Heat Transfer: Why Temperature is Not the Whole Story
Thermodynamics: The Science of Heat
Heat therapy involves transferring sufficient heat to your body to raise the temperature of your tissues and core. This temperature increase activates heat shock proteins and strengthens your cells and tissues. I want to teach you how to reap these benefits, but I don't just want to prescribe a protocol—I want you to understand the concepts. Facts memorized are easily forgotten, but concepts understood will stick with you, enabling you to adapt and apply your knowledge far beyond the basics. So, let's start with some fundamentals.
Temperature Is Not the Whole Story
Here's something that might surprise you: temperature isn't always just... temperature.
Sauna Myth: One of the most persistent sauna myths involves the air temperature of infrared saunas. You might have heard that infrared saunas must reach a specific air temperature—commonly 170°F—to be effective.
I get it. We humans love numbers and quantifiable goals—clear targets we can aim for. But what if I told you that you receive more heat faster in an infrared sauna at 130°F than in a traditional sauna at 170°F? And that while a traditional sauna is appropriately used at 170°F, an infrared sauna at this air temperature would deliver a potentially dangerous amount of heat?
Think about this: Anyone who has used saunas, or survived a summer in the South knows what 110°F air feels like— lying in the shade or in a sauna it is pleasantly warm, maybe inducing a light sweat. But water at 110°F? You'd pull your hand back in pain.
"Room temperature," where we feel neither hot nor cold, hovers around 70°F. But if you've ever checked a "cold" swimming pool's temperature, you know that 70°F water feels unpleasantly cold even on a scorching day. This is the pool day where moms watch from the side as kids insist "it's not cold" through chattering teeth and blue lips. Swimming pools are usually maintained between the high 70s and low 80s, and even water in the 80s feels refreshingly cool rather than warm.
Why such a difference in our perception? The difference is that water is a much better heat conductor than air—it transfers heat to us, and takes heat away from us, much more efficiently than air. In fact, air is one of the worst heat conductors, which is precisely why it makes the best insulator. When you pay a premium for that down or PrimaLoft® ski jacket, you're essentially paying for air. Those materials trap air better than any others, preventing heat transfer from you to the environment.
Your perception of heat depends on the rate at which heat is transferred to you. Temperature is just a small part of the story. This is crucial in understanding why air temperature in an infrared sauna doesn't accurately measure the amount of heat you're receiving. Infrared doesn't heat you through the air—it heats you directly, with light.
Methods of Heat Transfer
There are three primary methods of heat transfer:
Conduction – A hot object transfers heat by direct contact with another object. Think of touching a hot pan.
Convection – Heat transferred by hot air or water. Traditional saunas primarily rely on this method.
Radiation – Heat transfer by light. The energy of a photon (light wave) is absorbed and converted to heat. This is how infrared saunas work.
Don't be alarmed by the word "radiation." Radiant heat is not ionizing radiation. Ionizing radiation, like X-rays and gamma rays, refers to the shortest wavelengths, past blue and UV light on the electromagnetic spectrum. That type can cause damage and should be limited. Radiant heat, however, is heating by light waves, primarily infrared, which falls on the opposite side of the visual spectrum from UV, past red, as the wavelengths get longer and safer. It's what you feel when you experience the warmth from the sun on your skin on a cold day.
Infrared
Infrared heats us through a process called resonant absorption. The photons are absorbed by chromophores in the skin and converted to heat. The primary chromophores in human skin are melanin, hemoglobin, and water. Different wavelengths are preferentially absorbed by different chromophores. When you get to the long wavelengths of far infrared, they're almost entirely absorbed by water molecules. And guess what we're made of? About 60% water by weight, on average. Water fills our cells and bathes our tissues, forming a fluid medium that allows cells to move and communicate. When far infrared is absorbed, nearly 100% of its energy is converted to heat. Heat and water—it's that pure. Not surprisingly, there are virtually no significant risks or side effects. It really couldn't be more ideal for a sauna.
