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Porosity is the term used in the science of hair care to describe how easily water and other matter can diffuse back and forth through the cuticle layer and into or out of the cortex. Hair is much like a sponge, capable of absorbing water and other substances from the environment, and also susceptible to losing precious moisture and lipids to the environment. Maintaining an optimal balance of moisture in your hair preserves its suppleness, strength, and shine. This is especially important for those of us with curly hair, as it greatly influences the health and beauty of our tresses.

Low Porosity: Hair described as having low porosity is characterized by a very tightly bound cuticle layer, where the individual cuticle scales lie flat and overlap one another. Low porosity hair is often quite shiny, especially if it is a darker color. Overall this type of hair is considered to be quite healthy. If your hair repels water when you attempt to wet it, that is a good indication that it has low porosity. It can be quite difficult to process, because it resists penetration of the chemicals being used.

Normal Porosity: Hair possessing average porosity will generally require the least amount of maintenance. It allows moisture to pass into the cortex as needed, but resists permitting too much water to penetrate. Repeated works by various research groups have found that healthy hair of average porosity can absorb water up to a maximum of 31.1% by weight. Normal porosity hair has a tendency to hold styles well.

High Porosity: High porosity is an unfortunate result of damage to the hair. Chemical processes, harsh treatment, and environmental exposure are all responsible for causing cumulative, irreversible damage to the cuticle layer. This damage creates gaps and holes in the surface of the hair shaft—essentially chinks in its armor. Hair with this type of uneven, pitted and rough surface is prone to damage from more and more sources, resulting in a cascade of effects that culminate in unmanageable and unlovely locks.

In a similar fashion, curly hair has a tendency to be naturally higher in porosity than straight hair.

Hair Porosity Test: In order to test your hair’s porosity, you will need only a bowl of water.
Healthy hair should float because it has a sealed cuticle and therefore does not absorb water.
Excessively porous hair has open cuticle which will soak up water and sink. If this happens during your test, your hair needs a deep conditioning treatment in a salon. Most important essentials for treatments and products are those with protein and moisture. Protein is necessary to fill the holes in porous hair as well as to strengthen it while moisture is necessary to improve shine and manageability.