Breathability is a material property that ensures the transmission of body perspiration as water vapour from the body to the surface of a worn material. A high level of breathability can be achieved using functional underwear in combination with waterproof membranes. Such are especially designed for motion-intensive sports. While water molecules do not penetrate the material from above, water vapour can cross the membrane to the surface from below. Breathability values are indicated as MVP (g/m2/24h), or material resistance as RET (m2.Pa/W). The values are considered indicative.