Human hair is a biological filament composed primarily of keratin, a tough structural protein. This protein is tightly packed into a cylindrical shaft that extends from the hair follicle embedded beneath the skin. Understanding the physical dimensions of hair involves quantifying its size, including its minute diameter and its potential linear length.
Defining Hair Thickness (Diameter)
The most precise measure of hair size is its diameter, or thickness, quantified in micrometers (µm). The diameter of a single human hair fiber varies widely among individuals, generally falling in a range between 17 and 181 micrometers. However, most people have hair that measures between 50 and 100 micrometers in thickness.
To put this tiny measurement into perspective, an average hair strand is about one-sixth the thickness of a standard sheet of office paper. A hair measuring 100 micrometers is roughly the same width as a particle of fine dust or pollen. The measurement is not a perfect circle, as the cross-section of a hair fiber is often slightly elliptical, which influences how it feels and appears.
The diameter measurement is significant because it directly relates to the hair’s mechanical properties, such as its strength and resistance to breakage. Thicker hair shafts contain more keratin mass and tend to be more robust than finer strands.
Variables Influencing Hair Size
The wide range of hair diameters observed across the human population is primarily a result of several biological and environmental factors. Genetics plays the most significant role, establishing the fundamental parameters for hair thickness from birth. This is evident in the general differences observed across various population groups.
For instance, hair from East Asian populations typically presents the largest average diameter, often ranging from 80 to 120 micrometers. Conversely, hair of African descent tends to be the finest, usually measuring between 40 and 90 micrometers, while Caucasian hair falls into an intermediate range. Age is another factor, as hair tends to become finer and thinner over a person’s lifespan due to changes in the hair growth cycle and follicle size.
The location on the body also dictates hair size, with the thickest, longest terminal hairs found on the scalp, eyebrows, and eyelashes. In contrast, the fine, nearly invisible vellus hair covering most of the rest of the body has a much smaller diameter. Additionally, factors like climate and natural hair color can cause minor variations in thickness.
Growth Rate and Length Potential
Beyond thickness, the linear dimension of hair is determined by its growth rate and the duration of its active growth phase. Human scalp hair grows at an average rate of approximately one centimeter (or about half an inch) per month. This rate can vary slightly, with some studies reporting growth between 0.6 and 3.36 centimeters monthly.
The maximum length a hair can achieve is not determined by the growth rate but by the duration of the anagen phase, which is the active growing stage of the hair cycle. This phase is genetically predetermined and lasts between two to seven years for scalp hair. Hair on other parts of the body, such as eyebrows and eyelashes, has a much shorter anagen phase, which is why they cannot grow as long as scalp hair.
Once the anagen phase ends, the hair enters the short transitional catagen phase and then the resting telogen phase before it is shed. A shorter anagen phase means that even a fast-growing hair will be shed sooner, limiting its potential length.