Final answer:
Healthy scalp hair typically grows at an average rate of half an inch (1.25 cm) per month. This growth is part of a cycle that includes an active growth phase (anagen), a transitional phase (catagen), and a resting phase (telogen), after which the cycle repeats.
Step-by-step explanation:
The average growth of healthy scalp hair is about half an inch (1.25 cm) per month.
Hair growth occurs in three primary phases known as the anagen, catagen, and telogen phases. During the anagen phase, which can last from 2 to 7 years, cells divide rapidly at the root, causing the hair to grow. Hair growth averages at 0.3 mm per day during this time.
After the anagen phase, the catagen phase is a short transitional period lasting about 2 to 3 weeks before entering the telogen phase, a resting period. Consequently, hair is shed and replaced with new growth as part of a recurring cycle. The replacement of old hairs by new follicles produced in the basal cells maintains hair density, barring any imbalance causing excessive hair loss. Several factors can influence hair loss, including hormonal changes, dietary changes, age, and genetics.