On Friday, we posed the following back-to-school-themed Fermi
problem: Assuming you're not in a big lecture hall and the
professor shuts the door at the start of class, how long does it
take for you and your classmates to deplete the oxygen enough to
feel it? We promised a surprising answer, and here it is. You
decide if our back-of-the-envelope calculations are reasonable.
Let's build our classroom first. It's 16 feet wide and long, and
10 feet tall. In handy metric dimensions, that's: 5 meters by 5
meters by 3 meters, or 75 cubic meters. A cubic meter is 1000
liters, so now we've got 75,000 liters of fresh air. The oxygen
content of air is about 21 percent, and at about 17.5 percent
you'll run from the room screaming. To get from fresh and
breathable to absolutely stifling, take the difference between
21 percent of 75,000 liters and 17.5 percent of 75,000 liters.
That gives us 2,625 liters of oxygen to get through. How much
oxygen does a human consume? It was tough finding a reliable
source, but this press