Cooling off... Naturally!

Unlike us, chocolate rabbits can only melt when heated.  Photo by Pi Consti

Unlike us, chocolate rabbits can only melt when heated.

Photo by Pi Consti

Summer hasn’t officially begun but the heat is already here to stay in Connecticut. When asked “How do you keep cool in the hot weather?” most people might say “Air conditioning!” However, this answer ignores an even more important source of cooling: Your own body!

Humans are warm-blooded, or endothermic. This means that we have a stable body temperature independent of the environment that we’re in. Internally, our heat is mostly produced by chemical reactions involved in metabolic pathways or muscular movement. The human body works best at around its average temperature: 98.7oF. If it goes much higher, there’s the risk for illness, or even death! Fortunately, evolution has gifted humans with a variety of methods to keep us chill, even when the AC is turned off.

Blood flow

Managing blood flow is one of the ways your body cools itself. When your internal temperature starts to rise, signals are sent to the muscles surrounding small arteries, called arterioles. These thin muscle layers help arteries keep their shape. When they loosen, they increase blood flow and redirect blood to the tiny capillaries of the skin. Heat transfers from blood to skin to air, helping you cool off. The increase of blood flowing in the skin is also why many people get red faces when exercising or hot!

Unfortunately, this method isn’t perfect. When the air is close to or warmer than human body temperature you won’t be able to lose much heat this way. When temperatures start to climb, it’s time to get sweaty!


Sweating is the most effective form of heat loss for humans. It works so well because of the principles of evaporation. In a fluid (and other types of matter), molecules are always in motion. The warmer something is, the faster the component particles move around. Evaporation occurs when water molecules gain enough energy to escape from the fluid and turn into vapor. They take their energy and heat with them, leaving the remaining water slightly cooler.

Photo by Minghong

Photo by Minghong

When a lot of evaporation occurs at once the cooling effect can be pretty impressive. Evaporative cooling is why you feel more comfortable when you dump water over your shoulders on a hot day, and it’s why your body produces sweat.

Humans have an average of 2 – 4 million sweat glands. Different people sweat different amounts, depending on age, gender, fitness, and genetics. Fit people actually start sweating faster than unfit people, because their body is more closely in tune with its heat regulation needs.

Sweating works best in dry climates, since water evaporates most effectively when there isn’t much vapor already in the air.  In very humid environments with less evaporation, cooling the body down becomes a lot harder. Clothing that absorbs sweat and doesn’t absorb much sunlight, like cotton, is best to wear under these conditions.

An average person sweats between 0.7 to 1.5 liters per hour when working out. Since the average person only has 4.7 to 5.5 liters of blood, it’s very important to drink water when you get sweating!

How other animals do it

Both horse and rider are probably sweating here. Horses are one of the few animals that sweat over their entire bodies like humans do.  Photo by Goki

Both horse and rider are probably sweating here. Horses are one of the few animals that sweat over their entire bodies like humans do.

Photo by Goki

It’s fairly unusual for animals to use sweat as their primary means of cooling. Rather than sweating, many mammals pant when overheated. Panting involves taking short quick breaths. This increases evaporation in the lungs, letting animals expel heat. Birds can pant too and use gular fluttering to increase its effectiveness. They vibrate the muscles in their upper throat, increasing blood flow and evaporation in the throat and mouth.

Animals living in hot climates often have specific anatomy adaptations to deal with the heat. Camels decrease the insulating effects of fat by concentrating their fat reserves in their hump (humps don’t actually store water!). Long limbs, ears, or other extremities are also common for animals in hot climates. When a hare or an elephant get hot they can direct blood through their big ears. The large surface area of the ears makes them great for radiating heat away from the body. Elephants can even flap their ears to create a cooling breeze!

If anatomy and physiology doesn’t do the trick, animals can avoid overheating by just avoiding the heat altogether. Many desert animals stay underground during the hot day and only venture out during the cooler and more hospitable night. For animals that need to cool off but are awake during the day, a quick dip in the water might help instead! This is a method that works great for humans too, in case you find yourself getting tired of the sweaty life.

-  Kate Dzikiewicz, Paul Griswold Howes Fellow