The Absorption of Water by Hair

Woman with wet hair
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Q: Does hair absorb water when you wash your hair or when you walk in the rain? Or does the water just stay on the surface of your hair?
A: Yes, hair absorbs water. The water doesn't just stay on the surface of the hair to evaporate later. There's much more happening, and most people are insufficiently aware of this.
The structure of the hair fiber allows water to penetrate during washing or exposure to humid environments and rain. The outer layer of the hair, the cuticle, consists of overlapping scales that can open. These scales resemble shingles and allow water molecules to enter the hair and reach the inner layers.
Hair has a porous structure and can absorb water to some extent, but the precise amount varies based on different factors. On average, human hair can absorb about 30% of its dry weight in water. This 30% is an average, and the water absorption will differ among individuals due to factors such as hair porosity and hair health.
Hair porosity is crucial in determining its water-absorbing capabilities and is categorized into three types: low porosity, normal porosity, and high porosity. Hair with low porosity has a more closed outer layer, providing more resistance to water absorption. It will absorb water more slowly and less compared to other hair types. Hair with normal porosity has a more balanced outer layer, allowing it to absorb and retain an average amount of moisture. Hair with high porosity has a more open outer layer, making it prone to absorbing water quickly and in larger quantities. However, it may struggle to retain that moisture.
The water temperature during hair washing plays a significant role. Warm water helps to open the hair cuticles, facilitating water absorption and ensuring a more thorough hair cleaning. However, extremely hot water can strip the hair of its natural oils, leading to dryness. Conversely, cold water tends to close the cuticles, creating a smoother surface but limiting the amount of water absorption.
Wet hair is often considered weaker than dry hair due to changes in physical properties when exposed to water. A hair strand consists of three layers: the medulla (innermost layer), the cortex (middle layer), and the cuticle (outermost and protective layer). The outer layer, as mentioned earlier, consists of overlapping scales. When the hair is dry, these scales lie flat, providing a smooth surface and contributing to the strength and protection of the hair.
The overlapping scales of the outer layer of hair
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However, when hair becomes wet, it swells. The water penetrates the outer layer, causing the scales to lift, and the hair starts to swell. This swelling is a natural and reversible process. But, as the scales lift, the middle layer of the hair, the cortex, is exposed. And when this middle layer is exposed, the hair becomes more vulnerable. Some degree of swelling is necessary for effective hair cleaning, but excessive swelling can be problematic.
The swelling of the hair fiber also leads to an increase in diameter and volume, and thus a larger hair surface. Wet hair is therefore more sensitive than dry hair. Increased friction between individual hair strands makes wet hair more susceptible to tangling and breakage, for example, during combing or drying with a towel. To minimize damage to wet hair, it should be handled with care. Use a wide-tooth comb to detangle wet hair, avoid aggressive brushing, and pat the hair dry with a soft towel.
Wet hair is also more elastic than dry hair. While elasticity is a desirable quality because it allows the hair to stretch and bounce back, excessive stretching can lead to damage. Wet hair can be stretched beyond its normal limit, making it more prone to breakage, especially during activities like combing or brushing.
See also:
What exactly is hair made of and how does it grow?
What is keratin in our hair and what is it good for?