You've probably heard that you need to moisturize your hair. Maybe you've spent time choosing between "hydrating" and "moisturizing" products, or built a routine around "protein-moisture balance" or "sealing in moisture."

But what if I told you these words are marketing? That the hair care industry uses words like "moisturize" to sell products, even though cosmetic chemists know that's not what's actually happening?

When I learned this, it shocked me and undermined everything I'd previously thought about hair-care.

TL;DR

"Hydrating" and "moisturizing" are marketing words, not scientific ones. Hair's water content is set by the humidity around you, not products. What these so called "moisturizing" products actually do is condition, coating and smoothing hair to make it feel better. The water content has nothing to do with it.

The Moisture Conspiracy

In cosmetic chemistry, "moisturizing" and "hydrating" are considered "consumer terminology." Meaning they aren't technically true, but they are used because of marketing^[1][2]. In short, they sell products.

How you'll start to see so-called moisturizing products after reading this post

I first learned the truth through chemist Dr. Michelle Wong's blog post Hair, hydration and water: the real science and video Is hydration destroying your hair? The REAL science. She says she's attended hair science lectures where scientists openly admitted they don't think telling consumers the truth is worth it, but because consumers won't believe it and will just buy other products instead ^[3].

Cosmetic scientist Dr. Trefor Evans writes in Practical Modern Hair Science that while consumers demand products that "moisturize" or "hydrate", the scientific literature shows that the water content of damaged hair isn't depleted and that hair's actual water content is overwhelmingly determined not by products, but by the humidity of the environment. But words like "moisturize" persist because they sell.

"Moisturizing" shampoo is going to sell more products than just describing it technically as conditioning gentle shampoo.

But in this post I'm gonna assume you're not the consumer they think just won't understand the truth. Because I think there are consumers that genuinely want to know what's going on inside their hair.

How hair actually handles water

Hair constantly absorbs and releases water from the air around it. The humidity of the air is what mostly determines how much water is in your hair at any given moment. Products can't change this much ^[2].

You've probably felt this without realizing it. Think about how your hair behaves on a really humid day. That's when the water content in your hair is at its highest. And for most people it's also when their hair looks and feels the worst. Frizzy, puffy, rough, hard to manage.

If more literal moisture really is the secret to great hair, the most humid days would be your best hair days.

What Consumers Call "Dry" vs. What Science Calls "Dry"

Humans can't directly sense moisture levels. Instead we estimate it from other signals, mainly texture and temperature^[4]. Smooth feels "moisturized." Rough feels "dry." This makes it very easy to confuse conditioning with hydration.

Which brings us to damaged hair. When hair is damaged, the protective fat layer gets stripped and the cuticle becomes compromised. This actually causes hair to take in more water, not less. That extra water causes swelling, which lifts the cuticles and makes the surface rough and prone to tangling ^[5]. You can read more about this in my post Hair Porosity Explained.

So damaged hair feels "dry" not because it lacks water, but because it is rough due to raised chipped cuticles and lack of a lubricating protective fat layer.

This is backed up by multiple studies. In one, scientists controlled the water content of hair tresses using humidity levels, then asked panelists to rate them. The hair with four times less water was rated as more moisturized, smoother, less tangled, and less damaged by three out of four panelists ^[5][2]. A separate study found the same pattern with bleached hair: even though bleached hair had measurably higher water content than undamaged hair, panelists consistently rated it as feeling drier and less moisturized ^[4].

So What Do Moisturizing Products Really Do?

They condition. When cosmetic chemists formulate a "moisturizing" product, conditioning is what they're actually optimizing for ^[6]. So ingredients that lubricate, smooth, and coat the hair surface to make it look better, feel softer, and tangle less. Many of these ingredients also protect against damage.

The "moisturizing" and "hydrating" products do work. The label is just wrong about why.

So if your hair feels dry, the question isn't "how do I add more moisture?" It's "what kind of conditioning does my hair need?" and "what's on my hair that could make it feel rough/unpleasant?" That's a much more useful question, and we'll get into it later.

Nothing Can Seal Moisture In or Out

This follows directly from everything above. If products can't meaningfully change hair's water content, they also can't seal it in or out. Oils and "sealing" products form little deposits on the hair surface, but water vapor molecules are far too small to be blocked by them^[3].

Cationic conditioners bind to the negatively charged surface of damaged hair

That coating is not water-tight, if we zoom in we see plenty of gaps for water vapor to get in

This is why "seal in your moisture" as a routine step isn't doing what people think it's doing. The oil is acting as a conditioner, lubricating, smoothing, and protecting your strands. It doesn't need to alter water content to do this.

Protein/Moisture Balance Doesn't Make Sense

Once you understand that "moisturizing" means conditioning, a lot of common hair advice falls apart. Take "protein/moisture balance." The reason protein products seem to conflict with moisture products is simpler than people think: products that tout protein as an active tend to be lightly conditioning (to prioritize body/volume), while "moisturizing" products are heavily conditioning.

So "protein overload" is probably just under-conditioning, sometimes combined with buildup. You likely don't need to audit your routine for protein, you need more conditioning. I dive into this in detail in The Mystery of Protein Overload: A Scientific Investigation

Take Olaplex. Some people reported that the original Olaplex formula gave them "protein overload", even though it contains no protein. The original formula was just lightly conditioning. Used without enough additional conditioning, your hair could end up under-conditioned, making it dry and brittle. The new formulation is more conditioning to prevent this issue.

Not All Conditioning Is Equal

Oil is a conditioner, but it's not a great one, which is why modern hair conditioning products were invented. When hair is damaged, the surface becomes negatively charged and repels oil, especially when wet. Most modern conditioners are positively charged, which allows them to bind directly to damaged hair in a way oil can't. Silicones and mineral oil are also significantly better at reducing friction than natural oils.

Healthy hair has a fatty surface layer (F-layer). Oils adhere to this layer because like attracts like.

When the F-layer is lost, negatively charged groups are exposed and the surface becomes more water-loving. Oils are no longer attracted and may be repelled by water if hair is wet.

Positively charged (cationic) conditioners bind to the negatively charged surface, forming a protective coating that protects the hair and makes it look/feel better

Once you know this, product labels start to make more sense:

• Volumizing: lightly conditioning with the shampoos higher cleansing, that's how they avoid weighing hair down
• Moisturizing/hydrating: heavily conditioning with the shampoos usually gentler
• Damaged or color-treated: higher in cationic (positively charged) conditioners, which bind better to compromised hair surfaces

So if your hair feels dry and you're relying mainly on oils, that might be why. You don't need more literal moisture, you need better conditioning.

Unfortunately, many curly brands prioritize having "natural" ingredients over having effective ingredients. There are exceptions of course, but I'd say 80% of curly products I find in the local store are "natural" and seem to utilize more butters/oils than modern conditioners.

What We Don't Know Yet

There are a few places where I want to be honest that the science is thin, particularly around coily hair. There is some preliminary research suggesting coily hair may get stronger when wet ^[7], and that humectants in leave-ins may reduce breakage in coily hair ^[3]. But whether that's because of moisture effects or something else like film forming isn't clear yet. I hope more research gets done here.

The other interesting unknown is bond repair products. Some research suggests citric acid may improve damaged hair partly by reducing its water content, essentially crowding out water that was causing swelling and weakness. If that's true it's the opposite of what "moisturizing" implies, and another reason to be skeptical of moisture as an organizing principle for hair care. I discuss this a bit in A Skeptics Guide to Bond Repair

Want to learn more?

Chemist Dr. Michelle Wong has a video called "Is Hydration Ruining Your Hair?" with an accompanying blog post that goes deeper on this topic. It's what first led me to Dr. Trefor Evans' work. If you want the most technical version, the Adsorption chapter in Practical Modern Hair Science by Dr. Trefor Evans is a great place to start. It's available for free through Tri-Princeton, a respected hair science research institute.

Putting It Into Practice

When you see hair content talking about literally increasing water content in hair, you can treat it as a red flag: either it's just marketing, or the source isn't up to date on the science. The advice might still work, just not for the reasons given.

For the "dry hair" problem specifically, it helps to look for the actual cause rather than reaching for something labeled "moisturizing":

• Hair feels dry from buildup: clarify
• Hair feels dry from damage: look for conditioners formulated for damaged hair, particularly cationic conditioners
• Hair feels dry for unknown reasons: try more conditioning of any kind and see what happens

Example

Recently I saw a Reddit post in a subreddit I moderate (/r/swavyhair) from a woman who said "the crown area feels dry yet heavy."

Now let's dissect that with what we know now:

• Her hair is not lacking moisture in a literal sense
• It's hair near the root, so it's unlikely to be damaged
• She describes it as "dry yet heavy", which means under-conditioning is not likely
• So what's left is most likely buildup causing it to feel "dry"

In conclusion, it seems like this hair is over-conditioned. Good recommendations were to use stronger shampoos, clarify more often, and/or use less conditioning products.

That's way simpler than trying to increase the moisture level. In fact more moisturizing products would likely only increase her over-conditioning problem.

The Truth is Out There

Well if you've gotten this far you now know that truth behind "moisture" claims in marketing. You're more knowledgeable than probably 99.99% of hair-care consumers.

I follow a lot of hair influencers and brands on Instagram so I see people talking about "moisturizing" hair multiple times a day. The bad news is that the "moisturization" of hair stuff is so ingrained into our culture that now you'll notice it everywhere.

It's now up to us to spread the word, we're only going up against basically every single hair product brand out there...

About the Author

Melissa McEwen is the creator of CurlsBot. She is a software developer with training in science writing and a B.S. in Agricultural Science. Her writing has appeared in publications such as NPR and Quartz.