How does infrared heat actually work?

Everything warmer than absolute zero gives off radiation. Your body does, a radiator does and the sun does. That radiation is electromagnetic, just like visible light and radio waves, and the only real difference between these types is the wavelength. Visible light runs from about 380 to 780 nanometres, from violet at one end to deep red at the other. Anything just a little longer than red we call infrared, which literally means below red.

So infrared is not a separate kind of energy and certainly not a mysterious force. It is the same phenomenon as light, only with waves too long for your eyes to pick up. Your skin does pick them up. There the energy is turned into heat, and that is exactly what you feel when you stand by a wood stove or walk in the sun on a cold day.

The wavelength decides how matter reacts to the radiation. Water absorbs long wavelengths particularly well, and your skin consists largely of water. As a result, far infrared is almost entirely absorbed in the outer layer of the skin. Shorter waves, such as near infrared, are absorbed less strongly by water and so travel a little further before releasing their energy.

That is the core of this whole topic. Every difference between a sauna blanket, an infrared lamp and a red light panel comes down to two questions: which wavelengths does the device give off, and what does your tissue do with them. The rest is packaging.

Internationally, infrared is divided into three bands. That division comes from the standards for light and radiation and is marked with the letters A, B and C. In product texts you more often see the abbreviations NIR for near infrared and FIR for far infrared.

The wavelength a warm surface gives off depends on how hot that surface is. The hotter, the shorter the wave. A filament of well over two thousand degrees peaks around 1200 nanometres and so sits in near infrared. That is why a heat lamp visibly glows orange red: it is so close to the visible range that some light is left over.

The heating surface in a sauna blanket only gets a limited number of degrees warmer than the setting you choose. At that temperature the peak lies somewhere around 8000 to 9000 nanometres, right in far infrared. That is not a design choice or a brand secret but simple physics. Every heat source working at sauna temperature automatically gives off far infrared.

Heat moves in three ways, and it helps to keep them apart.

• Conduction: heat passes through direct contact, like a hot water bottle against your back or the inner layer of a blanket against your arm.
• Convection: moving air or water carries the heat, like a radiator warming the room air or the hot air in a traditional sauna.
• Radiation: the energy travels as waves through space, without anything needing to be in between. That is how the sun warms your face while the outside air stays cold.

A traditional sauna relies mainly on convection. The stove makes the air hot and that hot air hits your skin and your airways. An infrared blanket relies mainly on radiation, topped up with conduction where the material touches you. That one difference explains almost everything people find feels different.

Because the air you breathe does not need to be extremely hot with infrared, many people find it less stuffy than a traditional sauna. Your skin warms up, but your airways take in cooler air. That also explains why an infrared cabin at fifty degrees feels as intense to some people as a sauna at eighty degrees. The thermometer measures the air, not the radiation falling on your skin.

With a blanket, something else plays a part. As soon as you lie in it, you trap a layer of air between you and the material. That layer warms up too and cannot escape. So a blanket is not a pure radiator but a combination of radiation, conduction and a small enclosed warm space. It is exactly that combination that makes you start sweating relatively quickly, even though the temperature is lower than in a sauna.

A consequence few people expect: the set temperature of two devices says little about how warm they feel. A cabin with a lot of distance between you and the panels delivers less radiation to your skin at the same setting than a blanket that lies against you everywhere. Radiation drops quickly as distance increases. How that works out in practice is covered in an infrared sauna blanket and a traditional sauna compared.

This is the topic with the most exaggeration. Online you regularly read that infrared penetrates four to six centimetres deep into the body. That figure has been copied for years without any measurement behind it, and it does not hold up.

What really happens is this. Far infrared is almost entirely absorbed in the first few tenths of a millimetre of your skin. That is the epidermis and the start of the layer below it. Near infrared goes further, but even there you are talking about a few millimetres before most of the energy has been absorbed. No wavelength in this range travels centimetres through tissue without losing almost all its energy along the way.

The fact that the radiation does not go deep does not mean only your skin gets warm. As soon as your skin warms up, two things happen at once. The heat slowly conducts to the tissue below, and the blood vessels in the skin widen so more blood flows past. That blood takes heat with it and spreads it through your body. After a quarter of an hour your core temperature is therefore slightly higher, while the radiation itself never got further than the top layer of your skin.

In other words: your body does the transport, not the radiation. That is a less spectacular story than heat going six centimetres deep, but it is the story that is correct. For your experience it hardly matters anyway, because the warmth you feel is the same.

Wattage is the figure that appears most often in adverts and says the least without context. Watt is power: how much energy a device converts per second. Whether that is a lot or a little depends entirely on how much surface that power is spread over and how well the heat is held in. Three hundred watts over a small cushion is intense. Three hundred watts over a blanket of almost two metres is not.

A blanket that loses its heat to the room has to keep reheating to stay at its setting. That costs power and also causes fluctuations: the element switches on, gets hot, switches off, cools down, switches on again. You feel that as waves of heat instead of an even setting. A blanket with a thicker insulation layer needs to reheat much less, stays more stable and so feels calmer, even with a lower number on the box.

The same goes for the layout of the heating tracks. If there is a lot of space between the tracks, you get strips that are clearly warmer than the rest. Cold spots are the most common complaint with cheap models and have nothing to do with the number of watts. So anyone comparing on wattage is often comparing on the wrong figure.

Finally, the temperature itself. The setting you choose is what the sensor measures, and on almost all models that sensor sits at the heating surface. Your skin never gets as warm as that figure. Two blankets on the same setting can therefore feel noticeably different.

Red light and infrared are often mentioned in the same breath, yet they are two different things. Red light is visible and sits at the end of what your eyes can perceive, around 630 to 660 nanometres. Infrared starts just beyond that and is invisible. A red light panel works with LEDs tuned to a few narrow wavelengths. A sauna blanket works with a heating surface that gives off a broad band of long waves.

The practical difference is big. With red light it is about the light itself and the device stays relatively cool; you sit in front of it with bare skin and feel only a little warmth. With infrared heat it is all about the warmth and there is nothing to see. They are meant for different moments and do not replace each other.

The confusion mainly comes from both types of device being sold under the label infrared. Technically that is half true: the infrared LEDs in a panel sit in the IR-A band, while a blanket works in the IR-C band. They share a word but not much more than that.

Want to know exactly where those nanometres come from? The article on wavelengths in red light and infrared goes into it in more depth. For an overview of what red light therapy involves, see the knowledge base on red light therapy.

Now that you know what the figures mean, you can judge a product page a lot faster. The order below works well in practice and takes you a few minutes.

In the end, the physics behind infrared is not complicated. A warm surface radiates, you absorb that radiation and your body reacts the way it reacts to any heat source. Everything a manufacturer can influence lies on the practical side: the heat distribution, the insulation, how finely you can adjust it and how reliably the safety features work.

Want to go through that practical side step by step? The buying guide for infrared sauna blankets will help you further. Mainly curious about what happens in your body during a session? Read what an infrared sauna blanket does to your body.