* Highly processed hair: This includes hair that has been frequently bleached, permed, relaxed, or chemically straightened. These processes damage the hair's cuticle (outer layer), creating gaps and openings that allow moisture to be absorbed and lost quickly.
* Damaged hair: Heat styling (blow drying, flat ironing, curling), environmental factors (sun, wind, pollution), and harsh products can all damage the cuticle, leading to increased porosity.
* Naturally curly/coily hair: The natural structure of curly and coily hair often makes it more porous. The bends and twists in the hair strand lift the cuticle, making it easier for moisture to enter and escape. Type 4 hair (kinky/coily) tends to be the most porous of all hair types.