During air or any other gas compression, many undesirable pollutant passes into compressed gas from atmosphere or compressor. These are water, oil, dirt and solid particles, microorganisms, rust and pipescale. Of course there are oil-free compressors, which do not load air to be compressed with additional pollutants, but they are unable to prevent entering pollutant to compressed air from atmosphere. Their disadvantage is much higher expenditure vs. oil lubricated compressors.
Manufacturers of compressed air consumming instruments declare required quality of used air. These quality classes are determined by ISO8573-1:2010 standard. Moisture reduction can achieved by refrigeration dryer or adsorption dryer, while solid particle and oil content entering can prevented with coalescent filters. There are sterile filters which avoid microorganism penetration into compressed air.