Because it holds moisture from the atmosphere, wool has a high water content. It also has a high nitrogen content, and these qualities make wool very difficult to ignite. It requires more oxygen and higher temperatures to ignite than cotton, rayon, polyester, or acrylic to ignite (wool won’t ignite until 1,382 degrees Fahrenheit, while cotton burns at 491 and polyester melts at 485 degrees), and the structure of its fiber causes wool to self-extinguish once lit. This makes wool the best fabric to wear if you’re going to be hanging out by a campfire, but also the best for home furnishings like mattresses or carpets.
Modern homes—and the furniture in them—burn way hotter, and faster, than homes did 50 years ago. One of the main reasons is that furnishings that used to be made of natural fibers like wool are now made from petroleum-based materials that are quick to ignite. (And because those materials are so flammable, they’re heavily treated with flame-retardants, which are toxic—retardants can mess with your fertility, among other things. Read more about the truth about flame retardant furniture.)