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Our hair starts out with an innate, natural porosity, and our environment slowly increases it over time. This explains why your roots will always be less porous than the tips of your hair.

If you’re not familiar with porosity, you’re not alone. It’s a concept that, until fairly recently, was rarely referenced beyond hair industry insiders and scientists. The birth of the Curly Girl Method, as well as a broader shift towards conscious consumerism in the beauty world, has brought porosity to the attention of everyone who wants to understand and take charge of her hair care.

Porosity is the measure of empty space in a material. A sponge, for example, is highly porous—it's literally designed to absorb water. When we refer to hair porosity, we are talking about how much your hair likes to absorb water and other fluids.

There is more to your hair’s absorbency than the degree of empty space in your hair. The type of material covering the surface of your hair is also an important factor. Imagine if a porous sponge was inserted into a ziplock bag. It doesn't matter that the sponge itself is porous and has lots of void space. The ziplock bag is not porous; it repels water, so it provides a protective barrier to the sponge. The same phenomenon applies to your hair. The cortex, or central component of your hair, will have a certain natural porosity, but on top of that surface are your hair cuticles, which act as a barrier. See our previous blog post on cuticles to learn more.

Cuticles look like shingles on a roof. Your hair has multiple layers of cuticles.

They're bonded together by something called the cuticle-cuticle cell-membrane complex, which acts like cement. It's a fancy name for a compilation of proteins and lipids (fats). A crucial part of this natural hair cement is a component called 18-MEA. 18-MEA is hydrophobic; or put more plainly, it repels water. To illustrate 18-MEA’s effect on hair, imagine taking a piece of paper and covering it with wax. Wax, like 18-MEA, repels water. The more of this 18-MEA you lose, the less likely your hair is to repel water.

As with thickness, porosity is a spectrum.

Your hair is not simply porous or non-porous; it’s likely somewhere in between. Our hair starts out with an innate, natural porosity, and our environment slowly increases it over time. This explains why your roots will always be less porous than the tips of your hair. The tips have been around longer and thus have experienced more damage, making them more porous.

When we talk about hair porosity, we're referring to the way in which your hair absorbs water and products. To test porosity, we perform a contact angle measurement. A contact angle describes how a drop of water sits on top of a material: whether it forms a droplet and rolls around, or it lies flat on the hair. If the water penetrates the hair, the contact angle is zero, which would indicate that the hair is highly porous. If the water became a full sphere and rolled around on the hair, the contact angle is high and there is no porosity. Your hair’s response to water lies somewhere in the middle. We use a piece of lab equipment called a surface tensiometer to accurately measure this contact angle and identify your hair’s precise behavior.

Basing hair care on porosity.

We can use products to temporarily reduce the porosity of our hair. 18-MEA is essentially a fat, so if we want to keep our hair hydrated, we can apply conditioners and oils (which repel water - oil and water don't mix) to our hair. On the other hand, if you have low porosity hair, applying oils will cause greasiness; the oil has nowhere to go and will form a film on your head.

Porosity changes with heat and pH. If you have low porosity hair and want to increase porosity temporarily so that your strands adsorb leave-in conditioner, you can apply heat (a hot shower will do!). Heat helps loosen the cuticles on your hair and lets the product seep in. Similarly, products with a high pH (basic products) open the cuticles and increase porosity. Low pH products (acidic products) close up the cuticles and lower porosity. Using acidic products without a professional is not recommended, as sudden changes in porosity can significantly damage your hair.

Porosity is not only an important factor in how water and products enter your hair but also in how water leaves your hair. High porosity hair has little protection, meaning that the water in your hair (and there is a lot of water in your hair, even when it's "dry") can evaporate quickly, leaving your hair feeling dry and brittle. People with high porosity hair will notice that their hair dries very quickly after a shower.

Dying/bleaching hair can increase hair porosity. This is because the process of chemically treating hair disturbs its chemistry. A hair professional can make the right calculation about your hair condition and use the proper products, with the right pH, to reduce the impact of chemical treatments.

Have questions about porosity? We'd love to help! Comment below, and we'll be sure to respond promptly.

So really, whether silicones wash out of your hair and whether they build up, depends on the restrictions you put on your hair routine. If you choose a sulfate-free or low-poo routine, you should avoid water insoluble silicones. We believe it's ok for most people to use shampoos containing sulfates. Based on our analysis, low-poo or no-poo shampoos are more harmful than helpful, but we’ll do a post soon on the topic.