Hair, a beautiful asset of the human body, is actually a keratinized protein filament, which has its roots located deep within the dermis (the upper layer of the skin).
The presence of hair is one of the distinguishing characteristics of mammals except that the hair found in nonhuman mammals is called wool or fur and not exactly hair. The hair is entirely made up of keratins, which are actually dead cells, therefore, the only 'alive' part of the hair are the roots.
The rate at which hair grows depends upon many factors, such as the genetic history of every individual. Other determining factors are diet and hormonal graph of the person.
Normally, the terminal hair show a faster growth rate when compared to the overall body hair. Hair on the scalp grows about 0.3 mm to 0.4 mm per day, and it comes up to around ½ inch a month and 6 inches per year.
Hair Growth Cycle
Growth of hair takes place in three stages, namely: anagen, catagen, and telogen. Anagen is the growth phase, while catagen is the transitional phase, and telogen is the resting phase.
In this phase, the cell division is active and quick for the formation of new hair. The hair grows as fast as about 1cm per day for more than 28 days. This is an active stage for the scalp hair for about 6 years.
The absence of this active phase can be a key reason behind why in some people the hair doesn't grow long. The other body hair do not grow beyond a certain limit, because they do not have a longer anagen phase.
Catagen is the transitional phase. Amongst all the hair, about 3% of the hair are in this stage at any given moment of time. During this phase, the hair stops growing, and the outer case of the roots contracts and starts to attach to the hair root. This formation is also called 'club hair'.
This is the final and resting phase. Telogen lasts for more than 100 days for terminal hair and longer for other body hair. Hair loss is loss of hair, which are in this phase, and around 30 to 100 hair tend to fall per day.
This can happen even as a consequence of several medications, childbearing, and weight loss. After the telogen phase, the growth cycle is complete and starts again with the anagen phase.
Rate of Hair Growth
A few factors, like genetics, diet, health condition, hormonal balance, etc., decide the rate at which hair grows. It is largely affected by the aging process in humans, and it gets slowed down significantly. The growth speed varies from person to person, therefore, any answer to this question will be more hypothetical and a mere prediction.
The average hair growth rate that is generally estimated is around 0.44 mm per day and ½ inch per month. In the book Chemical and Physical Behavior of Human Hair (Springer, 2002) by Clarence R. Robbins, it is cited that, "Hair grows at about 16cm/year (approximately 6.2 inches/year) on the vertex or the crown area of the scalp.
It grows at a slightly slower rate (around 14cm/year) in the temporal area and generally at even slower rates on other body regions (around 10cm/year) in the beard area."
Of course, there are a few factors that can fasten the growth of hair and provide a person with a healthy mane. Vitamins and essential oils are known to stimulate the growth cycle and make the hair grow faster.
A few other factors, like leading a healthy and active lifestyle, having a healthy and balanced diet, and taking proper hair care can really speed up the process of hair growing. Also, there are a few medical conditions that can affect the speed of hair growth, therefore, the growth rate varies in every individual.
Disclaimer: This content is for informative purposes only and should not be used as a replacement for expert medical advice.