What is hair porosity?
Hair porosity is used to describe how open or closed the hair cuticles are. Hair porosity can be altered by chemical treatments such as color and relaxers, which tend to open the cuticles.
Low porosity
Cuticles are closed. More difficult for products to enter the hair strand. Usually heat/steamer will help to open cuticles and allow products in.
Medium porosity
Hair porosity is right in the middle. Not too difficult to moisturize hair and usually does not dry out quickly from loss of moisture.
High porosity
Cuticles are open. Moisture enters and leaves the hair strand quickly. Usually need a sealant (oil or butter) to help the hair retain moisture.
How to test porosity?
Hair strand test in a glass of water is the easiest.
You place a strand of clean hair (right after washing) before addition of conditioner or any products. Leave the hair strand in water for 5 minutes. If it floats, this is low porosity hair meaning cuticles are closed and water is not easily absorbed. If it sinks, this is high porosity hair and cuticles are open. Moisture enters the hair strand easily and is able to make the hair sink.
It is important to know your hair porosity as it is will help to determine the types of techniques you need to use when applying products to your hair. For example, low porosity hair benefits from heat/steamer when using a deep conditioner. Whereas high porosity hair really needs to focus on good sealants (oils and butters) rather than on absorbing the moisture. However, in all honesty all porosity hair truly needs to seal in moisture.