Chemical damage to the hair by the permanent wave, or vice versa, straightening as well as bleaching and dyeing of the hair causes irreversible changes in the internal structure of hair. What kind of hair do we call chemically damaged? When the hair is being too often and "mercilessly" colored, brightened, chemically permed or weaved and this results in split hair, knots or loops - we call this hair chemically damaged and porous. Dyed hair is especially susceptible to the effects of UV rays, as well as these artifical pigments tend to faster oxidize and eventually fade. To ensure stabil colours of dyed hair you need to use specialty shampoos, conditioners and masks for coloured hair. These products have no oxidizing substances and contain many nutrients, moisturizes and restoring components. If the hair is often dyed or damaged by chemicals, an at least once a week nutrient mask application is requried. The surface of the chemically damaged hair is porous and rough, flaky outer layer has multiple cracks, longer hair ends are usually split. These hair is difficult to comb through, they knot and loop and easily break, it makes it difficult to create a sustainable hairstyle. These hair do not shine and lack moisture, softness and elasticity, they are more rigid and have different thickness throughout the length. These hair should not be blow dried by hot air and,  in case if so requried, special hair shaft heat protecton products shall applied prior.  Chemical damage causes may eventually increase hair deterioration if, besides the lack of proper hair care there are nutrition disorders of hair follicles present. Specialty leave-in products after each hair wash shall be applied to scalp skin and hair along their entire length in order to maintain at least superficially chemically damaged hair elasticity and shine.