Terminal Hair Growth Starts During Puberty
In adult life there are two types of hair: vellus and terminal. Vellus is peach fuzz. You can easily see vellus hair on children and women because they don’t have a lot of terminal hairs to cover them. Vellus hairs are short, fine, and light colored. This is in contrast to the thick, long, and dark terminal hairs that begin to grow during adolescence. Increases in androgenic hormones (ex. Testosterone) cause vellus hairs to become terminal hairs, especially in the pubic area and armpits. Men will grow more terminal hair on the face, chest, abdomen, legs, and arms; while women will maintain more vellus hairs into adulthood. The terminal hairs undergo the hair cycle.
Human hair grows in a cyclical pattern. There is a phase of active growth called anagen. Then comes a transitory phase called catagen. This is followed by an inactive phase named telogen.
Anagen is the active phase of the hair cycle.
Anagen is the most commonly encountered hair phase (about 90% of hairs) due to its long duration. This phase can vary widely in duration based on factors like age, season, sex, location on the body, hormonal status, and personal genetics. For example, the percentage of hairs in anagen (active phase) peaks in the spring and dips in late summer/early fall. A hair on your scalp might be in anagen anywhere from 2-6 years but a hair on your thigh would only be in anagen for 1-2 months. And thank god for that or we’d have a lot more thinning heads out there.
How fast does hair grow?
You can expect speedy gonzalez rates of growth on your scalp – about 0.35mm/day. On your thigh it’s down to 0.20mm/day, and your eyebrows are growing at a snail’s pace of 0.16mm/day. Oh and for the newly (pre) menopausal women who are starting to see hormone changes, it’s not in your head that you feel like you have to shave your legs more often. Women aged 40-45years old can see rates of 0.26mm/day of leg hair growth.
Permanent Hair Removal Affects Anagen
During this phase, you can expect the best results from hair removal methods that are meant to be permanent or longer lasting. This is because the hair matrix during anagen is very sensitive to injury or changes from drugs, growth factors, hormones, stress, and physical factors. If you can destroy the epithelial stem cells in anagen you will have permanent hair loss.
The length of the hair at the end of anagen will be the length of the hair for the life of that hair.
Catagen is the transition between anagen and telogen. It’s unknown what causes anagen phase to move into catagen phase. It lasts about 2 weeks and produces a “club hair.” This club hair isn’t attached to a blood supply and cannot grow any longer. This is the rarest phase to find seeing as only about 1% of hairs will be in this phase.
Telogen is the arrested phase of the hair cycle.
During the telogen phase of the hair cycle, hair is not actively growing. The hair will not grow anymore and can only be shed from here. Telogen hairs are basically dead and make up anywhere from 1-15% of hairs.
When subject to high levels of stress, many more hairs can be induced to enter telogen. This means more hair are in peril of being shed. Telogen effluvium is a condition where there is increased shedding of telogen hairs from resting hair follicles. It can be caused by physical or mental stress and is the second most common cause of alopecia (sudden hair loss). It will show up 3-4 months after the stressor.
It’s theorized that Rogaine (minoxidil) increases hair growth by causing hairs that are in telogen to shed and be replaced by new anagen hairs.
- Olsen, Elise A. “Methods of hair removal.” Journal of the American Academy of Dermatology2 (1999): 143-155.
- Wolff, Klaus, and Richard Allen Johnson. Fitzpatrick’s color atlas and synopsis of clinical dermatology. McGraw-Hill Medical, 2009.
- Hair growth cycle: http://www.baylaserhairremoval.com.au/SiteMedia/w3svc511/Uploads/Images/hair_growth_cycle.jpg