Human hair undergoes a continuous process of growth and renewal, influenced by various internal and external factors. Understanding this process provides insight into how hair grows.
Understanding Hair Growth Rates
On average, human hair grows approximately 0.5 to 1.7 centimeters per month, which translates to about 2.1 to 2.8 millimeters each week. This rate is an average, and individual growth varies based on several factors. Hair growth is a cyclical biological process, with each strand moving through distinct phases.
The hair growth cycle primarily consists of three main stages. The anagen phase is the active growth period, where hair follicles continuously produce new hair cells. For scalp hair, this phase typically lasts between two to eight years, with males often experiencing a shorter anagen phase of two to four years. At any given time, about 85% to 90% of the hairs on the scalp are in this active growing stage.
Following the anagen phase is the catagen phase, a brief transitional period lasting about two to three weeks. During this stage, hair growth slows significantly, and the hair follicle begins to shrink.
The final stage is the telogen phase, a resting period that typically lasts for about two to three months. In this phase, the hair is no longer growing and eventually detaches, making way for new hair to begin its growth cycle.
Factors Influencing Hair Growth
Genetics play a substantial role, determining growth rate, hair texture, thickness, and the anagen phase duration. Genetic predisposition influences how long hair can grow and whether an individual experiences slower growth or certain patterns of hair loss.
Age impacts hair growth, with the fastest rates observed between 15 and 30 years old. As individuals age, hair growth tends to slow, and hair follicles may shrink or cease producing hair, leading to thinning or baldness.
A balanced diet rich in proteins, essential vitamins like B12, D, biotin, C, and E, and minerals such as iron and zinc supports healthy hair growth. Hair is primarily composed of keratin, a protein, so adequate protein intake is important. Deficiencies in these nutrients can lead to slower growth or increased hair shedding.
Hormonal balance significantly affects hair follicles. Androgens, including testosterone and its more potent derivative, dihydrotestosterone (DHT), are particularly influential. While DHT stimulates the growth of body and facial hair, elevated levels can contribute to male pattern baldness by shortening the hair growth cycle and causing follicles to miniaturize.
Overall health conditions can impact hair growth. Autoimmune diseases like alopecia areata and lupus, thyroid disorders, and certain infections can disrupt the hair cycle and lead to hair loss. Chronic stress can prematurely push hair follicles into the resting or shedding phase, a condition known as telogen effluvium, as stress hormones like cortisol negatively affect hair growth stem cells.