Filtration efficiency of a fabric is given by its fiber density (or the spacing in between the fibers). The higher the density of fibres (tighter spacing in between the fibers), the higher the filtration efficiency. Permeability of a fabric depends on the thickness of its fibers – the thicker the fibers, the worse the permeability. Figure below illustrates the structure of cotton (left) compared to non-woven fabric (right). Cotton fiber thickness is cca 3.5 μm, whereas non-woven fiber thickness is <0,3 μm. A fabric composed of thin fibers with high density provides high filtration efficiency and high permeability at the same time.

Another parameter of fabric permeability is its ability to absorb liquids. Cotton is hydrophilic, i.e. absorbs liquids, which inhibits its permeability. Contrary, the fibers of non-woven fabric are hydrophobic, meaning they repel liquids, thus permeability is better.