Accurate quantitative evaluation of hair density can be difficult – certainly time consuming. In classic studies, hair density was defined by plucking hairs in a marked unit area of skin – primarily to define hairs in anagen and telogen, but in passing this also enabled hair density to be evaluated. These days density is commonly evaluated by a phototrichogram, a close up photo of individual hair fibers where the surface area is known and consistent between photos. Phototrichograms are less accurate than counting hairs one by one, but they are quicker. The average density of hair on an adult who does not have baldness is 200-300 hair fibers per square centimeter (cm2). Variability in this value occurs with hair color, age of the individual, location you are observing on the scalp, and the presence of any hair loss diseases. The crown is the region that is most densely packed with hair follicles. In adults the sides and back of the scalp may have a hair density as low as 150/cm2. For the crown of the scalp pre-puberty, the density is 250-400 hairs/cm2.

Hair follicle density is a different thing altogether. Hair follicle density refers to the density of hair follicle structures actually in the skin. At any one time there are more hair follicles than hair fibers as not all hair follicles contain hair fibers due to the growth cycle of the hair follicle. So the hair follicle density in the skin is higher than the density of hair fibers. The follicular density (number of hair follicles/cm2) decreases with age. At birth the follicle density has been shown in one study to be 1135/cm2, 3-12 months = 795/cm2, third decade 615/cm2, and 435/cm2 by 70-80 years of age.

Hair color can be taken as a rough guide to hair follicle density. Redheads have about 90,000 scalp hair follicles. Black, 108,000, while brown and blonde haired people can have up to 140,000. The number of scalp hairs changes with ethnicity too. Far east Asians can have as few as 80,000 scalp hair follicles.

The follicular density is very high at birth. All hair follicles form during embryonic development. As an embryo/newborn is relatively small so there is less surface area over which the follicles are spread. With growth the follicles must be spread over a larger unit area of skin. No new follicles are formed at any stage of life after birth. Also, some hair follicles will atrophy with time, particularly if individual has androgenetic alopecia. In addition, the figure dervied from histology is probably an over estimate of the true density due to the method of analysis. Hair follicle desnity is examined by cutting skin samples into thin slices and looking at them under a microscope. Because the angle of cut through the skin is very often off the vertical plane of the hair follicles, more follicles are cut through in any one histology section. Horizontal sections also have some inherant problems as the figure derived can change depending on how deep the histology section is cut from the skin. Hair follicles vary in length. In practice, very small follicles are irrelevant as they contain very fine hair fibers or none at all.

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Hair density references

Barman JM, Astore I, Pecoraro V. The normal trichogram of the adult. J Invest Dermatol. 1965;44:233-43.
Courtois M, Loussouarn G, Hourseau C, Grollier JF. Ageing and hair cycles. Br J Dermatol. 1995 Jan;132(1):86-93.
Rushton DH, de Brouwer B, de Coster W, van Neste DJ. Comparative evaluation of scalp hair by phototrichogram and unit area trichogram analysis within the same subjects. Acta Derm Venereol. 1993 Apr;73(2):150-3.
Hayashi S, Miyamoto I, Takeda K. Measurement of human hair growth by optical microscopy and image analysis. Br J Dermatol. 1991 Aug;125(2):123-9.
Uno H. The histopathology of hair loss. In: current concepts. Upjohn, Scope Publications, Kalamazoo, 1988; 123-146.
Pecoraro V, Astore I, Barman JM. Growth rate and hair density of the human axilla. A. Comparative study of normal males and females and pregnant and post-partum females. J Invest Dermatol. 1971 May;56(5):362-5.
Saitoh M, Uzuka M, Sakamoto M. Human hair cycle. J Invest Dermatol. 1970 Jan;54(1):65-81
Giacometti L. The anatomy of human scalp. Advances in Biology of Skin. 1965; 6: 107–120.