm I’m breathing 0% of recently-in-your-lung air, at 40,000ppm I’m not breathing any other air. That’s our formula:
fraction_from_your_lungs = (co2_level - 420) / 40000The levels in the space are usually between 700-1,200ppm, so my inhalation contains about 0.7%-2% of air from your lungs.
Alright, we’re done. You can take a deep breath now. In… and out…
Feel that? That’s air that was in someone else’s lung. You’re welcome.
Higher-order effects
There are some issues with this formula (what about re-breathing air? how do we really know about 40,000ppm for exhausted air?) but I don’t think that matters for relatively low CO₂ concentrations. Higher CO₂ levels also run into “this will hurt you”, so I didn’t really focus on that…
If you want to do a more complex analysis of the matter, tell me about it! I’d love to read it :)