Does rubber dust become magnetic? My question is: if a rubber belt sustains wear, is it possible for the associated dust to become magnetic?  A bit of background here: my workplace was recently contaminated by a fine layer of black dust. It is suspected that this black dust originated from the air supply. The unit that provides air to the building is driven by five rubber v-belts (Gates 5V1500).  The unit blower and shaft are out of alignment and slight friction on the belts has resulted in degradation with every revolution, distributing fine black dust in the building.  Now, the belts do not contain metal, only mainly rubber.  The dust is highly magnetic. The belts are not.  Why is this? Thanks.
 A: V-belts used to drive fans generally do not have steel reinforcement but instead use polyester or cotton. If the v-belt pulleys were made of steel, abrasion from the misalignment of the belts would have eroded the belt grooves and thrown dust from them into the air stream. This process would be greatly accelerated if there were any grit or abrasive dust in the air (are you perhaps in phoenix or tucson?). alternatively, the misalignment of the belt run may have brought the belts into contact with part of the fan housing or motor mount and rubbed iron particles off of them.  
it is also possible that the belt misalignment applied an axial thrust load to the fan shaft that its bearings were not intended to withstand; this would urge the fan shaft to slide sideways and could have brought the end of the shaft into interference with the housing or another metal part, and thereby ground iron from the shaft into dust. 
Careful inspection of the pulleys, shaft ends and the fan housing will probably indicate whether or not any of these answers are correct. Please let us know what you find!
A: Rubber isn't magnetic by itself.
There are two explanations for what you observe:


*

*If you clearly sure that it is dust from belt: Rubber for belts are not just rubber, it is compound material (can contain Ferromagnetic and paramagnetic particles), and by technical process of forming belt in mould small pieces of metal could be caught by hot rubber.
This particles are small, thats why the whole belt doesn't show its magnetic properties (it actually shows but it is too weak). But weight of dust is small enough that you can see its magnetic properties.

*It could be something from ventilation. For example chacoal, soot particles are magnetic too. Something from street.
p.s. Sorry for my bad english
A: Drive belts may sometimes be reinforced with steel fiber, much like the thread of a steel belted tire. Here is a pertinent passage from the Belt (mechanical) Wikipedia article:

V-belts may be homogeneously rubber or polymer throughout, or there may be fibers embedded in the rubber or polymer for strength and reinforcement. The fibers may be of textile materials such as cotton, polyamide (such as Nylon) or polyester or, for greatest strength, of steel or aramid (such as Twaron or Kevlar).

The belt in question may have been reinforced with steel fiber. Rubber itself is not magnetic.
A: It is possible that any ferromagnetic material(dust or slightly larger particles) in the vicinity of the operating V belts could
become embedded into the v belt,
be further pulverized by the belt beating it against the pulley,
wear the steel pulley,
wear the belt,
and eventually expel the (now)magnetic rubber mixture into the circulating air supply.
The belts could be the cause, or the result, or both.  If you really want to track this down, you need to look for the virgin source of the original magnetic material.
Get a sample of the interior of the belt, not the face, and grind it up and check if it is magnetic.
If yes, you have your answer.  If not, keep looking.  
Start with anything contained inside the housing where the belts run.
Some potential suspects are,
wear from a rubbing guard,
pulverized bearings or races from the misalignment,
dust from worn motor brushes,
dust from braking surfaces,
chips from set screws spun on shafting,
or some completely independent source of magnetic dust and the rubber in the air is just from normal belt wear and it joined the mixture as dust from both sources landed on the same surfaces.
A red herring.
