1. What Are the Three Layers Inside a Single Hair?
Imagine pulling out one hair and looking at it through a very large lens. The inside is divided into three layers. The outermost layer is the cuticle. It overlaps like scales to protect the inside. In healthy hair, there are 4 to 10 of these scales [1].
The middle layer is the cortex. It makes up 85 to 90% of the hair. This is where the color comes from [1]. The innermost core is the medulla. It is soft tissue. Fine hair may not have a medulla at all [1].
2. How Do Two Types of Melanin Decide Hair Color?
The color of hair is determined by a pigment called melanin, which is inside the cortex. There are two types of melanin [2]. One is eumelanin, which creates black to brown colors [2]. The other is pheomelanin, which creates yellow to red colors [2].
3. How Does the Shape of a Hair's Cross-Section Decide Curl?
Next, let’s talk about curls. The answer lies in shape. Imagine cutting a hair crosswise. Straight hair has a cross-section that is close to a circle [3]. Curly hair has a cross-section that is an ellipse [3]. Hair with a flat cross-section cannot grow straight easily; it twists or waves.
Why are the cross-sections different? It is because the shape of the hair root is different [3]. Hair is made in the hair root inside the scalp and pushed out. The root is like a mold for the hair. If the mold is round, the hair is round. If the mold is distorted, the hair is distorted. It is said that the stronger the curl, the greater the deformation of the hair root [3].
This explains a well-known phenomenon. Even if you use a straight perm or a hair iron to make hair straight, the original curl comes back over time [3]. You can only treat the part that has already grown out. The mold for the new hair that will grow has not changed. Thickness works the same way: the more cortex there is, the thicker the hair. It is known that Japanese people often have thicker hair.
4. Why Is Losing Hair Every Day Normal?
In the morning, you might find hair on your pillow. Or your brush might be tangled with many strands. It is worrying, but this is normal. Hair has a lifespan.
Hair grows at a specific rate. It grows about 0.3 millimeters per day [4]. That is about 1 centimeter per month [4]. In one year, it grows about 15 centimeters (about 6 inches) [4].
5. What Do Perms and Bleach Do to Hair?
When you get a perm, straight hair becomes curly. It does not wash out. What is happening? Hair is made of keratin protein. These keratin chains are connected by small bridge-like bonds. A perm manipulates these bridges.
First, the first solution cuts the bridges, making the hair soft [5]. Then, the hair is wrapped around a rod to change its shape. Second, the second solution reconnects the bridges in the new position [5]. By cutting, bending, and fixing, the shape is set. This is why it does not return to the original shape when washed. Straight perms do the same thing; they just bend in the opposite direction [5].
Bleaching is a different operation. It breaks down the melanin inside to lighten the color. The chemicals enter through gaps in the cuticle, so the outer layer is also stressed. Because both perms and bleach deliver chemicals to the inside of the hair, and because hair is dead and cannot heal itself, the hair takes a burden. Products that claim to "repair" damaged hair are mostly adding missing components from the outside to improve appearance and feel. They do not return the hair to its original state.
6. How Has the Habit of Washing Hair Changed Over Time?
Finally, here is a surprising fact. How often did people wash their hair in the past? The frequency of hair washing has changed over time.
In the Heian period, people washed their hair about once a year [6]. In the Edo period, women in Edo washed their hair 1 to 2 times a month [6]. Around 1955, people washed their hair once every 5 days [6]. In the 1980s, it was 2 to 3 times a week [6]. By the mid-1990s, women in their teens and twenties were washing their hair almost every day [6].
7. How Can You Check Facts About Hair at Home?
You can check most of this article’s content at home. Look at the root of a shed hair. It may have a whitish, swollen part. Note: You cannot judge whether it fell out naturally or broke off just by looking. Just observe the shape differences.
Calculate the age from the length. Hair grows about 15 centimeters per year [4]. When was your hair tip born? This is an average value, and it varies greatly depending on the person, age, and health [4].
Ask your family. Did you wash your hair every day when you were a child? You can check if the table in section 6 is true for them.