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The Science of Sweat

I sweat. After a particularly long workout, I tend to sweat a lot. Now I know that's a normal reaction to exercise, but I never stopped to think about why it happens.

Out of curiosity one day, I decided to take my temperature after a particularly strenuous routine. I was a bit surprised when I read the thermometer and it registered 102 degrees!

To put that in perspective, normal body temperature is 98.6 degrees. I was more than three degrees above normal and technically I had a fever. I wasn't ill, so why the temperature spike? I soon discovered that it was nothing to be alarmed about. It turns out; my high temperature was a completely normal reaction to any kind of physical exertion.

When you exercise, your heart beats faster and pumps more oxygen carrying blood to your muscles. As your muscles work and heat the blood up, your sweat glands step in and take water from the blood in your capillaries and pump it onto your skin. When the water evaporates it cools everything down. (In very dry climates, air conditioning systems known as "swamp coolers" use this same mechanism to cool down buildings.)

Serious competition can cause athletes temperatures to go as high as 105 degrees without causing harm. So the higher temperature I had wasn't a problem, it was actually normal. However, something else was still bothering me.

Whenever I finish a workout, for the first few minutes it always seems like I sweat more than at any time during my exercise program. My heartbeat returns to normal, but that just seems to kick the sweating into high gear.

I discovered the cause and it's really quite simple. After you exercise your muscles still have some heat built up in them. When your heartbeat returns to normal, it pumps less blood through your body. With a reduction in the cooling effect of blood circulation, the heat from your muscles cause the blood temperature to temporarily rise even higher. Then the sweat glands have to work harder to lower it and more sweat is the result. Once the heat from exercised muscles is dissipated, sweating returns to normal.

Since I was digging so deep, I decided to find out why sweat smells too.

There are two types of sweat glands. Eccrine glands are the most common. They're found throughout your body including your hands, the soles of your feet, your forehead and palms, and they open directly onto the surface of your skin.

The other type is aprocrine glands, which develop in areas with larger amounts of hair follicles such as your armpits, genitals and scalp. Sweat that comes from the aprocrine glands also contain some fatty acids and proteins, and it tends to have a yellowish color. If you've ever had a white t-shirt stained yellow under the armpits, now you know the reason. But back to the smell.

Sweat starts out practically odorless, then when it comes in contact with bacteria on your skin, it starts to decompose and the smell begins. Aprocrine sweat is typically the smellier one, because of the odor the fatty acids give off when bacteria break it down.


The Smell of Sweat The smell your sweat gives off can actually be used to spot medical conditions.

Ammonia smelling sweat can be an indicator of liver or kidney disease. A fruity smell may be a sign of diabetes. There's even a rare condition called fish-odor syndrome (trimethylaminuria) where sweat smells like rotting fish. People with that condition have a defective gene that prevents them from metabolizing TMA (trimethylamine).


You can reduce sweat with over the counter antiperspirants used on the underarms, hands and soles of your feet. Antiperspirants block sweat ducts with aluminum-based compounds.

The way they work is the aluminum ions are drawn into the cells and water passes in with the ions. As the cells start to swell from the water, they squeeze the sweat ducts closed.

But the sweat ducts don't stay swollen closed permanently. Eventually the cells become saturated and they can't take in any more water. Once the concentrations of water in and out of the cells reaches equilibrium, the water starts to move back out of the cells through a process called osmosis, and the cell's swelling goes back down. When the cells return to normal size, the sweat ducts open up again and it's time to reapply antiperspirant.

Of course that downside of blocking sweat ducts for some people is irritated, swollen and itchy skin. If that's the case with you, consider deodorants. Deodorants don't stop the sweat, they simply turn your skin acidic making it less attractive to bacteria, cutting down on the smell.

Now that you know all about it, it's time for you to go out and break a sweat!

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4/22/2007