The membrane permeation flux is defined as the volume flowing through the membrane per unit area per unit time. The SI unit used is m3/m2.s although other are often used as well.
For the case of transport of gases and vapors, the volume is strongly dependent on pressure and temperature. As such, gas fluxes are often given in terms of a "standard condition" which is defined as 0 oC and 1 atmosphere (1.0013 bar).