How to Care for Low-Porosity Hair, According to a Cosmetic Chemist

Find out how to gauge your hair's porosity and how to find the best products for low-porosity hair.

Hair Health Hotline: Low-Porosity Hair
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One of the easiest ways to find out whether someone's ever fallen down a hair-care rabbit hole is to ask them to describe their hair. Beyond stating that their hair is "blonde," "short," or "curly," they'll probably launch into a monologue about their hair's density, thickness, and elasticity.

They'll doubtless also touch on their hair's porosity, i.e. how easily it absorbs and retains water. Having low-porosity hair is a blessing and a curse: Strands retain water well, but they don't easily absorb water in the first place. If you're currently taking your own deep dive into semi-niche hair topics and are curious about the ins and outs of low-porosity hair, here's a guide to the topic from cosmetic chemist and educator Erica Douglas, aka Sister Scientist.

Q: I think I have low-porosity hair because I struggle to keep my hair moisturized. How can I find the best low-porosity hair products and what should my hair-care routine look like?

A: Low-porosity hair thrives with a combination of steam treatments and lightweight, water-based products, according to Douglas. But before you overhaul your routine, it's helpful to understand the difference between low- vs. high-porosity hair, and how to determine where you fall on the porosity spectrum.

What Is Low-Porosity Hair?

As mentioned, low-porosity hair doesn't easily take in or lose moisture compared to high-porosity hair. The reason why relates to the anatomy of your hair. The outermost layer of a hair strand is called the cuticle, and it's made up of scale-like structures, which Douglas likens to shingles on a roof. Those quote-unquote shingles "open and close evenly, and that's what allows moisture to enter the hair," she says.

If you have low-porosity hair, you're working with a relatively impervious roof that doesn't let in much water when it rains. "Low-porosity hair is hair where the cuticle layer is extremely dense and compact, and that makes it very hard for the hair to absorb water or moisture through that top layer of the hair," says Douglas. "This is also why low-porosity hair is usually extremely smooth."

Genetics play a role in your hair's porosity, but low porosity can affect the full range of straight to curly hair types, explains Douglas. "I would say that there has been some linkage between some ethnicities [and hair porosity] but it's not a very hardcore correlation," she says. "It's not 100 percent of the time. A lot of times, very straight hair tends to be lower porosity traditionally, but that's not a fact." In other words, it's possible to have low-porosity curls or straight hair that's porous.

Your hair's porosity is probably more of a reflection of your styling habits. "I think there's more correlation between how you style your hair and your porosity than what your natural, biological disposition is to a certain porosity level," says Douglas. Styling treatments (including permanent hair color, relaxers, and even styling products) increase the pH of hair, which causes the cuticle to open to an extreme degree, increasing hair's porosity, she notes. And hot tools can eat away at the cuticle layer over time, she adds. "As more damaged spots occur on that cuticle layer, you're basically creating more entry points for moisture." So, if you have low-porosity hair, there's a good chance you take a relatively minimal approach to styling.

Hair Health Hotline: Low-Porosity Hair
Courtesy of Erica Douglas

How to Figure Out If You Have Low-Porosity Hair

While it's not always 100 percent accurate, the "float test" is a pretty reliable way to figure out your hair's porosity, says Douglas. To try it, you'll take a strand of your hair from your comb, wash it with shampoo, allow it to dry, then place it in a glass of water. If it sinks to the bottom of the glass immediately, you probably have high-porosity hair, as Shape previously reported. Medium-porosity hair will float for a few seconds and then sink, while low-porosity hair will take even longer to sink to the bottom. A cool trick: If you cut the strand in half first, and the segment that was closer to your scalp takes longer to sink, that's an indication that the ends of your hair have a higher porosity than your roots, likely due to damage from styling, says Douglas.

That said, if you're not up for any science experiments, you can still get a sense of your hair's porosity. Think about how your hair behaves when you jump into a pool or take a shower. If it seems like it takes a while for your hair to soak up the water, you probably have low-porosity hair, says Douglas. On the other hand, if your hair quickly soaks up water and air dries in no time, you likely have high-porosity hair.

How to Moisturize Low-Porosity Hair

At this point you're probably wondering exactly how to moisturize low-porosity hair, given that water doesn't easily pass through its cuticle layer. The key is to take advantage of steam, a clarifying shampoo, and lightweight products.

On Wash Days

On wash days, steam is your friend, says Douglas. "Essentially, the size of the water molecules in steam are much smaller than liquid molecules in water," she explains. "Because the molecules in steam are much smaller, it's much easier to penetrate the cuticle. The more you can penetrate that cuticle with moisture, it opens up those layers to receive your conditioning treatments and to essentially pull in more moisture."

Spend 15 to 20 minutes treating your hair with steam before washing, a step that's the "key to unlocking very dense, compact hair, which is low porosity hair," says Douglas. If you don't own a steamer — a hair tool designed for this very purpose — you can turn on a hot shower and close the door of your bathroom to steam up the room, she says.

It can also be helpful to prep your hair any time you use deep conditioning treatments, says Douglas. "I like to use heat in addition to deep conditioner, whether it's by sitting under a dryer or using one of those thermal packs," she says.Doing so "helps the molecules of the products to be broken down so that they can absorb into the hair a little bit better."

Finally, consider using a clarifying shampoo on a biweekly or monthly basis, suggests Douglas. "It's so hard to get moisture into low-porosity hair. You want to make sure that you're removing as much build-up from the hair as possible in order to ensure that the hair is really going to be able to absorb and take in whatever conditioning treatments you are subjecting the hair to," she says.

In Between Washes

As for choosing styling products to use between washes, low-porosity hair tends to respond better to water-based products, says Douglas. If water is the first ingredient on a product's ingredient list, it's probably a water-based product. That's a quick way to tell that anywhere from 50 "to 90 percent of that formula or that product is water," she says. "Using products like that, especially as your first step in your styling regimen, really helps to ensure that you're getting as much water as close to the cuticle surface as possible." Silicone- or oil-based products are more likely to sit on top of rather than absorb into your hair.

In general, you're better off with lightweight products. "Low-porosity hair is not absorbing products as fast as high-porosity hair absorbs the product, so a lot of times you're going to get build-up faster," says Douglas. "Being able to prioritize water-based products and use very thin or thinner lightweight oils or silicones is a much better regimen than using the heavier oils and silicones with low-porosity hair."

While you can view low-porosity hair as a good thing — it's often a sign you have minimal damage — it presents its challenges. If your hair doesn't exactly welcome moisture with open arms, adjust your routine and find suitable lightweight hair products that can work in your favor.

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