ABSTRACT
Percentage porosity is the percentage of fabric surface area not occupied by yarn
or fiber. These are areas referred to as fabric interstices. UV rays pass directly
through a fabric interstice, striking the skin under the fabric without ever striking
a fiber, which might deter its passage. l Percentage cover is the most important factor, as it determines how much of
incident UV will strike a fiber as the ray moves from the face of the fabric to the
skin surface under the fabric. l Any fabric that has a percentage cover of 94% or less will have a UPF less than
15, the lowest value permitted for a fabric with a claim of UV protection. This
statement holds true no matter what the fiber content of the fabric is and what
chemicals have been applied. l Other fabric features that influence how photoprotective a fabric is are yarn
openness and fabric thickness, as these structural features determine the path of
scattered radiation from the fabric face to the fabric back. l Chemicals in fabrics that can alter UV rays that strike them include the fiber
polymers, pigments, dyes, optical whitening agents, and specially designed UV-
absorbing compounds (often called UV-cutting agents). Specific chemicals within
each of these classifications will have different interactions with solar UV radiation.
