Along with texture and density, porosity is a characteristic of hair that is important to consider when taking care of your tresses. Specifically, this attribute can impact how your hair reacts to henna and other dyes. As defined by Healthline, porosity refers to how well the hair can ingest and sustain moisture. It's dependent on the composition of the outer layer of the hair, which is made up of cuticles that can be flat or raised and sit far or close together.

If you have high porosity hair, then your strands are very porous. This means your locks absorb moisture easily but have a hard time maintaining it. As a result, your hair may absorb products easily but ultimately feel dry and frizzy. YouTuber BiancaReneeToday explains that hair porosity is usually genetic but that heat and chemical damage are known to increase porosity.

Low-porosity hair has difficulty taking in moisture, often taking longer to become saturated by water and more time to dry entirely than other porosity types. BiancaReneeToday mentions that products tend to sit on this hair type, leading to unwanted build-up and even making the hair feel greasy at times.

Medium porosity is typically the desired hair type because it moderately absorbs and retains moisture.