(Intro)

You’re probably quite happy that your armpit hair isn’t dragging on the floor. So it's good that there’s a system to prevent that. But what is that system?

Us humans grow hair all over our bodies, except on our palms and the soles of our feet. But some of it, like leg hair, stops growing, while the hair on our heads just seems to grow out forever. That’s because every hair on your body goes through the same cycle: growing for a while and then falling out. But each type of hair spends a different amount of time growing, and grows at a different speed.

Every hair begins the same way, in a phase of the cycle called anagen. During anagen, blood flow starts to ramp up at the base of the follicle, feeding oxygen to specialized stem cells. These cells begin rapidly dividing and producing keratinocytes, which form the root of the hair. As the expanding mass of keratinocytes is pushed toward the surface of the skin, the cells die, releasing a protein called keratin, which holds the strand of hair together. Eventually that strand pops out of your skin. So the visible part of the hair is entirely dead, which is why, thankfully, it does not hurt to get your hair cut, although try explaining that to a three-year-old.

During anagen, hair can grow up to 1 and a quarter centimeters every month, depending on where it’s located on your body. The second phase is called catagen, and lasts about two weeks. Here, the blood supply is cut off at the bottom of the follicle, which stops the production of new keratinocytes. So for that particular hair the party is over. The follicle then shrinks to about a sixth of its original size and then the existing hair strand is pushed closer to the surface.

The third phase is called telogen, otherwise known as the resting phase, where the follicle remains dormant for one to four months. Finally, the hair is released, or shed, when the follicle dilates, and starts the anagen phase again.

So, how long a hair on your body gets, depends on how long it’s in the anagen phase, and how fast it grows during that time. The hair on your scalp, for example, stays in anagen for two to six years, which is why it can grow so long. Other hair types, like eyebrows and eyelashes and body hair have a short anagen phase, only 30 to 45 days. But they also grow much more slowly, with eyebrows, for instance, growing only 4.2 millimeters every month.

This is my nice eyebrow closeup. Hey.

As for how your hairs know when to grow and when to stop, that’s something scientists are still trying to figure out. It is known that genetics can lead to longer or shorter anagen phases in certain hair types. But the current thinking is that your hairs get their instructions by way of chemical growth signals from stem cells in the skin. And considering how extremely inconvenient it would be for all of the hairs on your body and all mammals' bodies to just continue growing forever, it makes sense that there's a system for making sure they don’t grow too long.

And you can thank Griffin Taylor, a supporter on Patreon, for asking us this question! If you’d like to submit questions to be answered, or get these Quick Questions a few days early, you can check out patreon.com/scishow.