There are two criteria for a substance to be considered a contaminant in relation to a data center environment:
It must be potentially damaging to hardware or people.
It must be able to migrate to areas where it can cause damage.
Contaminants that affect people and equipment are typically airborne, so, obviously, it is important to limit the amount of potential contaminants that cycle through the data center air supply to prolong the life of all electronic devices. Potential contaminants can also be settled, making them harder to measure. Care must be taken that these aren't agitated by people or mechanical processes.
Measures should be taken to prevent air contaminants such as metal particles, atmospheric dust, solvent ...