Human hair growth

The growth of human hair occurs everywhere on the body except for the soles of the feet, the palms of the hands, the inside of the mouth, the lips, the backs of the ears, some external genital areas, the navel, and, apart from eyelashes, the eyelids.

Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen.

This varied construction gives the hair unique characteristics, serving specific purposes, mainly warmth (redundant in modern humans) and physical protection.

[2] Most humans develop the longest thickest hair on their scalps and (mostly observed in males) faces.

[14] Ultimately, the follicle is 1/6 its original length, causing the hair shaft to be pushed upward.

In most people, scalp hair growth will halt due to follicle devitalization after reaching a length of generally two or three feet (610mm - 914mm).

[15][better source needed] Most chemotherapy drugs work by attacking rapidly dividing cells.

Primary Cicatricial Alopecias (PCAs), commonly referred to as scarring alopecias, are a collective group of inflammatory disorders with distinctive pathological features that include the development of fibrosis, permanent destruction of the pilosebaceous unit of the hair follicle (HF), and irreversible hair loss.

Tight ponytails and other styles that require added tension to the hair are often what cause this disease.

Plucking or waxing one's eyebrows frequently, for example, can yield suppressed hair growth in the area.

Human hair follicles are very sensitive to the effects of radiation therapy administered to the head, most commonly used to treat cancerous growths within the brain.