The hygroscopicity of fiber usually refers to the ability of fiber material to absorb water from gaseous environment. The adsorption of water vapor (water vapor is a general term for water molecules and tiny water droplets (< 1 micron)) is essentially a dynamic process, that is, the fiber continuously absorbs water vapor and releases water vapor at the same time. If the water vapor is absorbed mainly, it is a moisture absorption process, and vice versa, the two processes will eventually reach a balance, but there are still differences.