Infrared sauna and health: Part 2. Human body reaction to heat

Our environment rapidly changes when we enter into an infrared sauna. From our normal indoor surroundings with temperature around 70 – 85 F (20 – 30 C) we step into a room with air heated up to 110 – 130 F (45 – 60 C). Not only the temperature is higher, but also many infrared heaters radiate infrared waves at our body, heating us up. A healthy organism has to respond to such change. But how it will respond?

There is such term as homeostasis, an ability to sustain a stable state in a changing environment. One example of homeostasis is an ability of our body to keep a constant internal temperature when external temperature varies. Normal temperature of a human body is 98.6 F (37 C). That means that our body tries to maintain this temperature even if outside temperature is –20 F or +100 F.

This trait of a human organism is very important, because even slight changes in the temperature of our body results in unpleasant and even dangerous effects. If our temperature rises or falls in a range of +/- 5.4 F (3 C) then we feel bad and ill. If change in temperature is greater it can be fatal for our health. So a human body will try to keep its temperature constant when placed in a hot sauna. To explain what will happen I need to tell first about heat balance of a human body and how it is achieved.

Human body energy balance

Even when resting or sleeping our body still produces heat as a result of metabolic processes which occur in our organism. Exercises add more to the heat production rate, because working muscles also produce heat. Amount of heat energy produced by a human body right after sleep when resting and 12 hours since last meal is called basal metabolic rate (MBR). It is about 80 – 90 W (a power of a typical light bulb). When we are more active our body can produce times more heat.

A human body must release heat to the environment or its temperature will gradually rise. It can be done via three ways of heat transfer: conduction, convection and radiation. In fact, all objects exchange heat with these mechanisms, not only a human body. Conduction is when the heat is transferred from an object with higher temperature to an object with lower temperature when they physically contact. Imagine, when you touch a metal spoon you heat it up with your body heat. In a similar way, when air around you has temperature lower than yours you lose heat into air. Air is constantly moving, so heated air flows away and cooler air is arriving. This is convection. And as molecules in all objects vibrate they radiate electromagnetic waves. A hot object radiate more energy than a cold one, therefore it loses more heat. This is radiation.

Besides these three methods of heat transfer, our body utilizes one more – loss of energy via evaporation of sweat from our skin. We lose heat because our body spends its heat to make a liquid sweat to transfer into gaseous form. If you place a drop of water on a very hot surface it will evaporate quickly, but surface temperature will decrease slightly. Our body cools itself evaporating sweat in a similar way.

Simultaneously, when our body transfers heat to surrounding objects, surrounding objects transfer heat to our body. This fact leads to this conclusion: to maintain constant temperature a human body must release more heat than it receives from the environment. The amount of heat that should be released is equal to the heat energy produced inside our body due to metabolic processes. As the environment thermal conditions change and amount of energy produced changes a human body has some means to adapt to these changes, regulating the degree of heat loss.

How body regulates heat loss

Body temperature can be divided into two kinds. First kind is a core temperature of the internal parts and organs of our body. Other temperature is a shell temperature, the temperature of a skin and underlying tissues. Why is it important to distinguish between these temperatures? Because the amount of heat our body loses via conduction, convection and radiation is determined primarily by our shell temperature, the temperature of our skin. The core temperature in a healthy human varies much less than shell temperature.

Another thing that is important is that heat must be somehow transferred from inner part of a body to skin to keep the core temperature unchanged. This transfer is done via our body blood flow. When body flows through internal organs, its temperature increases slightly, when it passes under the skin, it releases some heat into the environment. This way, bit by bit, our blood regulates our temperature.

Our body is exciting in terms of choosing the most simple and effective mechanisms to solve different problems. Thermoregulation is one more example of this. Imagine – a human body suddenly finds itself in a cold environment. Heat loss becomes too much and there is a danger of loosing temperature. What is the most simple and effective way to decrease this loss? The answer is to decrease blood flow through the skin. This is done by the process which is called vasoconstriction, a constriction of blood vessels (capillaries). When this happens, less blood flows on a body surface and less heat is dissipated into the cold environment.

The reverse process, vasodilatation, expansion of blood vessels, takes places when our body is in a hot environment. In these circumstances, blood flow through the skin surface increases and heat loss increases too. At thermoneutral conditions, blood flow through the skin is about 200-500 ml/min. In extreme cases, blood flow through the skin can reach as much as 5 liters per minute. Such intensive blood flow is a serious exercise for a cardiovascular system.

There exist other mechanisms for temperature regulation. In a cold environment, our body tries to increase amount of energy produced by shivering (when we shiver, our muscles contract and produce heat as a side effect). In a hot environment, a body begins to relax muscles and reduces the intensity of metabolic processes to decrease the heat output.

As outside temperature increases less and less heat can be lost via conduction, convection and radiation. To help this situation, our body uses sweating to lose heat even when external temperature is greater than core temperature of a body.

So, what happens to human body in infrared sauna?

Temperature in an infrared sauna is actually higher than a human body temperature and infrared heaters radiate more heat on our body than it radiates itself. And body suddenly begins to gain heat, instead of losing it. As a result body temperature begins to raise. This change is detected by thermoreceptors in a body and organism activates it’s mechanisms to increase heat loss.

As I described above, vasodilatation begins and more blood begins to flow through the skin. Heart rate can rise to pump such big amount of blood through the skin surface. But this don’t work, as outside temperature is still higher and body continues to gain heat. To lose heat the body begins to sweat. It helps, but to lose enough heat our body has to sweat very intensively.

High flow of blood through the skin means that less blood flows though other parts of the body, for example brain, muscles, stomach. That’s why it is not recommend to eat right before the sauna – high blood flow through the skin can interfere with digesting process. Also, a body tries to slow down metabolic processes and to relax muscles to produce less heat which can result in sleepiness and relaxation.

How infrared rays affect human body?

From what I read and what I know I don’t see that infrared can affect us in some special way other than heating us. I plan to address claims about “resonant absorption” and other effects of infrared on our health in later posts. Infrared radiation is just another way to transfer heat, and there is no evidence that it can do something more to us. Indeed, we are affected by infrared through all our live, as all objects radiate infrared waves to a certain degree. However, infrared heaters give us much more infrared than in our usual life and are very effective way of heating. Because of that we can keep air temperature of an infrared sauna less than in a traditional sauna and stay in a sauna longer.

In this part of the Infrared sauna and health series I described how our body reacts to the infrared sauna environment. In the third part of this series I will write about the connection between the reaction of our body to infrared sauna environment and health effects that an infrared sauna provides.

If you want to explore the field of human body thermoregulation yourself, here is some links:

I found articles above very interesting and good to understand thermoregulation processes which occur in our body.

Read other parts of Infrared sauna and health series: Part 1, Part 2, Part 3, Part 4.

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One Response to “Infrared sauna and health: Part 2. Human body reaction to heat”

  1. Ora-Jean Perry Says:

    Your article is three years old, so you may not even have this website anymore, but I am writing to you anyway. I bought a six-person far-infrared sauna because of my size. I have peripheral lymphedema (whole body fluid retention) in the advanced stage. I used the sauna four times, but instead of sweating from the knees down I blistered terribly. I have had open wounds from the blisters for over a year that will not heal. I have been hospitalized and almost died from infections in these wounds. I believe the sauna would have been great during the initial stage of lymphedema, but not in the advanced stage because the skin is too thick to sweat. I have the sauna up for sale for $2,500 (a new one this size is almost $6,000. It’s American-made). So far no takers. I haven’t turned it on in over a year and today it was disconnected from the 220 wiring. It’s no use to me. I’d be afraid to ever try it again. I wanted to raise my body temperature because it gets as low as 95.5o. The ads say it is great for people with fluid retention, but it isn’t.