Fortunately there isn't a substantial flow of air, just the amount that is bleeding out of the water drain slots. The rest of the air in, is just 'bunched up' inside the plumbing and stays fairly steady at a given altitude and speed. However, dust will make its way up such pipework eventually, in part by inertia. An altogether undesirable state of affairs.
I think we'd be wise to treat all data from these devices as suspect.
It brings to mind being in North Africa with a Viscount. We asked if we could get help clearing the P1's pitot line. I looked up to see a 'helpful' fella connecting a rubber hose to the head. By the time I'd got the window open and screamed Nooooooo!!!! There were shards of glass and lots of little cogs in my lap.