This is one of the most important properties of hair. Because of its elasticity, hair can resist forces that could change its shape, its volume or its length. Its elasticity lets it spring back to its original form without damage.When healthy hair is wetted and stretched, it can increase in length by up to 30% and still return to its original length when it is dried. Stretching it more than this will tend to damage it, however, leading to permanent lengthening and even breaking.The elasticity of hair depends on the long keratin fibres in the cortex. Chemical treatments of hair such as perming and bleaching can alter the cortex after repeated damage, and change the hair's elasticity. Hair with poor elasticity will stretch only to a limited extent. It will not curl, it will break easily when it is groomed and it cannot be permed satisfactorily.Both natural sunlight and artificial ultraviolet light break down chemicals in the hair and damage its elasticity in the same way that bleaching does, though to a much lesser degree.
Static electricity
When dry hair is rubbed, as it is whenever it is brushed or combed, static electric charge builds up on the hairs. This is especially noticeable in hot, dry weather. Hair has negative charges, which tend to push each other apart, and as a result charged hairs can never lie smoothly against each other. The result is 'fly away' hair, which stands out from the head and looks unmanageable.
Moisture content
At least 17% of humidity is required by the hair fibers to maintain a healthy appearance. Up to 35% of water retention is possible. Hair length may vary up to 2% depending on the air humidity. ( In 1783, Swiss physicist and geologist, Horace Bénédict de Saussure built the first hygrometer using a human hair to measure humidity. You even can make such one by yourselves! http://www.kidsgardening.com/growingideas/projects/aug03/pg1.html)
The moisture content of hair is greater when the atmosphere is moist and humid, and less when the air is dry. When hair is wet the cortex swells and the edges of the cuticle scales tend to lift.
The hair surface temporarily loses its smoothness. There is therefore more friction when wet hair is rubbed than when it is dry. This is what can lead to matting and tangles developing during over-vigorous shampooing (there is not greater static charge on wet hair than on dry).
Slight lift of the hair cuticle
Porosity
In a normal, undamaged hair shaft, very little water can get either into or out of the cortex. This is because the cuticle covering the cortex is intact, and is then almost (but not quite) waterproof.
When hair is permed or tinted, however, the chemicals have to penetrate the cortex in order to react with the keratin inside it. Increasing the temperature, or applying an alkaline lotion, separates the scales of the cuticle enough to allow the chemicals to pass through. After the processing is finished the scales gradually close up again.But if hair is processed too many times the cuticle scales may never return to their original tightness and the protection they once offered is lost. The cuticle can also be damaged in the same way by too much blow drying, curling irons that are too hot, and the effects of wind and sun. The hair becomes increasingly porous, and water can then pass in and out of the cortex.Over-porous hair is dry, and tends to develop split ends. The damaged cuticle is fragile, and the damage worsens as time goes by. The greater the damage, the more the cortex swells with water whenever the hair is washed, but the more water it loses when it dries. The repeated wetting and drying of the cortex gradually weakens the hair.
Hair damaged by perming
In a normal, undamaged hair shaft, very little water can get either into or out of the cortex. This is because the cuticle covering the cortex is intact, and is then almost (but not quite) waterproof.
When hair is permed or tinted, however, the chemicals have to penetrate the cortex in order to react with the keratin inside it. Increasing the temperature, or applying an alkaline lotion, separates the scales of the cuticle enough to allow the chemicals to pass through. After the processing is finished the scales gradually close up again.But if hair is processed too many times the cuticle scales may never return to their original tightness and the protection they once offered is lost. The cuticle can also be damaged in the same way by too much blow drying, curling irons that are too hot, and the effects of wind and sun. The hair becomes increasingly porous, and water can then pass in and out of the cortex.Over-porous hair is dry, and tends to develop split ends. The damaged cuticle is fragile, and the damage worsens as time goes by. The greater the damage, the more the cortex swells with water whenever the hair is washed, but the more water it loses when it dries. The repeated wetting and drying of the cortex gradually weakens the hair.
Hair damaged by perming Light reflector
A healthy cuticle is more than just a protective layer. Much of the shine that makes healthy hair so attractive is due to the cuticle. Intact cuticle cells are smooth and glossy, and reflect light from their surfaces. This, together with the pigment within the cortex, gives hair its characteristic appearance.Black hair reflects less light than blond hair does. Black hair appears glossier, however, Because the bright bands of reflected light сontrast more sharply with the darkness of the rest of the hair, as in the photograph below.
The hair shaft seen with the light microscope: light is reflected from the colorless cuticle and bent as it passes through the hair - this effect gives hair the color we see
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