What is infrared and how does it work?

Infrared waves are part of the invisible electromagnetic spectrum (EM). The EM spectrum is measured in microns. Infrared is often subdivided into "Near IR" (0.076 to 1.5 microns), "Mid IR" (1.5 to 5.6 microns), and "Far IR" (5.6 to 1000 microns)
 
Human bodies send and receive FIR waves. The range of far-infrared waves generated by our bodies is 6 to 20 microns. The optimal micron output range is between 7 to 14 microns. This range, sometimes called the "vital range," appears to have special regenerative effects on our bodies.
 
When far-infrared rays are absorbed by living things (e.g. people in a IR sauna) the temperature of the body rises but  not the surrounding air. Thus, you sweat and feel hot from the absorption of the IR rays, but the surrounding air remains at relatively low temperates (typically between 115 and 135 degrees F).