Why does our hair go grey?

When it comes to grey hair we're largely in the hands of our genetics, Nigel Russell a trichologist specialising in the study of hair and the scalp, told RNZ's Nights.

"If it's in the genes, then there's nothing really you can do about it."

It's not actually grey hair, so much as a lack of melanin, he said.

"Melanin produces colour in the hair follicle, so due to health issues and genealogy, the melanin slows down production."

Men and women go grey at similar ages, but it is more apparent in men, he said.

"A woman usually will have longer hair, whereas a male will have sideburns quite short, and the back of his hair quite short.

"And that's where the most the early signs of greyness appear, the temples, round about the ears and the nape - so they're going to be more exposed."

And there's no escaping, everybody's hair goes grey eventually, he said.

"I would say 99 percent of the population at some point there'll be suddenly some appearance of grey, if not a lot."

And the grey appears in the same places on the scalp for everyone, he said.

"It seems to be predominantly the temples, the frontal hairline, the parting and the nape, they always go grey first in everybody and I'm not sure why that is, it could be genetics, I'm not exactly sure, but that's where it starts.

"Then it just either stays there, or it can spread right through the hair."

Going white from fright is something of a myth, he said.

"Theoretically, it's impossible, because it's melanin production. It's already in the hair, the hair would have to fall out and then come back white - that I could believe."

Melanin production is affected by other health conditions, he said.

"The body is compensating, taking all those nutrients that would normally go to the hair and the scalp, and it's redirecting them to the heart and liver and the lungs to keep us alive. So that's just how the body works - it's very clever."