Hair covers most of the human body in different lengths, textures and thickness. Hair serves a number of features in humans, including insulation, protection, friction buffer and redirection of water and sweat from the body.
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The primary function of human hair is to insulate the human body. Hair does this in two ways: it serves as a physical barrier between external cold air and the skin, and it also traps warm air in between the skin and the hair, keeping the body warmer.
Protection from External Factors
Hair protects skin from external factors, such as sun damage and chapped skin from wind damage. It also blocks dust and dirt from settling on the skin.
Hair serves as a buffer against friction. This is one of the reasons that hair is present on the legs, genitals and underarms. It also soaks up sweat for easier evaporation so that the sweat isn't stuck to the body during physical activity, which can cause chapping and blistering.
Differentiation and Beautification
Hair is widely used in all mammals for differentiation and beautification. Male lions have manes, deer have decorative spots and many birds have coloring to distinguish males from females. Humans have hair for the same purpose, which is why men grow facial hair and women do not. Hair is also considered to add beauty, which is why women often have elaborate hairstyles and why men go to great lengths to battle baldness.
Redirect Sweat and Water
Hair also soaks up sweat from the skin so that it won't cause irritation from friction and it also redirects sweat so that certain parts of the body can dry faster and stay protected. Hair in the armpits pulls sweat from the armpits to cool the area and eyebrows redirect sweat from the forehead away form the eyes.