What is the term for the difficulty in moisture or chemicals penetrating the hair?

The term that accurately describes the difficulty of moisture or chemicals penetrating the hair is porosity. Porosity refers to the hair's ability to absorb and retain moisture or other products. It is determined by the condition of the hair cuticle, which is the outer layer that protects the inner structures. Hair can be categorized as low, medium, or high porosity based on how easily moisture penetrates it.

Low porosity hair has tightly packed cuticles that resist moisture entry, making it challenging for products to get absorbed. This can lead to buildup on the hair because it does not readily allow moisture to penetrate. High porosity hair, conversely, has gaps and holes in the cuticle, allowing moisture to enter easily, but it may also lose moisture just as rapidly.

Understanding porosity is essential for choosing the appropriate hair care products and treatment methods. Products designed to moisturize or chemically treat the hair need to be selected based on the hair's porosity level for effective results.