Particles- Rust, dirt, and metal shavings are just a few examples of possible particle contamination.
Water- Length and condition of distribution piping, drops, dead-ends, condensate drain traps, and the use of polymer tubing can contribute to excess moisture.
Oil- Even oil-free compressors can be contaminated with oil vapor (gaseous hydrocarbons) from intake air.
Microorganisms- An improperly maintained compressor may provide the conditions (water + oil + warm temperature) needed for microbial growth carbons) from intake air.