During the recent smokepocalypse, we were told to stay inside with the windows and doors closed until further notice. Does that really help? If you really could seal your house airtight, wouldn't you just run out of oxygen pretty quickly? —Air Hoardin'   

Time for another of our patented Dr. Know bogus mathfests, where our quantitative rigor is matched only by our qualitative stupidity! I'll be using imperial units so those of us old enough to remember the now-discredited brontosaurus from textbooks (or, in some cases, from real life) can follow along.

For argument's sake, we'll assume your house or apartment is outfitted with magical, elf-forged super-weather stripping, suitable for submarine screen doors or space travel. We'll further assume 9-foot ceilings and 1,000 square feet of floor space all to yourself—9,000 cubic feet of air, all told.

The average American adult uses about 19 cubic feet of oxygen a day. (If you exercise, you use more, but I did specify "American.") Dividing 9,000 by 19 gives you 474 days of air—pretty good, right?

Well, not quite—recall that only 20% our air is actually oxygen; the rest is nitrogen and other gases. Still, the remaining 1,800 cubic feet of pure O2 is enough to sustain you for about 95 days.

Or it would be, if you had it conveniently concentrated in an oxygen tank. Unfortunately, it's diffused unhelpfully throughout your increasingly suffocating panic room, and your lungs can only extract oxygen from this miasma down to a concentration of 10% or so. This brings your prognosis down to something like 48 days.

It gets worse! The real problem with a sealed room isn't running out of oxygen, it's the buildup of carbon dioxide. You exhale 15 cubic feet of carbon dioxide per day. Within six days, your bomb shelter's air will be an already-uncomfortable 1% CO2, and by day 24, your air will have reached 4% CO2, a level the Centers for Disease Control and Prevention has declared an immediate danger to life and health.

All that said, a week or 10 days of airtight lockdown—longer than you might suppose—should be eminently survivable. Movies and TV shows where two teenagers get locked in a walk-in cooler and have 12 hours to escape dying as virgins are, apparently, not as realistic as you thought. But I won't say anything if you don't.

Questions? Send them to dr.know@wweek.com