Summary

Bryson begins Chapter 13 with statistics about the lungs and breathing. Humans breathe in and out about 20,000 times a day, processing 4,000 gallons of air. That equals 500 million breaths over the course of a lifetime. We breathe in air through our nostrils and into our sinuses, which take up an enormous amount of room, although no one knows why. Our lungs are good at cleaning the air we breathe. People who live in cities, it is estimated, inhale 20 billion foreign particles a day, anything from dust to industrial pollutants. The body coughs or sneezes many of these particles out or they get caught in the mucus that lines the nasal passage. If they make it past our snot, tiny hairlike cilia that act like paddles swat invaders out of our lungs and into the throat, where they travel to our...