August Question of the Month – an irregular series

I’m wondering if you can tell me: what makes a hair stop growing? Why do, say, leg hairs or eyebrows stop growing,
but the hair on my head continues to grow?

Thank you for your question regarding differential growth rates of human hair types.

This is a very interesting question that has been addressed recently in a scholarly article – Human Head Hair Is Not Fur by AH Neufeld and GD Conroy. Evolutionary Anthropology, vol. 13:3, p. 89, June 2004.

Humans, like other mammals, have hair over most of their bodies. Head hair is different from hair elsewhere on the body because it continues to grow (although it is anatomically the same as other hair). When head hair is transplanted elsewhere, it grows longer than the hair on those parts of the body.

Here is a description of the growth process from that article:
“Both head hair and body fur grow in cycles. The hair follicle produces a strand of hair during its active growth phase, called anagen. Then the growth slows, and the follicle “rests” for a while, the telogen phase. Then comes exogen when the hair falls out, and the follicle begins growing a new strand of hair as the anagen phase begins again. Hair on the leg usually grows for 19 to 26 weeks and then falls out. Hair on the head keeps growing for two to six years.”

The article states that it is not known why head hair continues to grow, and further research is needed.

We hope this information is useful.

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