Janaya Anisy, of Villa Maria College, asks :-

How often do the hairs on my head fall out?

Vernon Choy, a biochemist at Agresearch, Ruakura, responded.

A human hair, on all parts of the body, is grown by a special group of cells called a hair follicle. Hair follicles develop in the unborn baby from about the middle of pregnancy. No new follicles are created after birth, and none are lost in adult life. However, as the person grows, the density of the follicles changes and the type of hair grown may change. Babies and young children have fine hair. Adults have thicker, coarser hair that is also more pigmented.

Scalp hair varies tremendously between races, between individuals of one race, and even within an individual. Asians have hair that is thick with a round cross-section, causing it to be straight. Europeans have hair that is elliptical and slender. Other races have hair that is flattened, resulting in kinky curls. Blondes have more hair (average 140,000 hairs) than brunettes (105,000 hairs) or redheads (90,000 hairs).

Scalp hair grows about 0.3 mm/day. It is important to understand that hair follicles are not always actively growing hair. They spend some time in active growth, some time in a resting period, and some time in-between active and resting. At the end of an active period, the follicle stops producing a hair, which remains anchored in the follicle until the follicle resumes growth after the resting period. When the follicles begins growing hair again, it grows a new hair, and the old hair drops out.

The growing period of hair follicles on your head is three years. Therefore, the hair in each follicle will drop out every three years. It is normal for 50 to 100 hairs a day to fall out.