Functional underwear, consisting of an undershirt and underpants, forms the first layer of clothing for a wide range of sports activities. Functional undershirts cling to the body like a second skin and regulate the temperature balance. They absorb sweat from the skin, distribute it over a large area and transport it to the next layer of clothing. Thanks to this temperature regulation, the so-called "first-layer shirts" cool during sporting activity and warm the body when at rest. Manufacturers such as Odlo, X-Bionic, Icebreaker or UYN rely on synthetic or natural fibers as well as combinations of both for the textile materials. The synthetic fibers often consist of materials such as polyamide, polyester, elastane or polypropylene. Among the natural fibers, merino wool is a popular product because it has an odor-neutralizing effect and can absorb enormous masses of moisture.