Hair porosity is the ability of your hair to absorb moisture and retain it. Dr. Isfahan Chambers-Harris explains it with a metaphor: "If you have a straw and put holes in it, you can take in the liquid easily, but you can't retain the liquid. Once you take it out of the liquid, it's all going to run out of the straw.”

There are three types of hair porosity: low, medium, and high. Founder of Too Groovy Salon, Robin Groover, explains that "Low porosity hair has cuticles that are bound very close together, normal porosity hair has cuticles that are slightly less bound, and high porosity hair has cuticles that are more widely spread out."

Knowing what type of porosity you have is crucial when it comes to picking the right hair care routine, as “it will guide you on how to pick out products to either help replenish moisture in your hair or deal with issues of dryness and breakage," states celebrity hairstylist Kari Williams. It also affects how your hair absorbs and retains dye.

Low-porosity hair takes a long time to dry, shows product build-up very visibly, and takes a longer time to get your hair fully wet when you're washing it. Dr. Isfahan Chambers-Harris states that "Low porosity [hair] is actually healthy hair that is not being moisturized correctly [...] so it becomes dry and tangled."

High-porosity hair tends to look and feel dry, it's usually frizzy and dries quickly — “[it] can take in water, but you can't retain the water," Dr. Chambers-Harris says. Frequently straightening, blow-drying, or bleaching your hair can damages your cuticles and cause high porosity.