Key takeaways
- Hair fibers have three regions: the cuticle (outer layer), the cortex (bulk of the fiber), and sometimes a medulla.
- When the cuticle is worn away or chemically damaged, the cortex is exposed and the fiber is more likely to fracture.
- The review describes damaged hair as more porous, but it does not evaluate consumer "porosity types," so we do not make claims about them here.
What it is
Hair is built in layers. The cuticle is a protective outer covering of overlapping cells, the cortex underneath provides most of the fiber's mass and strength, and a medulla is present in some hairs. The shape and orientation of cuticle cells influence how much friction hair experiences.
What the evidence says
A review of hair cosmetics describes how weathering and chemical treatment can lift, scale, or crack the cuticle. If the cuticle is lost, the cortex is exposed, and further damage can lead to fracture of the hair fiber. The same review reports that damaged hair is highly porous, and that a thinner cuticle layer in African hair has been associated with greater proneness to breakage. It also notes that hair cosmetics can reduce friction and water uptake, and may help limit breakage related to cuticle damage.
What the evidence does not establish
This is a narrative review, not a trial of specific products. It does not test whether any particular conditioner, oil, or routine prevents breakage in a given person, and it does not define or validate the low/medium/high "porosity" categories used in consumer marketing.
Who may want to pay attention
People who chemically treat, heat-style, or frequently manipulate their hair may want to think about cuticle wear. Persistent or sudden hair loss is worth discussing with a dermatologist.
Sources
- Gavazzoni Dias MFR. Hair cosmetics: an overview. Int J Trichology. 2015;7(1):2–15. Source
Last reviewed October 2026. Educational information only, not medical advice. Findings from one study or review do not apply identically to every person or product.