Temperature can certainly affect your hair's health and appearance, whether it is living in a dry, cold climate or extended use of a hot curling iron. However, there is no clear-cut evidence that temperature can actually alter your hair's growth rate, which tends to be about a half-inch per month for everyone. However, considering a few of the ways that temperature can affect your hair—and gaining a better understanding of your hair's natural growth process—can help you make informed choices about hair care and styling as they relate to temperature.
In general, too much heat can lead to brittle, damaged hair and split ends. The most common source of excessive heat exposure is extended use of heat-based styling tools such as curling irons or hair dryers. While these devices can be safe and healthy when used properly, over time they can have a negative effect. On the other hand, heat can also encourage blood flow to follicles in the scalp, and it can cause cuticle scales to open, making conditioning treatments more effective. While heat can damage or help your hair, depending on the situation, it generally does not affect the rate at which your hair grows.
Cold can also have an effect on hair. In a clinical context, cold can sometimes diminish the effects (good or bad) of chemotherapy or medical hair treatments by reducing blood flow to the scalp follicles. As an environmental factor, cold air can make hair more brittle, dry and fragile, as well as increasing static. While some people claim that hair growth increases in winter to provide additional insulation, medical studies have not shown this to be true.
Hair fibers consist of keratin proteins, which are also found in human fingernails and toenails and in animal hooves, horns and claws. Hair fibers contain three layers. The outside consists of a thin, transparent layer called the cuticle, which protects the hair. Inside the cuticle is the cortex, a thick layer with melanin, which determines your hair color. The cortex also makes your hair curly or straight. At the very center of the hair fiber is the medulla, which reflects light to create your hair's shine and tone. Within the skin, a follicle surrounds the root of each hair and sebaceous glands provide oil to moisturize and protect the hair.
Hair Growth Phases
Hair growth takes place in three distinct phases. Anagen is the initial phase, when hair actively grows for two to eight years. The length of anagen determines how long hair can grow, from a maximum of a foot or so to well over four feet. Following anagen, the hair follicle recedes for two to four weeks during the transitional catagen phase. The resting phase of telogen follows for two to four months. During telogen, the hair does not grow, but it remains attached to the follicle. Finally, anagen can begin again, and the growth process pushes out the old hair. This process leads the average person to lose 50 to 100 hairs every month, often in the shower or while brushing hair.
Tips for Hair Growth
While temperature can't provide an easy answer for hair growth issues, there are ways to help encourage and sustain hair growth. As a rule, try to stay generally healthy by eating a balanced diet and getting regular exercise, and care for your hair with consistent hygiene and grooming habits. Massaging your scalp during a thorough hair washing is a great way to promote healthy hair growth, because it stimulates the follicles and increases blood flow. When in doubt—and especially if you're considering an unusual treatment or supplement—be sure to do plenty of research and consult a hairstylist, pharmacist or physician you trust.