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Physical Properties of hair

Hair diameter

Most people describe their hair as either thick or thin. However, the number of hairs you have can affect the thickness as well as the size if each hair. The diameter of scalp hair does not have a typical size given that different scalps have different hair structures. There are a number of factors that can affect the diameter of hair as can be seen by the examples given below:

• Genetics determine the structure and size of your hair and this can be extremely varied from person to person.
• Hair colour can also be a determining factor for example; red hair is thinner than black hair. (see hair types)
• Age is another issue. Babies and young children have finer hair than adults. As a person matures their hair becomes thicker and stronger. Then in older age hair can become thinner and finer again.
• Atmosphere can also affect the hairs diameter, such as heat and humidity. (see moisture content)
• Hair products and chemicals can also affect the hairs diameter. Chemicals such as perms, dyes, and bleaches can increase the diameter, as can certain shampoos, conditioners, and styling products.

The human hair diameter can be so varied that measurements have been known to range from 20 to 180 µm (millionths of a meter).

Hair Texture

Commonly hair texture is termed as thin, fine, medium or thick, coarse and wiry. Hair texture can change with the addition of products, chemicals or hair styling.

The texture or feel of hair can depend on a number of factors:

• The diameter of the individual hairs - the larger the hair diameter the coarser it will feel.
• The different types of hair: some types feel soft, smooth, and silky. Others dry, wiry and rough.
• The texture of hair can also be affected by the extent of weathering on the hair. See footnote.
• Hair texture is also affected by the external elements it has been exposed to. Applications of hair products give hair a different feel from that of hair which has been freshly washed and conditioned. In addition to this, hair that has been chemically treated often has a different texture to hair which has not. Applications of heat such as blow drying, tongs and straightening irons can also have an effect.

Note: “Weathering” is a term used to describe the progressive deterioration of the hair. Firstly the cuticle layer, then eventually the cortex. This is caused from exposure to continual daily wear and tear.

Hair shape

As well as hair appearance being termed in colour or texture, it can also be referred to as straight, curly, wavy or even frizzy. Hair shape can vary widely in people. What determines the shape of the hair is the shape of the hair follicle and its position in the scalp. This shaping and positioning of the hair follicle is genetically determined. The follicle is fundamental in giving the hair its shape from the moment it is manufactured and, as hair is considerd biologically dead tissue, the hair tends to keep this original shape. This shape is never entirely circular. Curly hair is shaped like an elongated oval and grows at an angle to the scalp. It also has a different biological structure from that of straight hair.

Curly hair is often likely to be dry in comparison to straight hair and tends not to shine. In most cases, the reason for this is the hairs lack of smoothness. Oils present on the hair, secreted by the sebaceous glands, travel down the hair more easily when hair is straight, this helps to keep moisture in and the hair soft and smooth. The smoother the hair, the more it will shine. This is due to the level of light reflected from the surface of the hair. Very curly hair can be dry, often frizzy and difficult to manage. See Hair Care

Moisture content

Atmosphere can affect the moisture content of hair:

• Water content in the hair is greater when the air is moist and humid.
• Water content in the hair is less when the air is dry.

The reason why hair changes in different atmospheres is down to the moisture in the air. For example, damp and humid conditions can cause wavy hair to become curlier or even frizz. This is due to the hair increasing in water content and reducing its static electricity content. Curly hair becomes curlier the wetter it gets. When hair is wet, the cuticle scales tend to lift, the cortex can swell and the hair surface temporarily loses its smoothness.
It is also important to note that different hair types have different moisture absorbing abilities, the more Porosity the hair has the more moisture can penetrate it.

Porosity

Soft, smooth and silky hair is likely to have less porosity than other dry, wiry and rough hair. This is largely due to the cuticle layer of the hair resisting the penetration of the moisture in the air. The hair can also become increasingly porous, such as when it is subjected to chemical processing or damage by external elements. The more porous the hair is the dryer it will feel, as more moisture can penetrate the cortex. If this is the case, more moisture can then be lost leading, ultimately, to weaker hair. Conditioners, of course, can help prevent this moisture loss (see conditioners).

Elasticity

 
Refers to the hairs ability to stretch and return to its original shape without breaking. The Degree of elasticity in the hair can be measured by how much it is able to be stretched whilst still being able to spring back to its original form without breaking. Due to its incredible elasticity hair can be very resistant. When healthy hair is wet and stretched it can increase in length by up to 30% and still return to its original length when it is dried. However, stretching it more than this can lead to snapping or breaking.

There are a number of factors that can affect the elasticity of hair, the most common of which are mentioned below:

• The elasticity of hair depends on the long keratin fibres found in the cortex layer of the hair.
• Chemical treatments on hair such as colouring, bleaching and permanent waving can alter the cortex and, after repeated damage, change the hair's elasticity.
• Ultraviolet rays from natural sunlight or artificial light can either break down chemicals in the hair or
• damage its elasticity in the same way that bleaching does, though to a much lesser degree.

Hair with poor elasticity will stretch only to a limited extent. It can break easily when it is groomed and cannot be styled properly.

Hair strength

In general, strong hair is considered a sign of healthy hair. However, on the whole, hair is remarkably strong and resilient. The key factors in supplying the hair its' strength and durability are the disulphide bonds. The middle layer of the hair, the cortex, is made up of polypeptide chains cross-linked with each other by side bonds. These bonds that link up the polypeptide chains of the hair are disulfide bonds, hydrogen bonds and salt bonds. Below you will find greater detail pertaining to each of these three types of bond:

• Disulfide bonds are chemical side bonds. These bonds are stronger and fewer than hydrogen and salt bonds and cannot be broken by heat or water. A disulfide bond is joined to the sulphur atoms of two cysteine atoms to create cystine. Although the disulfide bonds are fewer than the physical bonds they are the key factors in supplying the hair its' strength and durability.
• Hydrogen bonds contribute to approximately one third of the hair's strength. The hydrogen bond is a weak physical side bond that is easily broken by water or heat. Hydrogen bonds can be reformed by drying or cooling the hair. These bonds are very abundant in the hair.

Salt bonds are also weak physical side bonds that are easily broken by either, weak alkaline or acid solutions and changes in pH. These bonds can be reformed by normalizing the pH level of the hair.

The breaking and altering of these bonds is implemented to change the hair in physical and chemical processes. For example by water as in wetting the hair then drying it in a different shape, this process then re-sets the bonds.

In chemical processing for example, an application of alkaline or acid such as perming. As perms break the bonds then hydrogen peroxide applied will then fix the bonds in the altered form.

Static Electricity

Static electricity is the imbalance of positive and negative charges. As hair can act as a conductor static electricity can build up on the hairs and its effect on the hair is best described in the following way:
If an item, such as a woolly hat, rubs against your hair, electrons move from the hair to the hat and a static charge builds up on the hair. As the static charge builds, each of the hairs end up with the same positive charge. Items with the same charge repel each other and, as such, each hair tries to get as far as physically possible from all other surrounding hairs. In order to do this they stand up on end and away from one another. This is how static electricity causes or contributes to a bad hair day! This can also happen whenever the hair is rubbed, brushed or combed, especially when the hair is dry. Subsequently, charged hairs can never lie smoothly against each other. The result is 'fly away' hair which stands out from the head and looks unmanageable.
Static electricity on the hair is commonly noticed more in the winter when the air is dryer. During the summer the air is generally more humid and the water in the air means that less of a static charge builds up on the hair. Shampoos with added conditioning agents, conditioners and some styling products leave the surfaces of the hair smooth. As a result, there is less friction when the hairs are rubbed together, therefore less static electricity builds up on them and the 'fly away' problem is less. Also, many hair products can contain anti-static properties.
See Hair Care

Hair Types

Three types of hair grow on the human body

• Lanugo: found on unborn babies. The hair grows around 12 weeks after conception, all over the body. It is a soft, fine downy type of hair and is all the same length. The hairs are lost around 4 weeks prior to birth.
• Vellus: These are short in length, up to 2cm, and are generally fairer hairs; they do not have Sebaceous glands and are replaced by terminal hairs in the pubic and axillaries at puberty. For men these also appear in the beard area.
• Terminal hairs: These are the longer generally thicker hairs found on the head and, for some, on the face and body. Follicles with sebaceous glands produce them and growth is hormonally influenced. Terminal hairs can regress to vellus hairs as a person ages.

Hair Colour and Pigment

As well as texture, hair is commonly typed according to its colour, for example: someone who is blonde-haired, a red head, a brunette or even if they have mousey hair. The most frequent chemical process that is used on hair is to change its natural colour to something different. There are many different shades of colour that can be applied to the hair ranging from natural colours, used for the covering of grey hair, to radically bright colours used as a styling statement.

The pigment that gives colour to hair and skin is called Melanin. The melanin is produced by an assembly of cells called melanocytes. These cells are present in the hair bulb during the growth phase. Particles within the melanocyte, premelanosome and melanosome produce pigment. The completion of melanin formation occurs when melanosome gives way to melanin granules. The hair gets its colour from two types of melanin that create the variety of hair colours. Eumelanin is brown/black in colour and is the most common type of melanin. This form of melanin gives colour to hair shades from black to brown. Phaeomelanin is reddish-yellow in colour and gives the red, yellow and ginger shades of hair colour. The type and amount of melanin present in an individual is dependent upon their inherited genes.

The factors that determine the colour of hair are:

• Type of pigment
• Amount of pigment
• Distribution of pigment within the hair

Greying Hair

Greying hair is actually white. The hair whitening process is known as canities. Hair appears grey due to the mixture of pigmented hair and non pigmented (white) hair. The amount of pigment granules in the hair gradually begin to decrease with age. A person typically begins to grey between late twenties and forty years of age. The reason for this is that the cells known as melanocytes begin to slow down production and therefore produce less melanin (pigment). This is part of the natural aging process in humans. It has to be noted, however, that some serious illnesses or emotional conditions may also cause the hair to grey prematurely (See Premature Greying below).

Premature Greying

There are some instances when the hair will lose its pigment prematurely. This may occur for a number of reasons

• Presence of a particular gene or genetic occurrence
• Medical conditions - such as vitiligo
• Inflammation of the hair follicle, which can damage the melanocytes- this is often temporary and can be reversed (See Hair Nutrition).

There has been some research to show that greying hair could potentially be reversed (See Grey Hair Today – Gone Tomorrow!)

Albinism

This describes an inherited condition in which there is no pigment present in the hair and skin, leading to the appearance of very pale skin and white hair.

Vitiligo

Vitiligo is an acquired depigmenting disorder due to the destruction of pigment cells called melanocytes. The cause and progression of the condition remains largely unknown, however, autoimmunity remains the most popular theory. This is thought to be the case because of the association of vitiligo with autoimmune thyroid diseases and the increased prevalence of autoantibodies including thyroid autoantibodies.