We cannot overstress the meaning of clean when referring to clean film and bags. In this case clean is not dirt as we know it, but particulate contamination. Any foreign substance, including bits of the bag or film itself, are considered particulate contamination. In order for the reader to become more aware of the meaning of particulate contamination.First, you must become aware of the term micron when measuring size. One thousandth of one inch (.001 or 1 mil) equals twenty five microns or .0000394 inches. Measurements of contamination usually base their measurements using particles measuring one half micron (.5 microns) or .0000196 inches.As an example of relative size, a human hair measures approximately 40 microns in diameter while dead skin particles that shed from a human body can measure anywhere from 1 to 1, 000 microns. Particles of dust are about 1 micron with bacteria any where from 0.3 to 10 microns. Now remember, our cleanliness levels are based on of a micron.Federal Standard 209E basically classifies a number units of cleanliest levels which are as follows: