Oil and lubricants are major contaminants in compressed air systems. Oil and lubricants are most troublesome when they combine with moisture and solid contaminants. This forms a sludge that can clog instruments and tools. The best defense against contaminants in your air lines is filtering out any solids, moisture, and lubricants present in the air. Typical city air can contain more than four million dust particles per cubic foot of air.
Moisture is present in intake air as humidity.
During the compression process air gets heated, then cooled as it's distributed. This causes condensation of water vapor. Condensed moisture can cause rust in pipelines or will freeze in exposed lines outdoors. In general, particle size, desired efficiency, allowable pressure drop, and element life are the primary considerations in filter design and selection.