Beard Hair Biology: Structure, Density, Growth Rate

Structure of Beard Hair: The structure of male beard hair is intriguingly complex and markedly different from scalp hair. The variability in shape encompasses various profiles, including elliptical, asymmetrical, oblong, and trilobal. Longitudinal grooves in the hair fiber increase its stiffness. When compared to scalp hair, facial hair has been found to have almost twice as many cuticle layers, although the thickness of individual cuticle cells remains the same at about 0.5 µm. Interestingly, despite the similarity in gross surface structure between facial and scalp hair, the cuticle cells in scalp fibers have a more orderly scale pattern, with larger areas exposed. In contrast, cylindrical fibers and other irregular shapes in facial hair exhibit wider and more oblong-shaped cuticle cells.

The longitudinal sections of beard hair often reveal a continuous or discontinuous medulla. This medulla is composed of material joined by a parallel filamentous network along the fiber’s length. Unlike beard hair, scalp hairs are far less extensively medullated. Further studies have shown that melanin pigment granules are present not only in the cortical cells but also in the cuticle cells and medulla of beard hairs. Consequently, beard hair fibers can be quite strongly pigmented, and may look a richer color compared to scalp hair in the same individual.

Composition of Beard Hair: Caucasian facial and scalp hair exhibit significant differences in amino acid profiles, particularly in cystine content, with beard hair containing approximately 18% less cystine compared to scalp hair. This finding is surprising since beard hair has more cuticle layers, typically rich in cystine. However, the relatively high medullation in beard hair, with medullary cells showing very little cystine, may explain this difference. Additionally, beard hair was found to be richer in amino acids such as aspartic acid, lysine, and tyrosine, and poorer in threonine, serine, and valine. Beard hair’s medulla cells express at least 12 different hair keratins, along with an unexpected 12 epithelial keratins. Only limited studies have been conducted for other ethnic groups and while there may be some difference in amino acid content values, the numbers do not seem to be significantly different.

Beard Hair Diameter: Beard hair exhibits a diverse range of thicknesses, a phenomenon that is quite intriguing. Even in individuals boasting a full beard, numerous vellus hairs coexist alongside the terminal beard hairs. This pattern indicates that some follicles within the beard region remain unresponsive to androgen hormone activity, retaining their vellus hair follicle characteristics from birth throughout the person’s life. Consequently, the diameter of beard hair can vary significantly within the same individual. Excluding the beard’s vellus hairs, generally acknowledged to have a cross-section diameter of less than 25 µm, the thickness of terminal beard hairs has been proposed to span from 90 to 230 µm. This thickness is on average more substantial than that of scalp hair, which usually falls within a range of 80 to 120 µm.

Angle of Growth: Beard hair on the face seldom grows perpendicularly, straight out above the skin surface, though there are the occasional exceptions on the anterior curvature of the chin. The specific angle at which the hair shaft emerges from the epidermis differs among individuals and even varies across different facial regions of the same person. Generally, this angle ranges from 31 to 59 degrees. However, near the anterior cervical triangle of the neck, the hair may grow almost flat or, at times, nestle within a skin furrow (something more likely to be seen in more elderly men). The average growth angle is subject to personal variation; while some individuals may have a substantial angle in their beard hair growth, others may exhibit growth close to the skin’s surface, including on the cheeks. Recognizing these disparities in typical growth angles is important, not only for identifying those susceptible to ingrown beard hairs, but also for razor manufacturers as they develop products. The angle of growth is a critical factor in determining how closely a shave can trim the hair against the skin.

Beard Hair Growth Density: The growth density of beard hair varies across different regions of the face. In Caucasian men, the densest spacing occurs on the upper lip and front of the chin, while the least dense areas are found on the lower part of the cheek. The values of hair density indicate hairs per Square centimeter are; upper cheek, 30- 80, lower cheek, 20- 40, upper lip, 80-110, and chin center, 70-120. The average male shaves a beard area of around 300 cm².

An adult male’s beard area can contain anywhere between 6000 and 25000 terminal hair fibers, with the difference linked to factors such as family genetics, nutrition, and health. Remarkably, there are no significant differences in hair follicle density in the beard region across different ethnicities. Rather, the apparent difference in beard growth between men of different ethnicities depends on how much follicles respond to androgen hormones, how they enlarge during puberty, and begin to produce pigmented, terminal hair fibers. A separate article on this website discusses beard development differences.

The final beard growth doesn’t hit its peak density until around 30 years of age. Beyond 30 years density does begin to decline slightly, but loss of density is generally not noticeable until very old age.

Beard Hair Growth Rate: The process of hair growth is governed by intricate regulatory mechanisms, and it progresses through cyclical phases comprising active hair growth (anagen), resting (telogen), and a temporary transitional phase (catagen). Roughly 70% of beard hairs are in the anagen phase at any given time, a percentage slightly lower than that found on the scalp. It should be noted that, beard hair follicle growth cycle durations vary depending on their facial location with shorter growth cycles on the cheeks for example.

The growth rate of beard hair is notable for its rapid pace. Averaging between 0.27 and 0.47 mm per day (depending on which reports you read), it is approximately on par with the growth rate of scalp hairs and about twice as rapid as that of eyebrow hairs. However, this rate is also influenced by extrinsic and intrinsic factors, such as hair pigmentation. For instance, white beard hair has been observed to grow more than twice as fast (1.12 mm per day) as pigmented hair (0.47 mm per day) in the same individuals.

Several other elements substantially impact beard hair growth rates. A British study revealed seasonal variations in beard hair growth, with rates peaking during the summer months. Growth rates were at their lowest in January and February (winter), rising steadily to an increase of up to 60% and reaching a zenith in July (summer). Beard hair’s typical maximum length ranges anywhere from 50-300 mm; however, some men possess the genetic ability to grow their beard hair down to the ground and even further, a trait that appears to run in families.

Interestingly, sleep deprivation over 48 hours has been linked to a 19% decrease in beard hair growth in one study. This phenomenon is likely related to hormonal disturbances, such as the depression of growth hormone release, decreased availability of dihydrotestosterone, or increased sympathetic tone and/or corticoids. Rather old investigations exploring whether beard hair growth rates differed between night and day found no substantial variations.

Conclusion: The studies on beard hair reveal fascinating insights into its complex structure, composition, and growth properties. These findings expand our understanding of hair biology and provide valuable information for various applications in hair care, medical research, and perhaps even forensics. The distinct characteristics of beard hair compared to scalp hair raise intriguing questions and opportunities for further research. Understanding these differences may unlock new pathways for the treatment and care of hair diseases, connecting these aspects to broader scientific and medical contexts.

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