ELI5, how can we lose an average 100 hairs a day and not go bald?

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If we’re sprouting new hairs from the empty follicles, and the new guys grow at the same rate as the others, they’d take forever to catch up to the length of the rest of your hair. Make it make sense, please.

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You’ve been growing new hairs all your life, so there are hairs at all various lengths on your head. The 100 hairs that fall out today are replaced by a 100 hairs that start growing two months ago. The hairs that grow out of the newly-emptied follicles will replace hairs that fall out in two months. It’s just a constant, steady stream of old hairs being replaced by new hairs. We just don’t notice because the average person has about 100,000 hairs on their head, so a loss of 100 hairs (0.1%) is basically undetectable.

For starters, a hundred hairs is not a lot when you consider how many are on your head. You have roughly a hundred thousand. So you are losing 1/1000 of your hair every day.

You are correct that the new ones grow at the same rate, so for those hundred you just lost, you gained a hundred more and broke even.

The reason it’s not noticeable is because those hairs come from all over your head, not from one place. In other words, it’s not falling out in clumps, but in single strands. It would be like plucking a hundred blades of grass out of a field, but spread across the whole field. You wouldn’t notice it at all.