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Hair 101

  • Writer: Leilani W
    Leilani W
  • Oct 11, 2025
  • 15 min read

Hair plays a big role in our lives. It shapes our identity, expresses culture, reflects personal style, and can even carry emotional weight. We see this in the iconic hairstyles that define eras and in celebrities whose hair has become a symbol of their careers. Yet, despite its powerful presence in our lives, most of us never stop to think about what hair actually is, why it varies from person to person, or how it grows.


I've been excited for this entry, probably because it's something I have some previous knowledge of, or because it's something that everyone relates to on some level. For today, we'll be looking at all things related to hair, from its structure to the things that you might be doing that unintentionally damage it.



Hair Structure

As you've probably heard before, our hair is made up of dead cells, but more specifically, dead, keratinized cells. Keratinized cells are those that produce keratin, which can also be found on our nails, skin, and even animals' fur, claws, or feathers.

Our hair can be split up into 3 different parts: the hair shaft, the hair follicle, and the hair bulb.


Hair Shaft

Let's first start looking at everything above the skin! The hair that you see coming out of your scalp all the way to the tips is called the hair shaft. The hair shaft is composed of 3 layers: the cuticle, cortex, and medulla.

→The cuticle is the outermost layer. It is made up of overlapping scales that face downwards (kinda like rooftop tiles, or fish scales). They are shaped this way to help minimize friction between the hairs. Its main function is serving as a protective layer from outside stressors, including heat tools, the sun, hard water, or hair brushes and combs.

→The cortex is the layer found in the middle. It is the thickest layer and holds several chains of keratin proteins held by disulfide bonds, which are responsible for the hair's strength, elasticity, and texture. Also found here are bundles of melanin granules, which are responsible for our hair color. The color of your hair is determined by two different types of melanin: eumelanin, which is responsible for black or brown hair, and pheomelanin, which creates auburn or red hair. The distribution of pigment varies per person.

→ Finally, the medulla is the innermost layer of the hair shaft. It is not found in all hairs, though; it is mostly present in thicker hairs and absent in thinner ones. If present, there are several patterns, including continuous (one unbroken line of color), interrupted/intermittent (pigment line broken at regular intervals), fragmented/segmented (pigmented line unevenly spaced), and solid (pigmented area fills both the medulla and cortex, commonly found in animals). It is very porous, meaning it contains large voids with air, water, or oils. No one really knows what its function or purpose is.

Diagram showing the layers of the hair shaft: the cuticle, the cortex, and the medulla

Hair Follicle & Hair Bulb

Going below the skin, we'll reach the hair root (part located in and down the skin). The hair follicle is a tubular structure that surrounds and supports the hair root. It is surrounded by the epidermis (the uppermost layer of skin) and extends down to the dermis (the second layer of skin) [I'll go more in depth on the skin itself in a later entry!]. The hair bulb is part of the hair follicle. It is basically the growth center, located at the bottom of the hair follicle. As cells divide, the older cells are pushed upwards, forming the hair shaft.


The hair follicle consists of 2 layers: the epithelial root sheath (epithelial tissue, or surface lining tissue, that surrounds the hair) and connective tissue sheath (outer layer that is a portion of the dermis). Reaching the hair bulb, this connective tissue sheath makes a dip inwards at the bottom of the hair follicle, forming the hair/dermal papilla, an area where the blood supply and nutrients enter the hair follicle. Surrounding the hair papilla is the matrix, an area with very active basal cells (root cells that divide to grow new hair) that undergo mitosis (rapid cell division). In the matrix, we'll also find melanocytes, which are cells that produce and inject pigment (melatonin) into the keratinocytes, giving hair its color. Keratinocytes are the main cells in the matrix that divide rapidly and form keratin, which then hardens to produce the actual hair.


Every hair follicle has a muscle called the arrector pili muscle attached to it, which makes your hair stand up, causing goosebumps. Lastly, hair follicles also have a sebaceous gland attached next to their base. The sebaceous gland produces sebum (a natural oil that moisturizes the scalp and hair, gives it shine, and protects it).

Diagram of a hair structure showing the hair shaft, the hair follicle, and the hair bulb


Hair Types

Each person's hair is unique. Whether it's curly, straight, wavy, silky, coarse, black, blonde, or brown, these are all characteristics we see in hair. But what causes these differences?


Before we talk about factors that cause these traits, let's first look at the variety of hair types. There are 4 main categories: straight (1), wavy (2), curly (3), and coily (4). Each category can be divided into 3 subtypes (A, B, and C) depending on their texture, thickness, and curl pattern.

Diagram of different hair types, ranging from straight to coily hair.

Hair Follicle Shape

Hair type is determined by the shape of your hair follicle:

-In straight hair, the hair follicle is round.

-In wavy hair, the follicle has a circular or oval shape.

-In curly hair, the hair follicle has an oval shape.

-In coiled hair, the hair follicle is almost flat or more elliptical/oval.

So, the rounder the follicle, the straighter the hair is because the strand grows evenly in all directions, and the more oval/elliptical the follicle, the curlier or coiler the hair is because the asymmetry makes the strand bend as it exits the hair root.


Hair Position Angle

Surprisingly, even the angle at which your hair follicles are positioned can play a factor.

-Straight hair follicles are positioned vertically (straight up and down)

-Wavy hair follicles are positioned at a slight angle (this just means the hair follicle is leaning a bit towards a specific direction)

-Curly hair follicles are positioned moderately angled (a bit more than wavy hair follicles)

-Coiled hair follicles are positioned at a sharp angle, almost horizontally.

So, the more vertically the hair follicle is orientated in the scalp, the straighter the hair looks because it tends to grow straight out, and the sharper the angle the hair follicle is, the curlier or coiler the hair is because the hair emerges already tilted and tends to bend as it grows.

Keep in mind, though, that the angle of the hair follicle mostly determines the direction or how the curl pattern looks on the scalp, not necessarily the hair type.


Genetics

Well, we all expected this. Your DNA, your genetic makeup, plays a huge role in determining your hair type. Gebetics is what determines the shape of your hair follicle, and as mentioned earlier, that's what determines if you have straight, wavy, curly, or coiled hair.

The same goes for your hair’s thickness, density, growth rate, and oil production.


When looking at genes, we also observe a correlation between hair types and ethnic backgrounds. For example, specific genes seen in European individuals are associated with blonde, straight hair; East Asian ethnic backgrounds, such as Han, Tibetan, Mongolian, and Li (originating in or close to China), have a strong association with straight, black hair; and in African ancestry, we see dark, curly, coily hair.


Environment

Yes, external influences in your surroundings can play a factor in what hair type you have. The climate where you live, medications, and industrial locations are a few examples. A warmer climate, for example, can cause your body to sweat profusely, often leading to oily hair, while overexposure to sun rays can dry out your hair. On the contrary, in colder climates, you can expect to see the hair being more brittle and also drier due to the lack of moisture in the air. High moisture concentrations, as seen in humid climates, can actually affect the hair shaft and cause it to swell and frizz.


Body Hair Types

Before we move on, though, I think we should also look at the differences in hair found throughout our bodies. You’ve probably noticed how hair at different places on your body is significantly different. Some being thicker, shorter, or less pigmented than others.


Ok, so there are 2 main types of body hair characteristics (as in function and appearance): terminal hair and vellus hair. Here's some info about each of them:

  • Terminal hairs: the dark, thick hair that you can find throughout your body, such as on your scalp, eyelashes, eyebrows, armpits, and pubic region. It protects our bodies from sun rays, germs, debris, and regulates our temperature. Men typically have more hair than women because they produce more androgen hormones, which affects the quantity of terminal hairs. Compared to vellus hair, terminal hair grows faster and is buried deeper into the skin.


  • Vellus hairs: these are the hairs that kind of look like peach fuzz, being fine and light. They cover all of our bodies except for the soles of our feet, palms, eyelids, and lips.

    They look invisible because they lack pigment; you can only really see them in close inspection or under a bright light. They help with regulating your body temperature and with perceiving sensations (e.g., goosebumps).

    You've probably noticed your hair change during puberty, like the hairs on your armpits, around your genitals, and face to coarse, dark hair. That's because of an increase in androgens (a sex hormone mentioned earlier), which causes vellus hairs to become terminal hairs.


Hair Growth Cycle

Our hair has 4 distinct phases of life—if you will: anagen, catagen, telogen, and exogen. Let's examine each of them.


  1. Anagen:

    -Aka, the “active phase”, it’s the stage where your hair grows and is visible above the skin. It’s the longest stage, lasting around 3 to 5 years, but for some it can last way longer (like 8 years). Your matrix cells in the hair bulb divide quickly, adding to the hair shaft and pushing it upwards. This phase is different for each type of hair found on your body. For example, your eyebrow hairs and pubic hairs are in the anagen phase for a shorter time than your scalp hairs. Around 85% to 90% of your scalp hairs are in this phase right now. As we age, this phase slowly shortens, making hair growth slower and shortening our hair length.



  2. Catagen:

    -Aka, the “transition phase”, is the stage that marks the end of active hair growth. Here, your hair separates itself from the dermal papilla, losing its blood supply and the matrix cells that generate the shaft. Your epithelial root sheath shrinks and attaches to the root of the hair, creating a club hair (a fully grown hair that has stopped growing). This is the hair’s way of preparing to shed. It lasts around 10 days to 3 weeks, and only about 1% to 3% of your scalp hairs are in this phase.


  3. Telogen:

    -Aka, the “resting phase”, is the stage where your hair remains in the hair follicle, but no growth is occurring. The time that it lasts for is quite unpredictable, ranging from a few days to a few years, but it most commonly lasts around 2 to 3 months. About 9% to 15% of your hair is in this phase at a time. By allowing the hairs to stay anchored but not grow in length, your scalp prepares for new hair growth, so while this is happening, other hairs are growing during the anagen phase.


  4. Exogen:

    -Aka, the “shedding phase”, is when shedding occurs. The hair is released from your follicle, falling out, allowing this cycle all over again. It’s completely normal to lose 50 to 100 hairs per day, often being aided by washing or brushing your hair. This phase can last around 2 to 5 months.


Hair Hazards

Now that we've established everything there is to know about hair, let's look at how we damage it. Knowing what to avoid doing with our hair can help us maintain healthy locks.


  1. Chemicals:

    You might unknowingly be applying dangerous, harmful chemicals to your hair found in your hair products. Some of these chemicals in question that you should keep an eye out for include:

    -Anything with "sulfate" (e.g., Ammonium lauryl sulfate, sodium lauryl sulfate, sodium laureth sulfate) → over 90% of hair care products contain them! Commonly seen in shampoos, and are what cause it to foam. Some concerns include frizziness, dryness, flakiness, irritation, and the potential to form carcinogenic compounds (cancer-causing), which can lead to kidney damage and hormone disruption.

    -Words ending in "-paraben"(e.g., propylparaben, butylparaben, ethylparaben, methylparaben) → used as preservatives to lengthen shelf-life by preventing the growth of bacteria and fungi. Concerns include disrupting your hormone system, which can lead to reproductive disorders, obesity, impacts on birth, and hormone-related cancer; skin irritation; and neurotoxicity (damage or harm to the brain, spinal cord, or nerves due to toxic substances).

    -Undisclosed fragrances and colors→ if the label doesn't list the ingredients involved with the scent, then they're definitely hiding something. These fragrances can include over 100 different chemicals; it's basically a loophole for manufacturers to add whatever they want into the product. Possible concerns include infertility, irritation, hair loss, asthma, and cancer. The same goes for synthetic colors, which are derived from petroleum and coal-tar. Avoid anything with the terms "FD&C" or "D&C" combined with a number.


    This isn't even including chemical treatments such as permanents, hair dyes, or relaxants, which can lead to hair thinning and hair loss. Repeating these treatments frequently can destroy your hair follicles, making it unable to regrow hair and eventually leading to bald spots.


  2. Heat:

    We are all aware that curling, straightening, blow-drying, or anything that involves applying heat to our hair is damaging. These tools remove the moisture from your hair and inflame and damage your hair follicles. Further, you should never flat-iron your hair when it's damp; steam from the heat touching the water can lead to "bubble hair", leaving it brittle and frizzy.


  3. Hair styles:

    This one surprised me, but I guess it makes sense that tightly pulling your hair for certain hairstyles over long periods of time can cause damage. Repeatedly putting stress on your hair follicles can lead to permanent damage, split strands, and hair loss. Hairstyles in question include braids, ponytails, buns, or any other pull-up hairstyles. Leaving weaves or hair extensions for a long time and attempting to remove them without professional help is also a big no-no.


  4. Brushing:

    I'm guilty of this. I find that brushing my hair is quite relaxing, but unfortunately over over-brushing your hair can also damage it. Brushing your hair more than "100 strokes a day" can lead to split ends (which I have personally noticed in my hair) and, over time, hair loss.


    I'm also going to add this here, but brushing your hair harshly when it's wet is also damaging it. Water makes your hair weaker, so tugging at it carelessly could be detrimental.


  5. Towel Dry:

    This one also surprised me because it's something so common to do after you shower. Wrapping your hair in a regular towel, which is commonly made of rough, coarse fabric, and rubbing it against your hair can lead to frizz, pulled strands, and breakage.


  6. Shampoo:

    Balance is key. If you add too much shampoo, lathering it all over the length of your hair, you could potentially strip your hair of its natural oils, leaving it dry and fragile. But avoiding using shampoo at all can lead to the buildup of these oils, leading to dandruff and other scalp issues.


  7. Washing:

    Over-washing your hair can also be damaging because you're excessively removing the natural oils essential for maintaining shiny, healthy hair. Of course, each hair type differs. For example, people with curly, coily hair should wash their hair a lot less frequently than straight, fine hair. And those who naturally have really oily hair can pass washing their hair every day. It really depends, but overall, it's not good to wash it daily.


  8. Pool:

    Chlorine. Despite its negative effects on hair, it is essential for keeping pools safe from bacteria. Through its natural oil-stripping habits, chlorine can cause dryness, split ends, brittleness, frizz, itchiness, and leave your hair looking dull.


Hair Care

Ok, but now, how do we fix/avoid these damages?

  • Use a heat protector when using heat tools!

  • Change up your hairstyle (to avoid constant pulling)

  • After a shower, let your hair dry a bit before brushing it

  • Always opt to air-dry your hair, but if you HAVE to dry your hair, then make sure to set your blow-dryer temperature low

  • Only apply shampoo to your scalp rather than all of your hair

  • Wear a swim cap when going into a pool, or just rinse your hair really well afterwards

  • Most importantly, know your hair type. The more you know about your hair and what works best for it, the better you can care for it.


As someone who loves to eat, I think it's important that we also mention certain foods that can improve our hair health. As dietitian Michelle Dodd put it, “Your food choices affect anything growing on or in your body.”

  1. Eggs (specifically egg yolks)→ provide protein and are high in biotin, a B vitamin essential for the production of keratin!

  2. Lean Proteins → meats, chicken, fish are all good sources of protein. Some plant sources include tofu, beans, lentils, and nuts.

  3. Omega-3 foods → fatty fish such as salmon, sardines, herring, and mackerel, and other foods like chia seeds, walnuts, pecans, almonds, and canola oil, are all high in omega-3, which has anti-inflammatory properties.

  4. Leafy Greens → spinach, lettuce, and kale all have vitamin A, essential for the production of sebum. Spinach, specifically, is loaded with other beneficial nutrients such as iron, folate, and vitamin C.

  5. Fruits and Veggies → many fall under this one, including:

    ‣High in Vitamin C — guava, sweet peppers, carrots, cruciferous vegetables like

    broccoli, oranges, grapefruit, squash

    ‣Berries — blueberries, strawberries, etc, are loaded with vitamin C too, providing great antioxidant properties

    ‣Sweet Potatoes — provide beta-carotene, which is converted to vitamin A by our bodies

    ‣Avocados — provide healthy fats and vitamin E, which is also an antioxidant

  6. Shellfish → oysters, clams, shrimp, and crab are good sources of zinc, which is essential for keratin to grow hair

  7. And of course, water (I know this isn't a food, but I'm still going to include it) → staying hydrated helps prevent dry hair and supports healthy hair growth. It's recommended to drink at least 2 liters of water a day.


Now, instead of focusing on one food, it's always best to eat a well-balanced meal that incorporates the needed nutritional values.



Final Thoughts

Well, this one was longer, but I think it was worth it. Hair plays such a prominent role in our lives without us even noticing. From its structure and different types, to how we care for it and protect it from damage, there’s so much more going on than just what we see in the mirror. Taking a little extra time to understand and care for our hair can make a huge difference, not just in terms of looks but also for our confidence and overall health.


Image of 3 types of hair next to each other.


Sources

Hair Structure

Hair Type

Hair Growth Cycle

Hair Hazards

Hair Care

4 Comments

Andre SC
Andre SC
Oct 12, 2025

Very Nice work, congratulation!

Like
Karina Wagner
Karina Wagner
Oct 12, 2025

Great information!

Like
machadocami02
Oct 12, 2025

very interesting!! super insightful entry!

Like
Leah Lazenby
Leah Lazenby
Oct 12, 2025

Beautiful insight queen

Like

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