The Physical and Chemical Properties of Hair

Human hair is very complex: it is made of keratin polymers rich in amino acids and trace elements, and is used in forensic science for disease diagnosis and blood group determination.

What are the Chemical Components of Hair?

Hair, a fascinating subject, is a natural polymer fiber formed by fully keratinized keratin cells. Dominated by cysteine, these cells are packed with keratin proteins made up of various amino acids, with cysteine making up a significant 15.5%. Intriguingly, these keratin cells in hair are devoid of any biological activity, practically lifeless.

Hair includes a plethora of trace elements that may be detected in over 20 different types, including iron, copper, iodine, fluorine, selenium, zinc, and arsenic. Notably, the concentration of these components is much higher in hair than in blood or urine. Moreover, hair also contains blood type substances.

Due to its keratin composition, hair is almost indestructible by nature. For instance, the ancient Egyptians, with a history of over 2000 years, were discovered with little more than dust and a few bones, and yet their untreated hair remained. Hair's tenacity lends itself to a variety of novel uses, including pollution detection, disease diagnosis, medicine dosage estimation, and blood type testing.

A example in point is the 2100-year-old female mummy discovered in Changsha, Hunan Province, China, in 1972, which sparked a worldwide uproar. Despite the long burial, the mummy was astonishingly well-preserved, with skin unbroken and hair remaining securely attached, allowing blood type determination via hair analysis to be effective.

Napoleon's mystery death in exile on the island of Saint Helena in 1874 has sparked controversy for nearly a century, with most hypotheses pointing to violence or cancer. However, an analysis of his hair in the 1960s revealed arsenic levels 40 times higher than normal, implying that mercury poisoning was the true cause of death.

Present-day doctors use hair metal and non-metal element concentrations to diagnose certain diseases. For example, children with stunted growth, lack of appetite, or even peculiar eating habits often have lower zinc levels in their hair. Calcium levels in the hair of elderly patients with coronary heart disease are 60% to 70% lower than in healthy individuals. Similarly, manganese and cadmium levels in psychiatric patients' hair are lower than normal, and hair analysis can be used to diagnose disorders such as Keshan disease (selenium deficiency) and Minamata disease (mercury toxicity).

What are the Different Colors of Hair?

Human hair color varies greatly due to factors like geography, ethnicity, genetics, and dietary habits. Typically, Caucasians have brown or light yellow hair; Africans mostly have dark brown hair; Asians predominantly have black hair with varying shades, with pure black lightening to gray or white. Red hair, occasionally seen among Chinese, is not necessarily indicative of mixed heritage, as red-haired individuals have been recorded in Chinese history, exemplified by "Red-haired Old Liu" in the classic "Water Margin."

Hair color, like height, weight, skin color, and eye color, varies by individual. The color of hair is determined mostly by the makeup of its constituents, which is influenced by elements such as the amount of pigment, the presence of air bubbles, and the structure of the hair cuticle.

At the hair root, the bulb cells contain no melanin, but the cells above the bulb, moving from the hair matrix, contain melanin in their sparse, thread-like divisions. The large cells at the top of the hair papilla, branching melanocytes that originate from the epidermis, distribute melanin to the undifferentiated cells of the hair cortex and medulla.

Human hair is mainly composed of cortex with a small amount of medulla, hence the depth of the black color in hair largely depends on the amount of melanin in the cortex and the presence of air bubbles inside and outside the cells. More melanin and fewer air bubbles result in darker hair; conversely, less melanin and more air bubbles make hair lighter or even white.

Scientific research has confirmed a correlation between hair color and the metal elements contained in hair tissue. Hair with equal levels of copper, iron, and melanin, for example, appears black; nickel-rich hair appears gray-white; titanium-rich hair appears golden yellow; molybdenum-rich hair appears reddish-brown; and copper and cobalt-rich hair appears red-brown. Excess copper causes green hair, but excess iron or severe protein deficiency causes red hair, demonstrating that hair color is controlled not only by inherited factors but also by individual health and food.

Why Does Hair Have Different Textures?

The natural texture of human hair, like hair color, varies according to geography, ethnicity, genetics, and food. Caucasians have wavy hair, Africans have curly hair, while Asians have straight hair.

In general, the shape of the hair shaft corresponds to its cross-sectional shape: wavy hair has an oval cross-section, curly hair has a flattened oval, and straight hair has a circular cross-section. People of African heritage can have wavy hair, Caucasians can have straight hair, and Asians can have both wavy and curly hair.

The arrangement of cells in hair is genetically determined and plays a key role in its texture and shape. The formation of various hair shapes is primarily due to the internal action of the hair's constituent components. The curliness of hair is often attributed to its keratinization process. Curly hair tends to be eccentrically positioned within the hair follicle, meaning one side of the root sheath is thicker than the other. On the thinner side, the cuticle and cortex cells begin keratinization earlier, hindering the hair's growth rate on that side. As a result, the hair curls towards the side where keratinization occurs earlier.

Furthermore, the cortex and cuticle consist of hard protein (rich in sulfur), while the medulla and inner root sheath are made of soft protein (lacking sulfur). The different types of keratin protein affect the timing of keratinization, influencing the hair's texture. If three hair follicles open into one pore or if one follicle produces two hairs, these conditions can alter the arrangement of keratin cells in the hair, resulting in curly growth.

Artificial hair perming involves rearranging the cell alignment within the hair, leading to a curly texture.

What Factors Affect the Thickness of Hair?

Hair thickness not only differs between individuals, but it also changes throughout time. Hair begins to grow three months after conception; the fetal hair is shed after birth, and the hair continues to grow, getting thicker. Hair grows thicker in adulthood, with each strand ranging between 0.05 and 0.1 millimeters in diameter. Hair can thin with age, which is particularly frequent in males with male pattern baldness.

Hair thickness varies by ethnic group as well. Asians, on average, have thicker hair than Caucasians and are less prone to baldness. Hair color, texture, and thickness are also affected by nutrition and metabolism. Protein shortage, for example, might result in hair that is thin, dry, brittle, lusterless, and easily broken.

What Are the Different Hair Quality Types?

Hair, like facial characteristics, stature, and skin color, is a distinguishing trait. It's easy to notice that people's hair quality differs. Some people have oily hair while others have dry hair. Hair quality can be divided into numerous categories:

1. Oily Hair: This kind seems greasy and shiny, as if coated in oil, with fine and fragile hair strands. Although much sebum can protect the hair from breaking, fine hair requires less sebum coverage, which results in an oily appearance. Sebaceous glands on the scalp are generally plentiful and active in people with oily hair.
2. Dry Hair: Dry hair is harsh, stiff, inelastic, and lifeless due to a lack of sebum. The strands are prone to breaking and splitting and frequently curl, with split ends or knotted bunches. Prolonged sun exposure, strong winds, dry air, and alkaline soap can all absorb or destroy the natural oils in the hair, causing dryness and damage. Pool and seawater containing chlorine can bleach and damage hair.
3. Normal Hair: This hair type is smooth and lustrous, not greasy or dry, and is simple to style and manage. It signifies typical, healthy hair.
4. Combination Hair: This kind has dry ends and an oily scalp, or a single strand with both dry and oily hair. It is more common in women during their reproductive years and is often associated with an abundance of dandruff.
5. Damaged Hair: Often resulting from improper perming or coloring, this hair type feels rough, has split ends, is dry, and difficult to manage. Surveys suggest that about 25% of damaged hair cases are due to inappropriate shampoo (hair wash) selection.

To determine your hair type, simply observe your hair the day after washing. If it appears limp and greasy, it's likely oily; if it feels smooth, it falls into one of the other categories, which are relatively easy to distinguish.