A micron is one one-millionth (0.000001) of one meter. Bacteria are typically less than one micron in their largest dimension. Human hair is 60 to 80 microns and the smallest visible object is about 40 microns across. "Sterile" filtrations generally involve depth filtration media (normally a depth filtration sheet and possibly a depth cartridge) followed by an absolute membrane. Depth filtration at sterile micron retentions may be desirable prior to sterile membrane filtration in order to protect the membrane cartridge. The membrane filter then becomes your insurance policy to ensure near absolute exclusion of particles smaller than the membrane rating.