With the arrival of July, the temperature rises everywhere, and more people are active outdoors. Sunbathing by the sea, going to a park with a large lawn and having a picnic with the whole family is beautiful to think about. However, a new study from the University of Copenhagen shows that wearing a hat as much as possible when performing long outdoor exercises in the summer, and the scalp is directly exposed to the sun, there is a potential risk of damaging brain function.
Although the conclusion sounds rather frightening, the researchers' starting point for conducting this study is well documented. About half of the world's population lives in areas where extreme heat or cold climatic conditions are likely to occur, and the thermal effects of the environment potentially affect people's lives and production. In the outdoor environment, when the direct top of the human head is exposed to solar radiation, it will produce a considerable heat load, and researchers have confirmed through this study that such conditions have potential damage to human health.
△Image source: https://www.sopadre.com/plan-your-trip/weather/
<h1 class="pgc-h-arrow-right" how does the brain dissipate heat when heat builds up >? </h1>
Human brain function is highly dependent on a stable supply of glucose and oxygen to the blood, but the brain is also highly susceptible to damage when conditions for maintaining the brain's thermal homeostasis change. The brain is enclosed in the skull and is highly isolated, but the brain must remove the heat generated by metabolism to maintain the temperature of the brain. In general, the brain dissipates heat primarily through the flow of arterial blood. In a resting state, 700 ml of blood flow per minute can take away 1 kilojoule of energy, which will help the brain reduce by 0.3 degrees Celsius.
Sunlight can penetrate the skin by a few millimeters and heat the tissues beneath the skin, so exposing the head directly to intense solar radiation for a long time could theoretically raise the brain temperature. In a high temperature environment, a lot of sweating will consume water and salt in the body, resulting in dehydration, and the reduction of body fluids and salt directly leads to insufficient blood supply to the brain. As a result, the brain's ability to promote heat dissipation through blood circulation is greatly reduced. In addition, special layered structures in the skull and cranial cavity that resemble insulated cups may greatly limit the passage of heat, which causes the heat dissipation of the brain to become worse.
Blood flows in the brain to help dissipate heat in the brain (Image: University of Southern California)
< h1 class="pgc-h-arrow-right" > not only affect heat dissipation, but also affect motor cognitive function</h1>
Whether intense sunlight really affects brain function and brain temperature, let's find out with researchers.
Eight healthy men participated in the study, and 24 hours before any of the experiments were conducted, all participants drank water adequately and abstained from alcohol and strenuous exercise. As shown in the figure below, the researchers simulated intense sunlight (about 1,000 watts per square meter) in the lab to test subjects for testing exercise cognition. The test is mainly divided into the following four aspects:
1. Simple movement task (test the pinch/adjust force according to the numbers indicated on the screen).
2. Simple cognitive task (displaying four numbers on a monitor, participants using a numeric keypad to calculate and type the sum of four numbers).
3. Combine motor cognition tasks (additive calculations based on four numbers on the screen, then typing in the results, and testing of thumb and index pinch/adjustment force).
4. Complex visual motion tracking tasks (complete tasks with cursors by applying pinching force, calculating the sum of numbers at the same time, typing the result, and pinching/adjusting forces using thumb and index finger through the strain gauge sensor).
△ Schematic diagram of simulated solar radiation. Head radiation includes not only a light above but also the head and neck.
(Image source: References)
The figure below shows that subjects whose heads are exposed to simulated sunlight for a long time (2 h) have significantly lower scores for complex motor and integrated motor cognitive tasks. Conversely, neither acute exposure (15 minutes) nor sustained lower body heating led to a decrease in motor cognitive abilities.
Not only does it affect heat dissipation, but sunlight radiation can also cause damage to the brain nerves (Image source: References)
The researchers found that prolonged exposure of the head and neck to the sun would cause the body core temperature (the temperature of the body measured by the rectal thermometer at about 10 cm when inserted into the sphincter) by 1 degree Celsius, which obviously had a considerable negative impact on the normal maintenance of bodily functions.
In addition, to further study the effects of sunlight radiation on cranial nerves, the researchers tested the brainstem auditory evoked potential of the subjects. What is a brainstem auditory evoked potential? Let's first explain what an auditory pathway is. As shown below, the sound wave vibrations felt by the outer and middle ears of our ears are transmitted to the inner ear, and the inner cochlea is transformed into a bioelectric signal and then transmitted through the auditory nerve all the way to the cerebral cortex, so that people can hear the sound.
Hearing is transmitted to the channel (Image source: Baidu Encyclopedia)
The auditory evoked potential of the brainstem is an objective indicator of damage to the brainstem. The instrument records the potential response associated with hearing within 18 milliseconds of sound generation, and then reflects the functional status of the relevant structures from the cochlea to the brainstem. Any lesion or injury involving the auditory pathway affects this indicator.
The auditory brainstem evoked potential can objectively and sensitively reflect the function of the central nervous system, and it is often changed when the brainstem is slightly damaged and clinically asymptomatic and sign-free. By measuring this indicator, the study found that when simulated sunlight radiation was applied for a long time (about 2 hours until the core temperature increased by 1 degree), the subjects' brainstem auditory evoked potentials would change, while under acute (15 minutes) exposure to sunlight radiation, no signs of being affected were shown.
Image source: https://www.sopadre.com/plan-your-trip/weather/
Not only that, the long-term direct sunlight in the simulated state also significantly changed the physiological and psychological indicators. This increased the subjects' heartbeat from 76 to 116 beats per minute, the core temperature of the body increased by 1 degree, and the average skin temperature increased by 2.3 degrees Celsius, especially the skin temperature of the head and neck measured at the occipital bone increased by as much as 5.4 degrees. These data clearly mean that subjects will have a strong sense of discomfort.
Overall, the study found that the heat of radiation applied to the head for a long time can affect the functioning of the brain to some extent, especially in complex motor and combined motor cognition tests, the subjects' abilities show a fairly significant attenuation. Direct exposure of the head and neck to radiant heat mainly affects the superficial parts of the brain. But even a mere increase in temperature at the superficial surface of the brain can damage the short-term ability of the human brain. For comparison, when radiation exposure to the lower body causes a similar increase in the temperature of the deep core of the body, it does not cause significant damage. In addition, short-term acute exposure to the head and neck can immediately increase the temperature outside the skull (skin), but there is no significant effect on brain function.
<h1 class= "pgc-h-arrow-right" > does it mean that you can't have a "bald head" to play happily in the summer? </h1>
We often say that today's sun is so poisonous that it hurts people's brain shells. Judging from the conclusions of this latest study, this sentence is really not just exaggerated rhetoric. Moreover, the pain in the brain shell is small, and the problem with the brain is quite serious.
However, given that the sample size of this study (8 people) is far from statistical significance, and its conclusions, although they sound shocking, are not contrary to common sense. Everyone knows that under the sun for a long time, there is also a great possibility of heat stroke without pain in the head. There are many similar recent studies, and the media often makes excessive or even inappropriate interpretations of the research itself in order to attract attention, and we are not guilty of listening to these statements. Of course, although there is no need to make a fuss, it is still a completely reliable "health" recommendation to pay attention to sun protection and avoid heat stroke in the hot summer.
There are a lot of outdoor workers who are seasonally exposed to high heat, which does have a certain impact on their health and work. For such special groups, the use of various forms of hats to shade the body can be a good way to reduce the heat damage caused by radiation exposure to the body. However, when using a hard hat such as a hard hat, it may also aggravate the heat damage due to the limited heat dissipation ability. Therefore, such personnel should ensure adequate drinking water and appropriate rest, avoid long-term labor under the hot sun, so as not to cause safety production accidents due to a short decline in brain function.
In the hot summer, it is good to enjoy the sunshine, and it is more important to do a good job of sun protection.
bibliography
Jacob F. Piil, Lasse Christiansen, Nathan B. Morris, C. Jacob Mikkelsen, Leonidas G. Ioannou, Andreas D. Flouris, Jesper Lundbye-Jensen & Lars Nybo. Direct exposure of the head to solar heat radiation impairs motor-cognitive performance. Scientific Reports volume 10, Article number: 7812 (2020).
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Producer: Popular Science China
Producer: Li Zhuosi (Doctor of Agronomy, Kyoto University)
Producer: Computer Network Information Center, Chinese Academy of Sciences