Fluids used for lubrication or power transmission, may contain many different types of contaminants. Some of these contaminants grind & wear at the surfaces of the moving parts, producing even more contaminants.
Contaminants can greatly affect system reliability & accuracy. Control of contamination is especially important in hydraulic & control systems. It is estimated that more than 70% of hydraulic system downtimes are attributable to damage or operational malfunction caused by the presence of contamination.
The level of fluid contamination in the system is the result of multiple factors that interact such as: the rate of contaminant entry, rate of wear debris production, filtration characteristics, system duty cycle, design of reservoir, rate of fluid loss & replacement, contamination tolerance to system components,etc
Common reasons for inadequate contamination control is lack of appreciation of the tremendous impact of contaminants on safe & economic operations, & lack of understanding of contamination control technology (filters, breathers, seals).
Some Sources & Types of System Contamination are:

Built-In Contamination introduced into the system during manufacture, assembly & testing of components (often overlooked). Dust, Dirt, Sand, Paint chips, weld slag, solvents, Fibers, Elastomers, Water,Air, etc.
Ingressed External ingression of atmospheric contamination, condensate water from air, etc.
Induced during operation Contaminants introduced during normal maintenance or system operation. Wear metals, Excessive/used additives (Al,Cr,Cu,Fe, Pb,Sn,Si, Na,Zn,Ba, Ph, etc), Water, Process fluids, Acids & Hydrocarbon by-products of combustion, Products from Filter breakdown, Sludges, Oil-oxidation products, microbial,etc.
New Fluid New fluid cleanliness levels may not meet system requirements ( especially critical hydraulic system). Typical new oils are class 22/18/16 & sometimes worse.
Elastomers & Seals Degradation of seals, gaskets, hoses, tapes, pastes, etc. Rubbers, Fibers, metals, ceramics,etc.

Failure to thoroughly clean piping, sumps etc afteroverhauling, repairing or replacing components.

Variety of contaminants can be left behind.