Science Questions

I tend to drive with all the windows closed and the recirculation function engaged. This way I keep out the fumes and dust. The flip side is I’m breathing in re-circulated and progressively stale air. My question is, if the car were a perfectly sealed container, how big would it have to be for me to survive in it for a day? I’m trying to figure out how long I can drive in a compact car without running the risk of passing out. Paul, Hong Kong

Dr Gisli Jenkins, University of Nottingham:

The easy answer to this question is that you will never run out of air.  You will just exchange the breath that you breathe in with the breath that you breathe out.  I guess what the question alludes to is how long you have to survive in that box before you die. What you are doing is you’re exchanging ambient air with exhaled air and the gaseous composition of the two airs is quite different. Ambient air has a CO2 concentration of about 0.5% and oxygen concentration of about 21%. Exhaled air has a CO2 concentration of about 5% and an oxygen concentration of about 13% so what you’ll do over time is you’ll reduce the oxygen level. The problem is not so much the reduction of oxygen but the increase in carbon dioxide. By the time that the carbon dioxide levels in the air that you breathe reach 15% you’ll effectively die. Assuming the box is about 4 cubic metres it would take about 16 hours or so. But you would actually start to feel ill and probably die a lot sooner than that. Actually it could be down to, sort of, five hours.

April 2008