The very dead center screen, before you put on your basket/chamber/valve...
is the very top of the air path.. and yes- dust can easily settle right there.., unless you leave a valve on the unit 24/7.
The filter at the bottom is to prevent dust that may be on a counter from going in and passing the fan. They are protecting the motor and bearings on that fan from dust going upwards beyond the fan etc... is the column of forced hot air.. that is the air path.
Some dust then does in the initial seconds of a subsequent filling can rise, and get stuck to the bags inner walls (due to tackiness).. and will not be seen easily by unaided vision.
A Vapor's "Air Path" refers more appropriately to the tube, channel, or pipe that transfers vapor into our mouth/lungs.
That being said, in the case of the Volcano.. the air path would be a bag made of inert and safe plastic...
and because it itself is a bag of vapor, it is also free of dust.
*interesting note.....
once a bag becomes old (I use mine for about a year), if it goes unused for a time, and then gets back into circulation....said bag will start flaking off tiny glittery chips/shards of dried sublimate, that while flavorful and effective to vape.... are distracting when tiny bits go in the mouth. (Easy cure is to hold a very "loaded" EZ Valve bag upwards toward sky while hitting it)
...........Bag Shards from a well used Volcano Bag