Hair greying: ethnic and geographical differences
Seventy-four percent of people between the ages of 45 and 65 have grey hair,
with an average intensity of 27%. Within this age group, men have more grey hairs than women. The age at which hair begins to turn grey and the increase in grey hair with age appear to be clearly linked to ethnic and geographical origin. At a similar age, subjects of Asian or African descent, who have the darkest hair, have fewer grey hairs than those of Caucasian origin, who have lighter hair tones. While men have on average significantly more grey hairs than women of the same age, their hair begins to go grey at the temples, whereas women’s grey hairs are more spread out .
The Eumelanin content of hair of different ethnic origins changes with age
Eumelanin content  is significantly higher in African American hair, followed by Asian and then Caucasian hair. French eumelanic hair in this study was found to be lighter than Asian and African American hair. The lighter colour is due to lower melanin content.
Total melanin content increases with age in pigmented hair in all three ethnic groups. This increase could be due to the gradual thinning of the hair and a reduction in water content.
Eumelanin quality changes with age in African American and Caucasian hair, but not in Asian hair. This could be linked to a reduction in the activity of the enzyme dopachrome tautomerase. These results suggest that the phenotype of the hair follicle melanocytes changes with age.
Hair fibre fragility
The hair’s resistance to breakage decreases as the fibre becomes worn. This begins with deterioration of its external membrane, the cuticle. The morphology of the cuticle is closely related to the overall shape of the fibre.
In straight hair, the thickness and number of cell layers forming the cuticle remain the same (usually six to eight cell layers) across the entire hair.
In very curly hair, however, the cuticle varies in thickness and can comprise as little as one or two cell layers. These few layers can therefore be lost very quickly due to repeated brushing.
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The hair shaft is a fibre made
up of three concentric layers.
Starting from the inside and
working outwards, these are:
The medulla or marrow, which
is made up of nucleus-free cells
and is not present in every hair;
The cortex, which is made up
of pigment-containing epithelial
cells and forms 80 % of the
hair shaft ;
The cuticle, an external layer
made up of flat cells overlaying
each other like tiles on a roof,
known as scales.
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The cuticle can break easily, exposing the cortex of the hair, which will in turn break very quickly, as it is no longer fully covered by the cuticle. The unique morphology of the cuticle partly explains the physical fragility of African hair. African hair usually breaks at a torsion twist, where the hair is very flat and therefore made up primarily of the cuticle..
Study of ageing in natural, untreated hair
L’Oréal researchers  were given access to a few strands of hair from a Chinese woman whose hair had grown to 4.3 metres in length overall.
This assessment of extremely long Asian hair (over 2.4 metres) showed that the hair became progressively more damaged the further away one moved from the root. This damage spread from the outer edge of the hair fibre into the middle, in successive stages:
• Firstly, the cuticle became steadily more worn down, without impacting the cortex, for approximately one metre
• Then, further on, there was significant damage to the cuticle, combined with a significant reduction in ceramides (structural lipids in the cell membrane) and the lipid acid 18-MEA, and a change in the keratin protein content.
 Panhard S, et al. Greying of the human hair: a worldwide survey, revisiting the ‘50’ rule of thumb. 2012, Br J Dermatol167 (4): 865–873
 Commo S, et al. Age-dependent changes in eumelanin composition in hairs of various ethnic origins. Int J Cosmet Sci. 2012; 34: 102–107
 Thibaut S, et al. Chronological ageing of human hair keratin fibres. Int J of Cosmet Sci. 2010; 32 (6):422- 434