"Dust particles"? Where did that information come from? Transmissions are multi machine-element devices. They contain bearings, gears, clutches, hydraulics, and torque converters. The only thing they don't have is chemical reactions (combustion).
Therefore, from a contamination standpoint, the oil is subject to clutch wear shavings, gear wear shavings, thermal oxidation products, and moisture from the atmospheric breather vent. This is why even the highest quality transmission oil needs to be changed at regular intervals, including the filter(s).
It's also why I run an API Group III synthetic instead of a Group IV or V. I'm not striving for extended oil change intervals due to the contaminant loading. And - importantly for Ram owners - deleting the trans oil cooler thermostat brings oil temp down to normal hot running temps. Don't let Ram $hit you that 160-200F trans oil temp is "normal". It certainly wasn't with the original system design, and the fact they f'd up the thermostat addition doesn't make it "normal". Just like the Hemi valve lifter ticking isn't "normal". These are both design DEFECTS the corporation deflects.