A glossy head of hair is the epitome of good health - witness those smiling, sexy women bouncing their tresses across our television screens advertising the latest hair care.
However much we describe those first grey hairs as silver or distinguished, in the privacy of our own bathrooms they can seem like the first glimpse of the grim reaper.
But does the state of our hair actually say anything about our health? Does going grey really mean that the rest of your body is ageing? Can split ends genuinely tell us whether our arteries are furred or our bones becoming brittle?
Watch the small ads, and you'll find increasing promotion for hair analysis techniques, to alert you to allergies and measure levels of trace elements and toxins, and then draw conclusions about your general health from them.
Hair analysis has also been used to detect drug use among athletes, to monitor environmental exposure to toxins such as mercury or nicotine, and to test whether babies have been exposed to drugs while in the womb.
Even more surprisingly, researchers in Australia recently reported the startling discovery that x-rays of hair from women with breast cancer or one of its genetic mutations look different from x-rays of hair from healthy women.
Dr David Fenton, consultant dermatologist at St Thomas Hospital, London, says changes in hair texture or appearance can sometimes indicate an underlying disease, but he cautions against some of the more extravagant claims for hair analysis made by commercial firms.
'Hair testing can give useful information but it is almost impossible to accurately determine when exposure took place as hair grows at different rates. Arsenic may stay in the hair for up to five years after exposure, so you could show that someone has been poisoned, but not when.'
Fenton is particularly dismissive of commercial operations that ask you to send hair samples in the post to test for allergies or to measure trace elements. 'It has been shown that you can post two samples of the same head of hair, from two different addresses to these allergy testing services, and get two completely different answers. These techniques are not a reliable diagnostic tool and are totally unreliable.'
Yet the findings of Veronica James, professor of chemistry at the Australian national university in Canberra, give pause for thought. She believes her team may have found a new and simple technique to screen women for breast cancer.
Reporting in the journal Nature, she says x-ray hair samples from women with breast cancer exhibited a characteristic change. Similar changes were seen among women at high risk of breast cancer because they carry the BrCa1 gene.
Far more research is needed and there is no information as to what might be behind these changes. But the prospect that the slim shaft of a hair might offer an early warning system for such hidden diseases as cancer begs the question of just what else it might reveal.
What our locks - or, rather their colour - don't appear to reveal is that your body parts are ageing, or that they are ageing faster than those with still vibrant tresses.
Those who go grey at a young age can take heart from a study in Copenhagen which followed 13,000 people over 16 years and found no correlation between visible signs of ageing like greying hair and wrinkles, and early death.
'When you go grey depends largely on genetics,' says Fenton. 'We don't tend to go grey at a younger age than our parents or grandparents but we do live longer, so maybe it seems that there are more grey-haired people around.'
Hair greys because over time the cells that produce hair pigment (melanocytes) become less active. Early greying may occasionally be a sign of an auto-immune condition called premature ageing syndrome, a rare condition that does result in early death.
Losing your hair rather than just its colour is rarely a sign of illness. Most people who lose their hair have 'male' pattern baldness - so-called because men are most susceptible, though women can get it too, especially after the menopause.
It's caused when an enzyme starts, with age, to convert the hormone testosterone on the scalp to its less useful version, dihydrotestosterone. This makes hair follicles shrink and then they disappear.
Women who've had a baby also tend to moult about six weeks after delivery, because of the wild hormonal fluctuations that occur during pregnancy and after childbirth.
A severe illness and cancer drugs can have the same effect, but if you've not been pregnant or ill, and are not genetically predisposed to baldness, then finding handfuls of hair in the shower could mean anaemia or an underactive thyroid, both of which are easily diagnosed by a blood test. A sudden increase in hairiness, on the other hand, may be a sign of a hormonal problem, such as ovarian cysts.
And finally, what about the apocryphal tales of people getting such a shock that they age instantly, and turn white overnight? That's not quite how it happens, says Fenton. Shock doesn't change hair colour, but it can cause sudden hair loss, which is known as alopecia.
'If you already have a sprinkling of white hair amid your natural, darker hair - the so-called salt-and-pepper look - the alopecia may affect the dark hairs more than the white so it looks as if you've turned white overnight. Actually, you've lost your dark hairs overnight, not gained more white ones.'