Why is it that our hair – and for men, our beard – goes white while the other hair on our head (eyebrows and eyelashes) stays its original colour?
Ron Dippold
San Diego, California, US
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Eyebrows and eyelashes can go white if you live long enough. About 15 per cent of people over the age of 70 have white lashes, and lots have white eyebrow hairs. My dad had both. But they generally stay darker longer than head hair – why? Greying isn’t fully understood, but the vague consensus seems to be that follicles – complex machines that grow hair – in the scalp and beard work much harder.
You are born with all your hair follicles for life. Cells called melanocytes in your follicles colour the growing hairs. Crucially, hair isn’t growing all the time. In the “on” (anagen) stage, a follicle grows and colours a new hair at about 0.15 to 0.5 millimetres a day. In the “off” (telogen) stage, a follicle rests and rebuilds itself while the dead hair sits there until it falls out. Thankfully, every follicle is on a separate schedule or we would occasionally lose all our hair at once.
Genetics introduces variability, but for scalp hair, the cycle is about two to eight years on, three to four months off. They are busy! For eyebrows, it is around four to seven months on, nine to 10 months off; for eyelashes, one to two months on, three to four months off. The growth stage sets the maximum length of hair – you don’t want your eyebrows or lashes to be too long or you would have trouble seeing. From these schedules, scalp hair follicles are working 10 times harder to crank out luxurious hair.
Eventually, the melanocytes just give up and your hair goes grey, then completely white. And the hardest working seem to wear out first. For most men, the growth stage for their facial hair is a bit shorter than that for their scalp hair, so the beard stays coloured a little longer.
Eyebrows and lashes are also some of the darkest pigmented hairs of the body, so they may start with more melanocytes, which may help them last longer.
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