1. Why Cutting Hair Does Not Hurt: The Visible Part Is Dead
You do not feel pain when a barber cuts your hair. The part of the hair you see is made of cells that are no longer alive. Its main component is a protein called keratin. Medical texts describe the shaft of the hair as being "made of dead protein."[1][2]
So, where is the living part? Hair is made in a sac-like organ under the skin called a follicle. New hair is born at the bottom of this follicle, in an area called the matrix.[1] The part sticking out of your skin is just the end that was pushed out.
Follicles are not found on your lips, palms, or soles of your feet.[1] You can look at your palm to see that no hair grows there. This shows that some places have follicles and others do not.
Arm hair and head hair are both made in follicles. They start the same way. So, why does only head hair get so long?
2. Hair Length Depends on How Long Hair Grows, Not How Fast
The answer is not about speed. It is about the "period" of time that hair keeps growing. Head hair grows 0.3 to 0.4 mm each day. In 30 days, this adds up to about 0.9 to 1.2 cm. This is why people say hair grows about 1 cm per month.[4]
The time when a hair grows is called the "growth phase." For head hair, this phase lasts between 2 and 6 years, or sometimes 5 to 7 years, depending on the source.[1][3] In contrast, the growth phase for arm hair (upper arm) is only about 22 to 28 days.[3]
Let us calculate what happens if we use the same speed for both. If arm hair grows at 0.4 mm a day for 28 days, it reaches about 11 mm. It stops growing just over 1 cm long. If head hair has a growth phase of 5 years, and grows at 0.4 mm a day, the total length is about 73 cm. If it grows at 0.3 mm a day, it is about 55 cm. One review states that a 5 to 7 year growth phase corresponds to hair lengths of about 50 to 110 cm.[3]
This review says that the length of the growth phase mainly decides the maximum length of a hair. Even in mice, body hair grows for about 16 days, while whiskers grow for more than 2 months. This shows that location matters within the same animal.[3]
The range of 50 to 110 cm is a theoretical guide. Real lengths change from person to person and body part to body part. So, what happens at the bottom of the follicle every day?
3. How Matrix Cells in the Hair Bulb Push Hair Upward
The bottom of the follicle is bulb-shaped and called the "hair bulb." A group of cells called "matrix cells" gathers here. These matrix cells are deeply connected to the "dermal papilla" just below them. The review explains that they divide under the influence of the papilla. Nutrients and oxygen arrive from tiny blood vessels.[6]
A report from RIKEN states that matrix cells are cells that "keratinize to produce hair." It says the dermal papilla interacts with matrix cells to make parts of the hair and follicle.[5]
Cells that divide and increase in number are pushed up from below by new cells. As they move, their insides fill with keratin and harden. Eventually, they become a line of dead cells that exits the skin. This is the "visible hair" we discussed earlier.
To keep growing hair, the body must make large amounts of keratin and related proteins. The review notes that this creates a heavy metabolic burden.[3]
The same review calculates that if the growth phase lasted about 30 years, hair could reach about 4 meters. It states that matrix cells divide about once per day.[3] This is a theoretical calculation based on estimates. No such long hair has actually been recorded.
What happens after the growth phase ends?
4. How the Hair Growth Cycle Lets New Hair Grow from the Same Follicle
After the growth phase comes a short transition called the "regression phase." This is followed by a short "resting phase." This cycle repeats independently for each single hair.[1]
At any given time, about 90% of follicles on your scalp are in the growth phase.[3] Because the growth phase is so long, most of your hair is still growing whenever you look at it.
Hair that has finished its resting phase falls out. About 50 to 100 hairs fall out every day. This number is shown as a normal range.[1] Even when hair falls out, new hair grows from the same follicle.
A report by RIKEN and POLA Chemical Industries says that during the growth phase, the follicle extends deep into the skin. During the regression phase, it shrinks toward the surface. In human skin, each follicle is at a different point in its cycle.[2]
Follicles are one of the few organs in adult mammals that regenerate periodically throughout life. Adult tissue stem cells (follicle stem cells) were first identified here.[2]
5. How to Measure Your Bangs to Check Growth in One Month
You can try this today. Use a ruler to measure the length of your bangs from the hairline to the tip. Write down the length and the date in a notebook. One month later, measure the same spot using the same method again. Do not cut your hair during this time.
Compare the difference between the measurement from one month ago and today. Because differences can appear based on where and how you measure, use the exact same location and method both times. If the difference is close to about 1 cm, it matches the numbers in this article. Even if it is different, individual variation and body part differences are normal.
You can also see that arm or leg hair does not keep growing like head hair. This helps you feel the difference in growth phase lengths on your own body.
If you want to read more, "Overview of Hair Growth" from the MSD Manual Home Edition [1] and RIKEN's press release [2] are available in Japanese.