Hair can be considered in two separate sections, the root and the shaft. The hair root is located inside of the follicle and exists below the portion of the skin while the shaft is the portion of hair that extends out from the skin. The root of the hair receives nutrients from the blood through the dermal papilla. The nutrients that are supplied to the root of the hair through the dermal papilla, combined with oil from the sebaceous gland, can contribute to hair’s strength. This strength of the hair shaft can contribute to how long it grows before it breaks. Also, keeping it from drying out can prevent it from breaking. As such, a dry environment or treating hair with a lot of chemicals can limit its length.
The hair that grows out of the hair follicle can be course or fine. Coarse hair is thicker because it has a thick central core of cells (called the medulla) that make it stronger and less likely to break due to environmental factors. This is a genetic trait and blonde or fine hair often lacks this dense core. It is thus easier for someone with thick (likely dark) hair to grow it very long. Although the average rate of hair growth is apparently half an inch per month, the rate at which hair grows is also genetically determined.
Hair also sheds, naturally. The normal cycle of a hair follicle lasts from 2-8 years. The first phase, also called anagen, is when the hair shaft grows out of the root. The second phase in the cycle is called catagen and is the period during which the hair follicle shrinks and the root detaches from the blood supply. Catagen lasts for a couple of weeks. The final phase, telogen, is when the hair does not grow, but is still attached. This phase lasts for several weeks. After telogen, the hair follicle moves back into anagen and begins to grow a new hair. It is the pushing out of the old hair that leads to shedding. A normal rate of shedding is 50-100 hairs a day. It is thus the length of this cycle that partially determines how long a hair can get. If one has inherited a shorter hair growth cycle of 2 years, and hair grows at an saverage of half an inch a month, the longest it could grow if it were strong is 12 inches. However, if someone had a very long hair growth cycle of 8 years and their hair were strong, it could grow 4 feet or more!
Super-long hair thus occurs only when there is a combination of several factors: a decent rate of growth, coarse hair, a non-drying environment, a long hair follicle shedding cycle, and, of course, the choice to not visit a barber.