What's in the air you breathe?

Take a deep breath. In a single intake of air, your lungs swell with roughly 25 sextillion molecules, ranging from days-old compounds, to those formed billions of years in the past. In fact, many of the molecules you're breathing were likely exhaled by members of ancient civilizations and innumerable humans since. But what exactly are we all breathing? Amy Hrdina and Jesse Kroll investigate. [Directed by Sharon Colman, narrated by Alexandra Panzer, music by Miguel d'Oliveira].