Untreated compressed air always contains contaminants because of the nature of the gas and how it is produced. The need for air treatment basically results from 3 characteristics of compressed air.
Liquid water - water aerosols - water vapor
As water is incompressible, the amount of moisture per m³ increases when air is compressed. The maximum amount of moisture per m³ air is however limited for a certain temperature. Condensation will thus be formed when air is compressed.
Compressed air will always contain some level of moisture depending on the environment and ambient temperature surrounding the compressor. Using the air while it contains moisture can cause a number of issues for your tools, other equipment and ultimately your business.