[INTRO] Hair — whether you've got a lot of it or absolutely none — it's one of those things that all of us have thought about at some point. And we've ended up with questions like: why does it change color? Why does it disappear? And yes the big one: Why is butt hair even a thing?

We've covered all of this and more over the years and now from your head to your toes, we're putting it all together. Let's start from the top, literally. Whether you're working on your thesis, adjusting to a new job, or just dealing with whatever life is these days, you've probably run into the rumors about grey hair. Specifically, that stress can make you go grey.
It seems fair enough given how much other stress does to your body, but is there any truth to it? Here's Olivia with the answer.

We've all seen those comparison photos. Some president starts out with a full head of dark hair, and a few years later they're totally grey. But stress doesn't really turn your hair white— at least not not the hair already on your head. It can help reveal grey hairs though, by increasing turnover and scientists think it might just speed up the greying process a little too.
Each hair gets its color from melanin. A pigment made by specialized cells called melanocytes in hair follicles. You can have grey or white hairs because the hair itself, which is mostly a colorless protein called keratin, is manufactured by different specialized cells in the follicle— which are called carotenocytles and they tend to live longer than the pigment producing ones. So hairs can keep growing, shedding, and growing anew even if those little pigment factories have stopped production. How long your melanocytes last and therefore when you go grey is mostly determined by your genetics. But your hair doesn't turn white the second some of those cells die. Since the follicles can't alter hair that's already grown, grey hairs don't start to show until the old colorful hair has fallen out, which usually happens every few years. But if something makes your hair fall out faster, like I don't know, being massively stressed out, then it might seem like you're suddenly greying.

Chronic stress is known to trigger telogen effluvium: a condition where a hair follicles jumps ahead to the final stages of its life cycle and the hair attached to it falls out. So instead of lasting years, hairs can fall out within a few months of a stressful event. The new hair starts growing again right away so it doesn't cause baldness. And it doesn't effect melanocytes, so for young people the hair that falls out grows back its normal color. But if you happen to be at the age where the next hairs in line will be grey anyway, it can speed up that turnover.

It's not that the stress turned your hair grey, it just bumped your grey hairs to the front of the line. That said, there is some research that suggests stress might make you go grey sooner. That's because stress can increase levels of free radials: atoms or molecules that have an unpaired electron which makes them unstable and very reactive. They basically try to steal an electron from anything they run into, which messes with key components of cells. A 2006 study found that too many free radicals can prematurely age cells because of their toxic effects on DNA, or even kill the cells outright- and that includes melanocytes. 

Other things known to increase free radicals in your body, like smoking, also happen to be associated with premature greying. But so far, no one has conducted a controlled experiment to conclusively demonstrate the connection between stress, free radicals, and the sudden appearance of grey hairs. So stress might make you go grey, or it might just reveal how grey your hair already is.

[Transition: Sci-Show]

Well that's kind of encouraging at least, although I wouldn't mind a cool white streak in my hair some day. Regardless of what color your hair is though, you've likely experienced the joys of frizz. Or at least, if you have relatively long hair. When it's humid out, hair just puffs way out there turning into a static-y mess. But weirdly enough, there is something we can learn about the atmosphere because of it. Here's one from Stef

[Transition: Why does hair get frizzy when it's humid?]

You know what they say, "It's not the heat, it's the humidity." And that's extra true if your hair is the frizzy type.

The higher the humidity, the more prone some of us are to a frizz-trastrophy. And it all has to do with how water interacts with the proteins in our hair. The really weird thing though is what hair has been able to teach us about atmospheric water.

Frizz is what you get when individual strand of hair change shape and stop matching the neighboring strands. This can mean getting curly, wavy, or otherwise infuriatingly irregular. Frizzing has to do with hair's composition. Hair is made up of a long fibrous protein called keratin, bundled layer upon layer in an elaborate structure. The keratin molecules stay together through a combination of strong and weak chemical interactions.

The really beefy chemical bonds are called disulfide bonds, in which sulfur atoms from neighboring protein strands join together in a pretty tight hold. And the weaker interactions are what are known as hydrogen bonds. Hydrogen bonds aren't true chemical bonds, rather they're a weak attraction between a slight positive charge and a slight negative charge. 

For hair, the hydrogen bonding happens between keratin molecules and water molecules. When a water molecule bonds to two different keratins, it can help hold them together to an extent, but that hold can be easily broken or changed. And when more water enters the picture, it can change how keratin sticks together. Getting your hair wet can break previous hydrogen bonds and form new ones, resulting in a new shape to your hair. And when your hair dries in a certain shape, it tends to stay, which is why going to bed with your hair wet is asking for epic bed head.

When there's lot of water in the air in the form of high humidity, the changes to our keratin molecules are more unpredictable. Frizz occurs when water is absorbed into dry hair in different areas at different rates. Some strands of hair may absorb water into the center, while others may absorb less or none at all, causing differences in swelling that affect hair shape. Alternatively, the higher moisture may cause some strands to form so many hydrogen bonds around the outer layers that the hair folds back on itself, forming curls.

And the water absorption is pretty random. No two adjacent hairs respond to airborne moisture the same way, resulting in the frizzy mess that we find so frustrating. They all get bent out of shape differently.

But the amount of water a hair absorbs is ultimately still proportionate to the relative humidity. Like, it's predictable enough to be useful for making measurements. Measuring humidity was a goal of scientists back in the day who wanted to know more about what caused rain and how predictable various weather phenomena could be. 

Enter the hygrometer, a device for measuring relative humidity. One of the first hygrometers was made in 1783 by Horace Bénédict de Saussure, and his worked using human hair. This may seem weird and kinda gross, but it's both effective and something you can try for yourself.

Because dry hair will absorb atmospheric moisture, you can reliably observe a long strand of hair to change shape as humidity changes. All you have to do is fix one end of a clean, de-oiled strand of hair in place, and hang a weight from the other end to keep it stretched. Then you can observe the length of the hair changing as humidity increases or decreases.

The proteins coil and uncoil as hydrogen bonds form and break, all as a function of how much water is in the air, so the resulting variation in the strand's length is proportionate to the humidity in the air. You can even calibrate relative humidity by using a hair dryer to simulate 0% and a wet rag for 100%, allowing you to get a sense of how far between those two extremes the air is on any given day. So, while it might be a seasonal annoyance, at least frizz has some practical applications.

I mean, knowing that you can study humidity with your hair probably won't help you the next time it's muggy, but at least frizzy hair isn't 100% bad. Now, when it comes to the hair on our heads, there's one other big question. Why do people lose it?

I mean, you might come into the world and grow a full, thick head of hair, but then you hit 30 or 50 or 70, and someone can suddenly see their reflection in your scalp. We've talked about that one, too, and how some treatments try and fix it. Here's one from Hank.

Why Do People Go Bald?
Just another fun part of getting older, as people age many of them start to lose their hair. Why? Turns out it's complicated, otherwise we would've fixed it by now, because it's basically the Holy Grail of beauty.

But lots of different factors affect whether or not you end up with a shiny bald head, but mainly it has to do with two things: hormones and genetics. Hormones are involved because they affect how your hair grows. More specifically, you can blame testosterone, because balding happens when hair follicles turn it into dihydrotestosterone, or DHT. Scientists don't know exactly why, but DHT makes certain hair follicles start to shrink. As they get smaller, the hair gets thinner, and, eventually, the follicle stops producing hair entirely.

Males typically have plenty of testosterone, which explains why balding typically affects biosex males more than females. In what's known as male-pattern baldness, hair loss starts at the temples and the top of the forehead and at the top of the scalp. Eventually, the receding hairline meets the growing bald patch.

But biosex females produce testosterone, too, just a lot less of it, and they make enough estrogen to mask its effects. After menopause, estrogen production slows, but they're still producing about the same amount of testosterone, so that's when the hair starts to get thinner. In female-pattern baldness, it's more of an evenly-distributed thinning of the hair across the scalp without much of a receding hairline.

So how do you figure out whether hair loss is in your future? Well, that is more complicated, and there's a lot that we still don't know. Mainly, it seems to be tied to genetics, and researchers have pinpointed a few different genes that might be involved. Some of these genes are on the X chromosome, which explains why males are so much more likely to go bald than females.

Males get only one X chromosome, so whatever version of the gene they inherit on that chromosome is gonna be expressed. Females, on the other hand, have two X chromosomes, so there's more of a chance that they'll inherit a non-balding version of the gene from one of their parents. But studies have shown that there are other genes on non-X chromosomes involved, too, like one genetic variant on chromosome 20, which made males seven times more likely to go bald, but, for some reason, doesn't seem to influence whether females experience hair loss.

So there are a lot of different pieces to this balding puzzle, which is why we haven't figured it out yet, but if you don't like the new look, there are some drugs that can help. Finasteride, for instance, works by blocking production of DHT, and minoxidil both increases blood flow to hair follicles and widens them, helping hair grow. Of course, you could also just embrace your newfound resemblance to Jean-Luc Picard. He did pretty well for himself.

Alright, now onto the rest of the body, because, although it's not always immediately obvious, we are covered in little hairs. We're pretty bald compared to most other mammals, and there are various ideas about why, but, overall, we have dense hair in just a few spots, including our heads, faces, armpits, and nether regions.

And those hairs are all so different. Like, the hair on your head can grow to be meters long, but your armpit hair will stop after a few centimeters. Why? Here's another one from Hank.

How Does Hair Know When to Stop Growing?
You're probably quite happy that your armpit hair isn't dragging on the floor, so it's good that there's a system to prevent that. But what is that system? Us humans grow hair all over our bodies, except on our palms and the soles of our feet. But some of it, like leg hair, stops growing, while the hair on our heads just seems to grow our forever. 

That's because every hair on your body goes through the same cycle, growing for awhile and then falling out, but each type of hair spends a different amount of time growing and grows at a different speed. Every hair begins the same way in a phase of the cycle called anagen. During anagen, blood flow starts to ramp up at the base of the follicle, feeding oxygen to specialized stem cells. These cells begin rapidly dividing and producing keratinocytes, which form the root of the hair.

As the expanding mass of keratinocytes is pushed toward the surface of the skin, the cells die, releasing a protein called keratin, which holds the strand of hair together. Eventually, that strand pops out of your skin, so the visible part of your hair is entirely dead, which is why, thankfully, it does not hurt to get your hair cut, though try explaining that to a three-year-old. During anagen, hair can grow up to one and a quarter centimeters every month, depending on where it's located on your body. 

The second phase is called catagen and lasts about two weeks. Here, the blood supply is cut off at the follicle, which stops the production of new keratinocytes. So, for that particular hair, the party is over. The follicle then shrinks to about a sixth of its original size, and then the existing hair strand is pushed closer to the surface. 

The third phase is called telogen, otherwise known as the resting phase, when the follicle remains dormant for one to four months. Finally, the hair is released or shed

when the follicle dilates and starts the anagen phase again. 

So, how long a hair on your body gets depends on how long it's in the anagen phase and how fast it grows during that time. The hair on your scalp, for example, stays in anagen for two to six years, which is why it can grow so long. Other hair types, like eyebrows and eyelashes and body hair, have a short anagen phase, only 30 to 45 days. But they also grow much more slowly, with eyebrows, for instance, growing only 4.2 millimeters every month. This is my eyebrow close-up. Hi.

As for how your hairs know when to grow and when to stop, that's something scientists are still trying to figure out. It is know that genetics can lead to longer or shorter anagen phases in certain hair types, but the current thinking is that your hairs get their instructions by way of chemical growth signals from stem cells in the skin. And, considering how extremely inconvenient it would be for all of the hairs on your body and all mammals' bodies to just continue growing forever, it makes sense that there's a system for making sure they don't grow too long.

Alright, we couldn't make this episode without including one from the SciShow Hall of Fame. Why, you may wonder, do we have butt hair? I'll just let Hank take it from here.

Why Do We Have Butt Hair?
We've covered a lot of topics, both trivial and profound, and we work very hard to capture both our fascination and excitement as well as our deep desire to always get things right. And, over the last year, you may have noticed a comment on, we think, every single SciShow video, asking us one question. We have ignored this question long enough. It is time we took it on. Litojohnny wants to know, why does he have hair around his anus?

Well, Johnny, the reason we haven't answered you is because, you know, despite the fact that everyone gets their own personal pocket-sized super computer and that we can send robots to Mars and convert the entire face of the planet to human use, we still do not really know why humans have butt hair. And it may not surprise you to learn that not a whole lot of research has been done on the "why" part of this question about butt hair, but a fair amount of study has gone into the medical problems that butt hair can cause.

For example, pilonidal disease is a chronic skin infection cause by hairs that get embedded near the top of the butt crack, which, if you want to impress your doctor, you can describe by its technical name, the intergluteal cleft. So, as the owner of a butt yourself, you probably know that butt hair does seem to have more downside than upside. So, given that, what, if any, purpose does it serve?

Well, there are a few theories out there, and maybe some enterprising scientist out there watching right now can do some research on them, but here's what's been proposed. Theory number one: Butt hair exists because there's just no significant evolutionary pressure against butt hair. Sure, it's sometimes inconvenient, and, depending on the moment in cultural history, it might be considered unsightly, but it appears that butt hair has never been a significant reason for one human not to make babies with another human. It's important to keep in mind that not every bit of our physiology needs and evolutionary purpose, so butt hair might just be another side effect of unintelligent design. 

Theory number two: Scent communication. Body odor definitely has a negative connotation in today's world, but there's little doubt that communication through scent has played an important role in the evolution of humans. After all, that's likely why we have body hair in the same areas where we produce body odors. The hair is there to hold onto sebaceous, or oily, secretions that have their own smell and are also consumed by bacteria that create even more smells.

Since we all produce different smells compounds and all have our own microbiomes, each individual human actually smells different. And, if our early human ancestors were anything like other animals, and they probably were, their personal smell probably helped with everything from broadcasting territorial rights to attracting mates. Butt hair, then, may be just another way our oldest human ancestors enhanced their smell profiles. 

Theory number three: Friction. In addition to giving off smells, humans have also always done a great deal of walking and running, and skin rubbing on skin, especially in areas where that skin may be moist and dirty, can cause irritation, rashes, and even serious, debilitating infection. 

It's even possible that those sebaceous or waxy secretions that help produce body odor are held in place by body hairs to provide an added benefit, acting like a kind of natural anti-chafing cream.

Now this theory, of the ones that we've talked about, is most appealing to me personally, but it's very difficult to test, because shaving or otherwise removing butt hair and then having someone run 20 miles on a treadmill is not a good experimental design, because there's no way to know whether any irritation by the lack of hair or whatever technique was used to remove the hair, none of which sound fun to me.

But I have come up with an alternative experimental design that I like quite a lot: just interviewing a few hundred runners about how much they need to worry about butt chafing, and then measure the density of their anal pelage to see if there's any correlation between whether they chafe and how hirsute their butts are, which is not an experiment that I want to do personally. But if there's an expert out there in anatomy and physiology who's up for tackling this prickly problem, please take it on. And if you get any useful data, definitely let us and Litojohnny know it went.

For the record, we made that video in 2016 and have yet to hear from any researchers, but, if you're out there, we're still interested. In the meantime, well, at least you have something to think about on your next trip to the bathroom. Thanks to everyone who's helped us make these episodes over the years. Whether you're a viewer or a full out patron, SciShow exists because of your curiosity about the world.

And if you like this, you'll probably also like our podcast, SciShow Tangents. It's hosted by some of the smart, talented people who've made SciShow happen, and it features mind-blowing facts, science poetry, and a lot of laughter. You can find it wherever you get your podcasts.

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