--Less hair keeps the head cool. People can lose up to 50 percent of their body heat from the top of their head. This is why it's important for everyone to wear hats and earmuffs during the winter months.
But when we don't want to get overheated, having less hair can make it easier for that heat to escape the head. Thick overlapping layers of hair "insulate the scalp," says Desmond Tobin of Bradford University in England. "Short hair will reduce this during hot weather," he says.
Things get even better if the hair is not totally flat. Curly hair, such as an afro, is 'raised' above the scalp and can actually permit wind eddies to cool the head, Tobin says.
--Hair grows slightly faster in the hot months, suggests at least one study. A 1991 article in the British Journal Of Dermatology explored "androgen-dependent" hair growth in bald men living in temperate climate. Androgen-dependent hair includes some scalp hair and other hair whose growth is influenced by hormones called androgens. By contrast, androgens do not affect the growth in other types of hair such as eyelashes and eyebrows.
According to Tobin, the 1991 study suggested that androgen-dependent hair growth is faster during the spring and summer months and slower during the winter months in temperate regions such the US. Although this study was conducted in men, the results may also apply to women.
One caveat, however, is that the results may not apply to children since their hormonal activity is very different from adults', at least before puberty.
--Scalp hair grows up to 0.45 millimeters per day, says Mike Philpott of the Center for Cutaneous Research at Queen Mary's School of Medicine and Dentistry in London. This daily rate translates to about half an inch per month.
--Cutting hair does not make it grow back faster, say Tobin and Philpott, so clever kids cannot use this excuse to delay a haircut.
--Of course, shorter hair makes it easier to spot ticks on the scalp, which is important for some areas with a prevalence of Lyme disease.
On the negative side: --Shorter hair, especially for those with lighter hair, receives more exposure to potentially damaging ultraviolet radiation during the summer months. Thicker hair may reduce the amount of UV hitting the scalp, as suggested by the existence of UV-induced scalp problems in bald men, says Philpott.
Hair does a very good job of blocking UV, Tobin notes. It will block ultraviolet rays from reaching the scalp skin as well, if not better, than sunscreen. Thus, hair that is kept very short, for example a buzz cut, will permit UV to gain more access to the scalp. For those with short haircuts, avoiding the direct sun, and wearing light-colored hats, is still the best way to prevent UV radiation from hitting the scalp.
The researchers offered some other interesting tidbits about hair:
--Blondes tend to have more hairs per unit area, with redheads having the least, and other hair colors in the middle, Tobin says.
--Does hair grow faster during the day than in the night? "There is evidence," Philpott says, "that hair growth follows a circadian [24-hour] rhythm. We have in fact found some of the circadian clock genes are expressed in hair follicles."
--"All hair twists as it grows," Philpott says, even in hair that looks completely straight. "The more twists in a hair fiber," he says, "the curlier it will be."
--The thickness of hair is generally between 60-90 microns (millionths of a meter) in European populations, but it can average 120 microns in some Asian populations. By comparison, a red blood cell is about 7-8 microns in size. The different thicknesses, in many cases, result from the varying shapes of hair when viewed edge-on.
Dr. Desmond J. Tobin
Reader in Cell Biology & Director of Postgraduate Research (BMS)
Dept. of Biomedical Sciences
University of Bradford, Bradford, England
Dr Mike Philpott
Centre for Cutaneous Research
Queen Marys School of Medicine and Dentistry
London E1 2AT