Hair porosity explained without the jargon

Porosity is how readily your hair takes in and lets go of water. It is largely a function of the cuticle: how tightly those overlapping scales lie against the shaft.

• Low porosity: cuticle lies flat and tight. Water is slow to get in and slow to leave.
• High porosity: cuticle is raised, chipped or gapped. Water enters quickly and escapes quickly.
• Normal porosity: somewhere in between, which is most people.

Porosity is partly genetic and heavily affected by damage. Bleaching and chemical processing raise porosity by disrupting the cuticle, which is why colour-treated hair usually behaves as high porosity even if it did not before.

The popular version says to drop a strand into a glass of water and see whether it sinks or floats. It is unreliable, and worth knowing why so you stop repeating it.

Whether a hair floats depends mostly on surface tension and on any residue or oil on the strand, not on internal porosity. A clean low-porosity hair can sink and an oily high-porosity hair can float. The test measures the wrong thing.

Signs of low porosity

• Water beads on the surface and hair takes a while to get properly wet.
• Products sit on top and take effort to work in.
• Hair takes a long time to air dry.
• Heavy products cause buildup quickly.

Signs of high porosity

• Hair soaks up water almost immediately.
• It dries quickly, sometimes faster than expected.
• It absorbs product readily but feels dry again soon after.
• It tangles easily and feels rough when you run fingers up the strand.
• Often accompanies colouring, bleaching or heat use.

If low porosity

• Use lighter products, since heavy ones sit on top and build up.
• Apply products to damp rather than soaking hair.
• Warmth helps. Applying oil or a mask with a warm towel improves how much gets in.
• Clarify occasionally, because buildup is your likelier problem than dryness.

If high porosity

• Layer: a leave-in for moisture, then something heavier such as an oil on top to slow the loss.
• Mildly acidic rinses help the cuticle lie flatter.
• Be careful with heat, which raises porosity further.
• Reduce friction, since a raised cuticle catches on itself and tangles.

The practical summary: low porosity needs less product applied more cleverly, high porosity needs sealing after moisturising. Everything else in the porosity conversation is elaboration.

Porosity can vary along a single strand. The roots are newer and usually lower porosity; the ends are years older and usually higher. A single label for your whole head is an approximation.

It also changes. Reduce heat and chemical processing and porosity improves as damaged length is trimmed away and replaced by new growth. It is a description of current condition, not a fixed identity.