You'll hear lots of people say you have to use sulfate-free shampoo to avoid damaging or stripping your hair. You'll also hear people say you have to use sulfates to get your hair clean.

They are both wrong. Almost everyone misunderstands this fundamental thing about shampoos, and I think it's why so many people can't find the right products.

I'm Michelle of Lab Muffin Beauty Science, chemistry PhD and cosmetic chemist, and person who usually talks more about skincare science, but we are doing hair today.

I've been paying more attention to hair science on the internet lately, and there are so many completely wrong explanations and strong opinions and just people yelling at each other.

But because I love pain, apparently, I am going to talk about it. I'm starting with sulfate versus sulfate-free shampoos.

I've talked about it a bit before, but I'm going into more depth because I think if we really dig into the chemistry of it, a lot of things about hair start making more sense.

But before we talk about sulfates, we have to talk about the state of hair science—not just internet hair science, but actual academic hair science.

It is a mess. There are three big reasons I think there's so much confusion. Reason one: hair is really diverse, so much more than skin.

Different hair just does not respond to hair care products the same way. Skincare seems complex, but when you break it down, it's actually pretty straightforward.

Maybe also it seems that way to me because I've been talking about it for so long. With skin type, there's oily and dry and in between.

Then you have concerns like fine lines, pigmentation, acne. It does take a while to understand your skin and find the right products for it.

I have to plug my ebook because I keep forgetting to do that. It walks you through that process. But when it comes down to it, ingredients tend to work pretty similarly for everyone.

if you have a hundred people use glycerin on their skin, you might have 60 people's skin get more hydrated and improve, 20 people's skin stays around the same, and the final 20 might get worse for some reason, like the glycerin helped some other ingredients penetrate and the skin got irritated, or they have an allergy.

There is still complexity, but you wouldn't have like half the people's skin dry out more. But that's kind of how it is with hair.

Even just with the shape of the strands, you have straight, curly, a million different levels and types of curly, kinky, tight coils. The coils might be rounder or flatter or a mix.

And shape makes a massive difference to how hair responds to different products. Variation in geometric shape is just not something we have to think about much with skin.

Take wet versus dry detangling, for example. The way water interacts with hair is weird. I'm not going to go into too much detail here because that's a whole topic for like five videos, but in short, it weakens the bonds gluing the inside of the hair together, which is why hair gets stretchy when it's wet.

But it adds stickiness to the outside of the hair, so hairs stick together more easily, and you need to use more force to get the comb through.

Water also raises the cuticle scales. These are the roof tile-looking cells on the surface, and that means they can bash against each other more and chip.

So all of this means it's less damaging to detangle hair when it's dry.

But that's just for straight hair. It is the opposite for curly hair. All of these effects still happen, but changing just the shape of the hair strand, that is enough to change the physics of how the hair interacts with a comb.

The cuticle scales still stand up more. The curly hair is still weaker on the inside and sticky on the outside.

The hair is still more fragile, but there are two big differences. First off, the hairs stick to each other less because there is less alignment or tessellation between curly hair strands compared to straight hair.

It's like how when you cook spaghetti, they stick together much more than with spiral pasta.

On top of that, the weakening of the bonds inside the hair actually makes the curls loosen in shape. A looser curl means that geometrically there's less chance of tangling and causing damage and breakage.

So, the change in the physics of how that curl interacts with the comb that can offset the damage from manipulating wet, fragile hair, and overall, you get less breakage.

But where is the line? Is it better to detangle, say, wavy hair when it's wet or dry? What if you use a brush versus a comb or a different type of brush?

What if you detangle it from the bottom up or just straight from the top? What happens if you partly dry the hair?

Those answers just don't seem to exist, and we are just talking about water and hair shape. We're not even talking about other ingredients, how often you wash your hair, how much damage you have, the type of damage you have, if you bleach or straighten your hair, or how long your hair is, or how oily it is.

There are a lot of variables, and this kind of overlaps with reason two: hair science is just really sparse.

We just do not know that much about hair. I used to assume that hair was really straightforward. I mean, hair is dead, skin is alive.

If you're doing tests on hair strands, it should be really easy. You just buy a bunch of hair. That is so much easier than finding volunteers.

But because of all this diversity with hair, every experiment you do—you just can't necessarily generalize this to other types of hair.

On top of that, there are tons of other variables that make a huge difference to hair, like what humidity the experiment started and ended up with.

When I go places with high humidity, it gets a little extra body. Okay, that alone can massively change the results, and we all live in places with different humidities.

And the humidity changes throughout the day, it changes in different rooms, so it isn't like if every study did experiments at 20% humidity, that would solve the problem.

Hair scientists don't even know the structure of hair in that much detail. The understanding is still evolving relatively quickly, and there just isn't that much consistency with terminology, even in the peer-reviewed literature.

So, for example, around the late '90s, some hair scientists just decided half the protein in hair wouldn't be called keratin anymore, and they have good reasons for that.

But not everyone got the memo, or maybe some hair scientists just didn't agree. So you'll still read things like "hair is 80% keratin by mass," even in this paper from 2017.

And this isn't a huge deal here because it is pretty clear they're using that old definition. But if a paper says, "We concluded that this ingredient works on keratin in hair," do they mean keratin, or do they mean keratin?

Reason 3: Hair products aren't just about individual ingredients.

The two main hair products, shampoo and conditioner, the way they work isn't about the individual ingredients. The overall formulation is really important.

And yeah, we say that a lot about skincare, but it is even more the case with hair care. And this is central to this debate about sulfates in shampoos.

Sulfates usually refers to sodium lauryl sulfate and sodium laureth sulfate. These are two of the cleaning ingredients or surfactants that you find in things like shampoos and cleansers.

Surfactants are special because they have a head that likes water and a tail that likes oil.

As you hopefully know from life, oil and water don't usually mix, so surfactants can help oil break up and mingle with water so you can rinse it away and end up with clean hair or skin.

The tails are pretty similar in surfactants, so most of the time we're talking about the heads.

Sulfates have sulfate head groups. Chemists are not imaginative when it comes to naming. And yes, if we compare sulfates on their own with other surfactants on their own, they have good cleaning ability.

But because it's hair science, there is more complexity. These four diagrams show four different ways that shampoos clean at a microscopic level.

Each of these diagrams is read from left to right. The pink tadpoles are the surfactants, and you can see that the surfactants are coordinating to clean the hair.

This is called a supramolecular process, where the molecules are interacting without going through a chemical reaction. It's a bit like molecular choreography.

Now, this isn't like four separate processes. All four of these are probably happening on your hair at the same time to different extents.

There are probably still more mechanisms still to be discovered. How much of these depends on the formula of the shampoo, because the pink tadpole surfactants won't all be the same.

They'll have different tendencies to interact in different ways. But it also depends on what sorts of stuff or soils you're cleaning off, because they are also part of this choreography.

So, for example, if you have bigger blobs of oil on your hair, then this one is probably happening more because some of that oil breaks off multiple times instead of all of it coming off at once like here.

But if you have big particles that don't break up easily, then it might all just be figure 4.

this one happens more at high surfactant concentrations, and that is another variable. And that's not just about the concentration of the surfactants in the formula.

At the start of the shampooing process, you've just put the shampoo on your hair. It's at maximum concentration. But if you add more water or you start rinsing, then the concentration will decrease, and you might have less of this happening.

And this is already getting pretty complicated, but it's still a massive oversimplification, because hair science will not give us a break.

If you look at the ingredients of almost any shampoo, it'll have three or more different surfactants.

That means that these pink tadpoles in the diagram aren't all the same. If you change the ratio of the ingredients or swap one of these out with something else, then the shampoo might work quite differently.

Plus, there's a bunch of other ingredients that aren't shown here, like polymers and oils, and those will also interact with the surfactants and affect how it cleans.

And on top of that, things like the texture of the shampoo, the opening of the bottle, that's going to change how much you dispense and how the shampoo spreads on hair.

So, in case you didn't notice, it is really complex.

And what that means is trying to predict what happens just by looking at whether some of these pink tadpoles are sulfates or not – that is just not going to tell you much.

It just isn't as simple as "it has sulfates, it'll clean better" or "it'll strip or degrease your hair." If it was, it would be so much easier for everyone to just find the right shampoo.

Same with the pH of shampoos. I see lots of people saying you have to check the pH and make sure it's acidic.

And again, that is just not a very meaningful piece of information when the whole thing is so complex.

it's a bit like trying to predict how a food tastes from how yellow it is. And even if you knew the full formula of a shampoo—every single ingredient, what concentration they're in, where they got the ingredients—it would still be pretty much impossible to predict how it's going to interact with itself, let alone with different types of soils on your hair at different stages of the shampooing process.

It takes a ridiculous amount of computational power to model a much simpler sort of system with way less moving parts than a shampoo.

This is why cosmetic formulators spend so much time just doing trial and error, making formulas and trying them out.

Changing little things about a formula can make a huge difference. Here's one example: Good Housekeeping did this test with 10 shampoo and conditioner pairings that were meant to protect color.

Some of these had sulfates and some didn't. There were salon and drugstore brands.

The set that stripped hair dye the least had sulfates. It's this Tresemmé set. They also have this article where hair scientists talk about how complex it is to formulate a shampoo.

But even with their own results as proof, and two scientists with PhDs explaining to them that it's not about sulfates versus no sulfates, their article on color stripping and shampoos still has this BS about sulfates "sudding" the hair and causing pigment loss.

But I think a lot of us have had the experience where we try a sulfate-free shampoo and it does feel like it's gentle or it doesn't clean enough.

The way you frame it probably depends on whether or not your hair likes that. And the reason for this, I think, is a sort of circular reasoning that happens quite a bit with products.

Formulators know that people looking for sulfate-free probably want something that cleans less.

They know there are all these widespread myths about sulfates being harsh. They've been going around since the 90s, like in this email forward, "SLS is used to scrub garage floors and it is very strong." Even if the formulator didn't know about all this, because I don't know if they've been hiding in a hole, it's probably on the product brief.

That's the description of the product that the formulator works off. It's usually written by marketing people based on consumer research.

It'll have the marketing claims that they plan to put on the products. So chances are the brief says "sulfate-free" as well as "gentle" and "not stripping." So there's a good chance that if you grab a random shampoo with sulfates and a random shampoo without sulfates, the one with sulfates will clean better.

It's not because just adding sulfates automatically makes a shampoo strip more.

We saw that it can be formulated to strip color the least. It's just because of intentional product design. The formulator formulated with sulfates and to make it clean more, they might have also changed some of the other ingredients to make it feel like your hair is cleaner.

But brands don't all have the same idea of what counts as gentle, and some brands choose not to follow this consumer trend, so there's going to be lots of exceptions.

It's like how if you grab a shampoo in a black and green bottle, there's a pretty good chance it'll smell minty because product designers have decided that is what men like.

This is exactly the same situation with foam and cleansers.

How well a product foams doesn't tell you about how well it cleans. What's happening on a molecular level is different.

How well something foams depends on how well it interacts with air, how well it traps air in a thin film. Cleaning is about how well it interacts with oil and dirt.

both cleaning and foam rely on surfactants, but 
apart from that, there's no real correlation   between which surfactants foam 
best and which ones clean best.

And there's so much that depends on 
the overall formula. There are lots of   other ingredients that help to create a nice foam.

Again, both are complicated supramolecular 
processes; there are so many moving parts. But if something doesn't foam when 
we use it, we tend to feel like it's   not cleaning properly.

It's a purely 
psychological thing, and I personally   still have this gut feeling, even though 
I know all of this chemistry behind it.

Humans did not evolve to be 
perfectly rational in our feelings, so formulators make sure that shampoos and other   cleaning products usually 
foam really nicely for us.

But if we don't see it, then it 
doesn't have to foam to convince us.

One example is detergents for dishwashers. They're 
actually kind of formulated the opposite way. They have to clean really well with as little 
foam as possible because foam causes pressure,   and if that builds up, then it can make 
the dishwasher leak and flood our kitchen.

Sploosh, sploosh. Bottom line: sulfate-free, sulfate- 
containing, doesn't really mean much.

How well a shampoo cleans is way too 
complicated to predict that easily. It's much more useful to actually look 
at what the shampoo is trying to tell you.

If it says clarifying, then it's a shampoo designed   to clean your hair better. It doesn't 
matter if it has sulfates or not.

Scientist explains: What everyone gets wrong about sulfates in shampoo

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