Keeping Cool: The Science of Sweat
While sweating, or perspiration, can be embarrassing, it is an important body function. Sweating, and a lack of sweating, can also be a helpful sign that there is something wrong with the body.
Sweating is the release of a salty liquid from the sweat glands. The liquid has one main purpose: as it evaporates, it helps to cool the body. Sweating is regulated by the autonomic, or sympathetic, nervous system. Signals, using the neurotransmitter acetylcholine, are sent to the sweat glands. The sweat is then released to the skin surface through ducts.
A person has 2 million to 4 million sweat glands, with the highest density of sweat glands on the palms of hands and soles of feet, according to the National Library of Medicine.
“From baby to adult, the number of sweat glands does not change. Therefore, babies have the highest number of sweat glands per square inch, i.e. 8 to 10-fold higher than adults,” said Dr. Eugene Bauer, chief medical officer at Dermira and former dean of Stanford University’s School of Medicine.
Sweat is odorless. Bacteria on the skin mixing with sweat is what produces body odor. Most sweat is colorless, too. Both odor and sweat stains are caused by the apocrine sweat glands.
“Yellow underarm stains are caused by your apocrine glands, which contain proteins and fatty acids and thus make underarm secretions thick and milky,” said Dr. Niket Sonpal of Touro College of Osteopathic Medicine in New York.
The secretions also attract bacteria from the skin, which breaks down the secretion and creates the bad smell.
It’s not unusual for some people to only perspire a little and for others to sweat a lot. “The normal range for sweating is very wide,” said Dr. Robert Sallis, co-director of the sports medicine fellowship at Kaiser Permanente Fontana Medical Center. “Some people might only sweat half a liter during an hour of intensive activity, while some might sweat 3 or 4 liters, and both are still within a normal range.”
A person’s size and gender can play a role in how they sweat and where they sweat. People who are out of shape, for example, tend to sweat in a central pattern, such as at the center of their chest or back. Those who are more in shape tend to sweat more evenly across their body. Also, women typically sweat less than men. “People used to think women were more likely to suffer heat stroke and therefore incapable of running marathons,” explained Sallis. “The truth is, women have fewer sweat glands, but they also have less muscle mass so they produce less heat (and therefore need to sweat less).”
Sweat loss vs. water intake
A person’s sweat rate is the amount of sweat lost during an activity. To prevent dehydration, marathoners, triathletes and other athletes find their sweat rate so they know how much water they need to drink during an activity.
Here is how to measure someone’s sweat rate:
- Weigh the body, nude, on a set of scales.
- Do an intensive physical activity for an hour, like running, ideally in the condition in which they will compete (hills, humidity, etc.).
- Weigh while nude again.
The amount of weight lost is the sweat rate. One pound (0.45 kilograms) of weight loss equals 16 ounces (0.47 liters) of sweat loss. “This rate can vary somewhat based on the intensity of the activity, humidity, blood sugar rate and other factors,” said Sallis. “Once this rate is established, it should be used as the amount of water the athlete needs to drink in that hour of activity, incrementally.”
Lack of sweat when a person is hot could be a symptom of many medical disorders. “There are a couple of things that come to mind when I hear that someone is hot, but not sweating,” Dr. Neha Pathak, WebMD’s medical editor, told Live Science. “The most concerning, depending on the rest of the story, is heat stroke.”
Heat stroke happens after prolonged exposure to high temperatures, explained Pathak. For younger people, it generally occurs after prolonged exercise or outdoor activity, without adequate hydration. For older people, it can happen without exertion because of underlying medical conditions, medications or other factors like physical disability. Hot and humid weather makes cooling off through sweat evaporation ineffective. The longer the exposure, the more dehydrated a person becomes. This can eventually lead to a body temperature above 104 F (40 C), along with feeling faint, dizzy, nauseous, and confused. “You can feel very hot without any sweating. Without treatment, there can be serious complications from heat stroke,” said Pathak.
There are many other reasons why someone may not sweat. For instance, there could be nerve damage. Conditions that can lead to this type of nerve damage include diabetes, alcoholism, Parkinson’s disease or direct damage to the skin, such as after trauma, said Bauer.
Sometimes skin diseases, such as psoriasis or heat rash, can interfere with normal sweat gland functioning, as well. Conditions called hypohidrotic ectodermal dysplasia and hypohidrosis, also known as anhidrosis, can also cause the sweat glands to malfunction.
Menopause can lead to the sensation of feeling hot or having episodes of “hot flashes” without associated sweating, as well, said Pathak.
A person who is not hot, but is still sweating, may have a problem with their sympathetic nervous system. “Many times, this excessive sweating is associated with stimuli, e.g. eating spicy foods or nervousness, but often there is no specific reason why this happens,” said Bauer.
Hyperhidrosis is a skin condition where the body sweats more than what is required to keep cool because of overactive sweat glands. It can affect the underarms (axillary hyperhidrosis), palms of the hands, soles of the feet and face, according to the Mayo Clinic.
If the heavy sweating is accompanied by chest pain, lightheadedness, chills, nausea or a body temperature of 104 F (40 C) or higher, seek immediate medical assistance.
“Researchers are looking into harvesting energy from human sweat,” said Pathak. “They have shown that they can use the electrochemical properties of sweat to power wearable biomedical devices, like heart rate and blood pressure monitors. These techniques are being used to develop power sources for wearable electronic devices. Their work shows that human sweat can be used as a biofuel!”
- National Library of Medicine: Biomechanical Energy Harvesting from Human Motion
- Mayo Clinic: In-depth, Excessive Sweating
- Prevention: 9 Things Your Sweat Is Trying to Tell You