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How does infrared radiation make your skin feel hotter?

The skin absorbs infrared radiation by causing various molecules making up the surface of your skin vibrate more rapidly. This increase in the kinetic energy (the energy of motion) nerves sense as "heat". The details of the process are pretty complicated but that is the basic mechanism.

Vince Calder


All radiation, including visible light and radio waves, carries energy. When radiation meets a material, such as skin, it can be absorbed, reflected, or it can pass through. Infrared radiation passes through the cells on your retina. This is why you cannot see it. Radio waves can pass through your entire body. Most visible light is reflected from a piece of white paper. Infrared radiation is ABSORBED by your skin.

When the radiation is absorbed, the energy enters your skin. This makes the molecules in your skin shake around more than usual. This is what a high temperature is. Another example of this effect is when you stand near a vent while the furnace is running. The vibrating molecules in the warm air cause your skin molecules to vibrate. This too makes your skin feel hotter. Absorbing energy can often cause temperature to increase.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

Your skin might be considered as having about 3 layers: inner, middle, and outer. Each has a successively cooler temperature, due to body heat flowing outwards. As a way of measuring the magnitude of this outbound heat flow, the nerves in your skin very sensitively measure temperature difference between middle and inner layers. (The outer layer is too dead, tough, and dangerous to be worth investing with nerves.)

When infrared light shines on your skin, some is absorbed in the middle layer, making it slightly warmer than it would have been, or at least subtracting a little from the usual outwards heatflow.

Deeper in the body, starting with the inner layer, blood flow carries heat around more vigorously. If positioned any deeper, this nerves-measuring-temperature-gradients business would not work too well.

I wonder if it is possible for some heat-sources, some wavelengths of infrared, to warm up the inner layer more than the middle layer, resulting in a feeling of cool instead of warm? It would probably have to be temporary to work, a brief pulse of radiation.

Jim Swenson

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