This article was previously published on January 23, 2012 and was reviewed and updated in October 2017.
If your face turns tomato red while working out, it’s probably a good sign that you are working out hard enough!
Why does this happen? First, let’s think about your body temperature. Humans, like all mammals, are homeothermic animals. This means that our body maintains its temperature within a narrow safe range, even though the temperature outside your body can vary quite a bit.
Did you know? The average body temperature of a human is 37℃, with normal fluctuations between 35.5℃ and 37.7℃ .
Your body temperature remains stable because of a constant balance between heat input into your body and heat output from your body to the atmosphere. It is important to remember that heat input includes not only the external heat that comes from the environment, but also internal heat produced by your body itself. This internal heat is produced from your body's own metabolic processes, and from the heat released during muscle contraction.
Did you know? More than 70% of the energy released during body metabolism chemical reactions is released as heat.
When you exercise, your body creates even more heat. That’s because your metabolic rate also increases.. But at some point, your heat production will exceed heat loss. As soon as this happens, your body temperature rises and can ultimately reach 40oC - 42oC, which we would normally consider as a high fever.
This increase in body temperature triggers two thermoregulatory events that help cool your body off. The first one is sweating, which lowers your body's temperature through what is called evaporative cooling. This is why we all sweat during an extensive workout.
Did you know? Horses and humans are two of the few animals capable of sweating. Other animals, such as cats and dogs, pant rather than sweat.
At the same time, more blood begins to flow to your skin, from where the excess heat can escape to the atmosphere. In order for this to happen, your skin blood vessels dilate (expand) to let more blood circulate through them and carry the excess heat to the surface of your skin. This phenomenon is called vasodilation.
Vasodilation of your blood vessels is what makes your skin red while working out. Because your face has many small capillaries close to the skin, it turns redder than other areas of your body.
Did you know? If you keep exercising regularly, you’ll probably sweat less and turn less red than you did at first. That’s because your routine will no longer cause your body to heat up as much.
The opposite happens when you’re cold. Your blood vessels contract (narrow) so that blood flow to your skin is reduced to conserve body heat. This is called vasoconstriction. If you are too cold, you may also start shivering, which is an involuntary, rapid contraction of your muscles. This extra muscle activity helps to generate body heat.
Your body is very smart! But don’t forget that there are times you should be careful to protect it. For example, exercising vigorously in a hot and humid environment, that can interrupt your normal cooling mechanisms and cause a potentially fatal condition called a heat stroke. So, if you’re exercising and you notice symptoms other than just a red face - for example, dizziness, fatigue or nausea - you might be in danger of getting overheated. Always make sure to drink cool water before, during and after your exercise.
Thermoregulatory: how animals regulate or control their temperature
Evaporative cooling: the process of evaporation causing cooling. In order to evaporate, liquid (like your sweat!) needs energy, and it takes the energy from the surface it is placed on (like your skin!).
Dilate: to expand or to get larger
Vasodilation: the process in which veins, arteries and capillaries become larger to increase surface area. When surface area increases, more heat dispersal can occur.
Contract: to decrease in size
Vasoconstriction: the process in which veins, arteries and capillaries become smaller to decrease surface area. When surface area decreases, less heat can escape.