The air resistance of paper may be used as an indirect indicator of Z-directional fluid permeance, as well as other
variables such as: degree of beating, absorbency (penetration of oil, water, etc.), apparent specific gravity, and filtering
efficiency for liquids or gases. Air resistance is influenced by the internal structure and also the surface finish of the
paper. Internal structure is controlled largely by the type and length of fibers, degree of hydration, orientation, and
compaction of the fibers; as well as the type and amount of fillers and sizing. The measurement of air resistance is a
useful control test for machine production; but due to the number and complexity of factors outlined above, careful
judgment should be used in the specification limits for air resistance