Do you breathe out only carbon dioxide

In addition to his day job as Woot's full-time "Debunker," Jeopardy! wunderkind Ken Jennings moonlights as an author of books, and this month he has a new one in stores. It's the fifth in his Junior Genius series, this time chock-full of amazing facts about The Human Body. To mark the occasion, he'll spend all of February debunking anatomical anachronisms and medical misinformation for us. Finally: the inside scoop about our own insides.

The Debunker: Do People Breathe in Oxygen and Breathe Out Carbon Dioxide?

Well, okay. Yes, you do breathe in oxygen and breathe out carbon dioxide, which is why you're still alive. Fine. But you also breathe in plenty of carbon dioxide and breathe out plenty of oxygen. It's a lot more complicated than the diagram you remember from third grade where the people are pumping out carbon dioxide and the trees are pumping out oxygen.

For one thing, Earth's atmosphere actually has more nitrogen in it than anything else, so the air you inhale and the air that you exhale is 78 percent nitrogen. Exhaled air is also considerably higher in water vapor - as high as five percent, which is why you can see your breath on a cold day.

But the crux of the matter is the oxygen-carbon dioxide tradeoff. Your lungs just aren't as good as you think they are at oxygen exchange. Every time you take a breath, which is 21 percent oxygen, your lungs are only able to extract about a fifth of that oxygen. The rest gets breathed right back out again. There wasn't much carbon dioxide in the air you breathed in: unless you are in a failing submarine or spaceship, it's around 0.04 percent. But when you breathe out, it's still pretty low, about 4 percent. There's not a huge difference between inhaled and exhaled air, chemically speaking. But it's that tiny change that keeps you from not being a corpse. Vive le difference.

Quick Quiz: Radiohead agreed to give songwriters Albert Hammond and Mike Hazelwood co-writing credit for what 1993 hit song, because of its close resemblance to the Hollies hit "The Air That I Breathe"?

Ken Jennings is the author of six books, most recently his Junior Genius Guides, Because I Said So!, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at or on Twitter as @KenJennings.