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Before air conditioning, curly hair helped keep people cool.

A thermal manikin wearing tightly curled (left) and straight (right) human hair wigs

Hair textures impact body temperature regulation

Researchers at Loughborough University studied the role of human hair textures in regulating body temperature and found that curly hair may have helped early humans stay cool while conserving water. This adaptation can explain how the human brain grew to modern-day sizes and sheds light on our evolutionary history.

Evan Pugh University Professor of Anthropology at Penn State, explained:

“Humans evolved in equatorial Africa, where the sun is overhead for much of the day, year in and year out,” said Nina Jablonski. Here the scalp and top of the head receive far more constant levels of intense solar radiation as heat. We wanted to understand how that affected the evolution of our hair. We found that tightly curled hair allowed humans to stay cool and actually conserve water.”

Using a “thermal manikin” to study heat transfer in the human head

The researchers used a thermal manikin to study heat transfer between human skin and the environment. This manikin was a human-shaped model that used electric power to simulate body heat. To examine how diverse hair textures affect heat gain from solar radiation, the scientists programmed the manikin to maintain a constant surface temperature of 95 degrees Fahrenheit (35 degrees Celsius), similar to the average surface temperature of skin. They set the manikin in a climate-controlled wind tunnel and took base measurements of body heat loss. They monitored the amount of electricity required by the manikin to maintain a constant temperature.

The scientists shined lamps on the manikin’s head to mimic solar radiation under four scalp hair conditions: none, straight, moderately curled, and tightly curled. They calculated the difference in total heat loss between the lamp measurements and the base measurements to determine the influx of solar radiation to the head. They also calculated heat loss at different wind speeds and after wetting the scalp to simulate sweating. They ran their results through a model to study how the diverse hair textures would affect heat gain in 86-degree Fahrenheit (30 degrees Celsius) heat and 60% relative humidity. This is similar to environments in equatorial Africa.

The researchers found that all hair reduced solar radiation to the scalp, but tightly curled hair provided the best protection from the sun’s radiative heat while minimizing the need to sweat to stay cool. They reported their findings yesterday (June 6) in the Proceedings of the National Academy of Sciences.

Hair protecting the brain from solar radiation

As early humans evolved to walk upright in equatorial Africa, the tops of their heads were exposed to more solar radiation, which can be harmful to the brain. The brain generates heat, especially as it grows larger, and too much heat can lead to dangerous conditions like heat stroke. To keep cool, humans developed efficient sweat glands, but sweating comes at a cost in lost water and electrolytes. Scalp hair likely evolved as a way to reduce the amount of heat gain from solar radiation, thereby keeping humans cool without the body having to expend extra resources.

“Around 2 million years ago we see Homo erectus, which had the same physical build as us but a smaller brain size. And by 1 million years ago, we’re basically at modern-day brain sizes, give or take. Something released a physical constraint that allowed our brains to grow. We think scalp hair provided a passive mechanism to reduce the amount of heat gained from solar radiation that our sweat glands couldn’t.”

“The work that’s been done on skin color and how melanin protects us from solar radiation can shape some of the decisions that a person makes in terms of the amount of sunscreen needed in certain environments. I imagine that similar decision making can occur with hair. When you think about the military or different athletes exercising in diverse environments, our findings give you a moment to reflect and think: is this hairstyle going to make me overheat more easily? Is this the way that I should optimally wear my hair?”

Image Credits

In-Article Image Credits

A thermal manikin wearing tightly curled (left) and straight (right) human hair wigs via Loughborough University by George Havenith with usage type - News Release Media

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A thermal manikin wearing tightly curled (left) and straight (right) human hair wigs via Loughborough University by George Havenith with usage type - News Release Media


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