Lack of hydration and loss of mass are different damages. The first is a problem with the cuticle, the outermost layer of the hair, and the hair is rough to the touch. Mass loss is a problem that occurs in the cortex, the innermost layer of the hair that ends up becoming porous. As a result, the wires become weak and brittle.
If the hair is rough, it needs nourishment and repair. If it is porous and fragile, it needs reconstruction and regeneration. Generally, chemical treatments, such as straightening and discoloration, damage the hair in both the inner and outer layers of the fiber.
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