Hair Growth Facts

Kevin McElwee is a junior research immunologist/dermatologist who completed his Ph.D. doctorate on the pathogenesis of alopecia areata in Scotland. He currently works in Marburg, Germany, and continues his research into alopecia areata, along with basic hair biology. Kevin shared the following answers to some common questions about long hair.

What is the maximum length to which hair can grow?

If you do not cut your hair then you can grow it quite long, but there is an upper limit to the maximum length that you can grow your scalp hair.

We need to draw on three aspects of hair biology research to define the maximum length that scalp can grow to. There are various papers that examine the time duration for scalp hair follicles are actively producing a hair fiber. The most frequently quoted time period is 1,000 days, but this figure is a rough approximation.

The time duration of anagen growth, plus knowing what length of hair fiber a scalp hair follicle can produce per day, will enable a calculation of the average maximum length that scalp hair can grow to. Perhaps to complete the calculation you may need to know the density of hair follicles on the scalp.

Research on humans suggest that active hair fiber growth production from scalp hair follicles may last from 500 to 1800 days. In normal haired individuals about 80% of hair follicles are in a growth phase with the remaining 20% in a regression or resting phase. Hair follicle density on the scalp is 200-300/cm square, that is about 100,000 to 150,000 hairs total. The scalp hair fiber growth rate is between 0.3 to 0.35mm a day. For the most part, the studies were done with male volunteers, but a few other studies indicate similar results for women too.
If you put it all together in a calculation, the research would suggest that the maximum length scalp hair can grow to is between 20 and 60 centimeters and the sum total length of hair produced each year is about 9 km! However, from (vintage photos) you can see that some people are able to grow their hair much longer. The upper limit to scalp hair length is determined by our genes. Some people have genes for short hair, others grow hair much longer.

Why does human hair on the head grow so long while hair on the rest of the body will grow to a specific length and stop?

The answer is probably rooted in the evolutionary origins of humans. Our ancestors were considerably hairier than ourselves and certainly our nearest primate ancestors the orangutans have a good crop of hair all over their bodies. There is a disease of that involves total body hypertrichosis called congenital hypertrichosis lanuginosa where hair follicles seem to revert to their ancestral terminal hair state. Individuals with this condition have inherited and genetic mutation and this disease can be passed from parent to child. It is very rare and only occurs in a few families around the world. However, it would seem that we are only one genetic step away from our ancestors in terms of having extensive body hair growth.

Clearly there are advantages to having hair on some regions of our body. Eyebrows protect the eyes from sweat and sun glare. Eyelashes protect the eyes by sweeping away dust and debris. Nasal hairs trap dust before reaching the lungs. These sites of terminal hair growth have probably been maintained to keep these advantages.

Scalp hair does give us one advantage of trapping heat. One third of our heat loss is through our scalps so terminal head hair does have a practical action. However, this does not explain why it should grow so long.

There are two hypotheses based on evolution to explain long head hair.

1) Many animals have "manes" as sexual attractant signals or to increase their apparent size during sexual contests. Some feel this was a factor in human evolution. Cotton top tamarins may be an example with their extensive, white, billowing hair that frames their small face. Mature male gorillas develop a gray "silver back" appearance that reflects their status in the group.

2) It has also been suggested that through human evolution infants needed something to hold onto while being carried by the mother. Long head hair would serve such a function. Note there is no real evidence in support of any hypothesis.
Be sure to visit Kevin's site, Keratin.com