Hair Composition, Function, and Characteristics

Scalp hair is composed of three main layers, each with a distinct role in maintaining the hair’s structure and function:

1. Cuticle (Outer Layer):
   The cuticle is the outermost layer of the hair shaft, made up of overlapping, transparent cells that form a protective shield. It protects the inner layers of the hair from damage caused by external factors, including UV rays, heat styling, and chemical treatments. A healthy cuticle gives hair its smooth texture and shine. Healthy cuticles lie flat—they are porous and can absorb and retain moisture. Unhealthy cuticles do not lie flat—they allow moisture to enter and escape easily, causing frizz and dryness
   
2. Cortex (Middle Layer):
   The cortex comprises the bulk of the hair strand and consists of tightly packed keratin protein fibers. This layer provides the hair with strength, elasticity, and color. The pigments in the cortex determine your natural hair color. It also retains moisture and contributes to the overall health and structure of the hair
   
3. Medulla (Innermost Layer):
   The medulla is the central core of the hair strand, mainly present in thick or coarse hair. It consists of soft keratin and may not always be present in finer hair. The medulla plays a minor role in the hair’s overall structure and function

Hair is composed mainly of a protein called keratin, which gives it its strength and durability. Keratin makes up about 95% of your hair and consists of 18 different types of amino acids. These amino acids play a crucial role in hair health by strengthening each strand and helping your hair retain moisture, keeping it smooth and hydrated. This is why keratin is often a key ingredient in hair care products.

Keratin versus Protein

• Keratin is a specialized protein that strengthens hair, skin, and nails
• Protein from food provides the amino acids needed to produce keratin
• You cannot eat keratin, but you can boost keratin production by eating a protein-rich diet with essential vitamins and minerals

Amino Acid Composition in Human Hair Keratin

• Cysteine: 17.5%
• Serine: 11.7%
• Glutamic acid: 11.1%
• Threonine: 6.9%
• Glycine: 6.5%
• Leucine: 6.1%
• Valine: 5.9%
• Arginine: 5.6%
• Aspartic acid: 5.0%
• Alanine: 4.8%
• Proline: 3.6%
• Isoleucine: 2.7%
• Tyrosine: 1.9%
• Phenylalanine: 1.4%
• Histidine: 0.8%
• Methionine: 0.5%

Hair also contains trace amounts of water, lipids (fats), and melanin, which provides pigmentation. Hair is non-living tissue once it exits the scalp, meaning it cannot repair itself like other body parts. This is why proper hair care is essential for maintaining both health and appearance.

Understanding the Role of Hair

Each hair strand on the human body is connected to its own nerve, blood supply, and a small muscle. These components work together to nourish the hair, sense changes in the environment, and even cause the hair to stand up when you feel cold or experience intense emotions, a reaction known as “goosebumps.”

While scalp hair is not essential for survival, its protective and sensory functions, as well as its cultural significance, make it a vital aspect of human life.

The primary functions of scalp hair include:

1. Protection: Scalp hair helps shield the scalp from environmental elements, such as ultraviolet (UV) radiation from the sun, reducing the risk of sunburn and skin damage. It also offers some insulation against cold temperatures by helping to retain body heat
   
2. Thermal Regulation: Hair on the scalp helps maintain body temperature by providing insulation. It also helps cool the body by trapping air and sweat near the scalp, which evaporates and dissipates heat
   
3. Sensory Function: Hair follicles are surrounded by nerve endings, making the scalp highly sensitive to touch. This allows the hair to act as a sensory organ, detecting changes in the environment, such as movement or temperature
   
4. Storehouse of Information: A wealth of information can be found in a person’s bloodstream by analyzing a strand of hair, including traces of medicine, drugs, alcohol, vitamins, and minerals
   
5. Social and Cultural Significance: Beyond physical functions, scalp hair has profound social, cultural, and aesthetic importance. It is often associated with identity, beauty, and self-expression, significantly influencing how individuals perceive themselves and others

Hair Strength

Human hair is surprisingly strong despite its delicate appearance. Each healthy hair strand has a tensile strength comparable to materials like steel or Kevlar, commonly used in bulletproof vests. On average, a single strand of hair can support up to 100 grams (about 3.5 ounces) of weight. When you consider that an average human scalp contains approximately 100,000 to 150,000 hair strands, all the hair on a person’s head could collectively bear the weight of 12 tons, which is equivalent to the weight of two elephants!

This impressive strength comes from the unique structure of hair. Hair is primarily made of keratin, a tough protein arranged in a tightly packed, layered structure. The outer layer, or cuticle, protects the inner cortex and medulla, contributing to the hair’s durability and resistance to breakage. However, factors such as over-styling, chemical treatments, and environmental damage can weaken the hair, making it more susceptible to breakage.  

Human hair is remarkably elastic and can stretch up to 30% of its original length without breaking or sustaining damage. This elasticity plays a key role in hair’s strength and resilience, allowing it to withstand daily styling and environmental factors.

Hair by the Numbers

The average scalp contains between 100,000 and 150,000 hair strands, but hair density and the thickness of each hair shaft can vary depending on ethnicity and genetics. 

• Blondes: They have the most hair, with an average of around 150,000 hairs
• Brown-haired individuals: They have an average of 110,000 hairs
• Black-haired individuals: Average approximately 100,000 hairs
• Redheads: They have the fewest hairs, with an average of 90,000 hairs

Hair Density

Hair density is defined as the number of hair strands per square centimeter of the scalp, which naturally varies among ethnic groups. 

On average:

• Caucasians have the highest hair density, with approximately 200 hairs per square centimeter
• Asians typically have about 150 hairs per square centimeter, totaling around 80,000 hair follicles on an adult scalp
• African Americans usually have a hair density of around 130 hairs per square centimeter, with approximately 60,000 hair follicles on the scalp

Hair Color

Hair color varies globally and is influenced by genetics, ethnicity, and geographic factors. Below is an overview of the most common hair colors and their prevalence across different racial and ethnic groups:

Black Hair

• Prevalence: Black is the most common hair color worldwide
• Ethnic Groups: These groups are predominantly represented among people of African, Asian, Indigenous Australian, Pacific Islander, and South Asian descent
• Characteristics: Typically associated with higher concentrations of eumelanin (a type of pigment), giving it its dark, rich color. Black hair tends to have a thicker and more coiled structure in African populations, while in Asian populations, it often has straighter or wavy textures

Brown Hair

• Prevalence: The second most common hair color worldwide
• Ethnic Groups: Common in people of European, Middle Eastern, North African, and South Asian descent
• Characteristics: Varies in shade from light brown to very dark brown. It is often found in Caucasians and can occur in various hair types, including straight, wavy, or curly

Blonde Hair

• Prevalence: Less common globally; natural blondes comprise about 2% of the world’s population
• Ethnic Groups: Found primarily in Northern European populations (e.g., Scandinavians, Germans, and Slavs) and certain isolated groups in Oceania, such as the Solomon Islands
• Characteristics: Caused by low levels of eumelanin. It is often more common in childhood and can darken with age

Red Hair

• Prevalence: The rarest natural hair color, occurring in approximately 1-2% of the global population
• Ethnic Groups: Most commonly associated with people of Celtic and Northwestern European descent, particularly in Ireland, Scotland, and Wales
• Characteristics: Caused by a mutation in the MC1R gene, leading to higher levels of pheomelanin and lower levels of eumelanin

Gray/White Hair

• Prevalence: Occurs naturally with aging across all ethnicities
• Ethnic Groups: Not tied to a specific racial or ethnic group but rather to the aging process, when melanin production slows or ceases
• Characteristics: Can appear earlier in some populations due to genetic predisposition, stress, or certain medical conditions

Geographic and Ethnic Hair Insights

1. Caucasian Populations: Show the widest variety of hair colors, including black, brown, blonde, and red. Europe is diverse in hair color due to genetic variations and evolutionary factors
2. Asian Populations: Predominantly have black hair, which is typically straighter and thicker than the hair of other groups
3. African Populations: Black hair is universal and is characterized by tightly coiled or kinky textures that help retain moisture in hot climates
4. Indigenous Populations of the Americas and Australia: Typically have black hair, although some Oceanic populations exhibit unique blonde hair variants due to genetic mutations
5. Middle Eastern and South Asian Populations: Predominantly have black or dark brown hair with occasional lighter shades in certain regions due to genetic diversity

Hair Characteristics

One’s hair type is influenced by genetics and ethnicity, with straight, wavy, curly, and coily hair distributed unevenly worldwide.  

Caucasians

• Hair Texture: Wide variety, including straight, wavy, and curly
• Hair Shaft Shape: Oval
• Hair Density: High, with the highest number of hairs compared to other groups
• Thickness: Medium to fine, with significant variation within the group
• Color: Broad spectrum, from blond to dark brown and black

African Americans

• Hair Texture: Coiled, curly, or kinky
• Hair Shaft Shape: Flattened elliptical, leading to tight curls
• Hair Density: Moderate to low, but appears fuller due to its texture
• Thickness: Thinner hair strands compared to Asians, but denser in appearance
• Strength: Fragile due to the bends and curls in the shaft, prone to breakage and dryness

Asians

• Hair Texture: Straight and smooth
• Hair Shaft Shape: Round, contributing to a uniform and sleek texture
• Hair Density: Moderate to low, with fewer strands but thicker hair shafts
• Thickness: Thickest hair strands of all ethnic groups. Because of its thickness, it is also the strongest and most resistant to damage
• Color: Generally dark brown to black

Indians

• Hair Texture: Straight to wavy
• Hair Shaft Shape: Oval, similar to Caucasian hair, but with more variation in texture
• Hair Density: Medium to high
• Thickness: Medium to thick
• Color: Dark brown to black, with strong natural shine

Middle Eastern

• Hair Texture: Wavy to curly, sometimes thick and voluminous
• Hair Shaft Shape: Oval, similar to Indian and Caucasian hair
• Hair Density: Medium to high
• Thickness: Thick, coarse hair strands
• Color: Generally dark brown to black

	Hair Texture	Hair Shaft Shape	Hair Density	Thickness	Color
Caucasians	Wide variety, including straight, wavy, and curly	Oval	High, with the highest number of hairs compared to other groups	Medium to fine, with significant variation within the group	Broad spectrum, from blond to dark brown and black
African Americans	Coiled, curly, or kinky	Round, contributing to a uniform and sleek texture	Moderate to low, but appears fuller due to its texture	Thickest hair strands of all ethnic groups. Because of its thickness, it is also the strongest and most resistant to damage	Generally dark brown to black
Asians	Straight and smooth	Flattened elliptical, leading to tight curls	Moderate to low, with fewer strands but thicker hair shafts	Thickest hair strands of all ethnic groups. Because of its thickness, it is also the strongest and most resistant to damage	Generally dark brown to black
Indians	Straight to wavy	Oval, similar to Caucasian hair, but with more variation in texture	Medium to high	Medium to thick	Generally dark brown to black
Middle Eastern	Wavy to curly, sometimes thick and voluminous	Oval, similar to Indian and Caucasian hair	Medium to high	Thick, coarse hair strands	Generally dark brown to black

Curly Hair

The percentage of people with curly hair varies by ethnicity, but it’s generally less than 20% globally. Curly hair is considered a dominant gene trait, meaning that if one parent has curly hair and the other has straight hair, the child will likely have curly hair.

Curly hair has unique needs because the natural oils from your scalp, which provide moisture and protection, have a harder time traveling down the twists and turns of curly strands compared to straight hair. This makes curly hair more prone to dryness and requires extra hydration. Research also shows that curlier hair tends to be more delicate, with smaller hair diameters and reduced strength, making it more vulnerable to damage from combing and styling. This can lead to breakage and frizz if not properly cared for.

To keep curly hair healthy, focus on regular conditioning and lubrication to minimize damage during grooming. However, striking a balance is essential – overloading curls with heavy products can weigh them down. Since curly, textured hair has lower tensile strength and less access to the scalp’s natural oils than straight hair, it requires a little extra care and attention to keep it strong, hydrated, and beautifully defined.