The definition of porosity is the ability of hair to absorb moisture, which is mainly due to how swollen or tight the cuticle layer is against the hair shaft.
Low porosity means low absorption of moisture
High porosity means high absorption of moisture
Understanding porosity is important to comprehending what effect various products have on the hair shaft. Coloring and chemical services, shampoos and treatments all have a different effect on healthy vs. damaged hair.
Porous hair absorbs product more rapidly and can release it more quickly – causing color to fade. Healthy hair absorbs the proper amount of product.
Hair becomes porous, dry or brittle because to the loss of natural lipids. Very porous hair, where the cuticle is wide open, is usually damaged from over-processing due to too much bleach, over-coloring, and straighteners or due to the over-use of heat from flat irons or blow dryers.
Overly porous hair tends to be damaged. Having the cuticle wide open is like an escape hatch for hair color. Even the best color line can fade more quickly due to unhealthy hair. Damaged hair looks dull and lifeless, and breaks easily.
External elements like blow dryers, flat irons, the sun, and even rough handling of delicate hair when it’s damp can also cause damage. Extreme damage can result in the cortex rupturing.