Horses cannot breath through their mouths, so I doubt much is getting in through the mouth and then swallowed during or just after a race, as the epiglottis is closed down over the esophagus so the horse can breath. So ingestion isn't anything I'd worry about, be it synthetic, turf or dirt.
Nostrils are a concern, obviously. Vets routinely find alot of dirt in the trachea and larger bronchi after races, but particles of any composition (dirt, smog, AWT, dust, whatever) have to be very, very tiny (there is a particular micron size) to get further down the airways than the "supply tubes".
From the little available that I have read vets are seeing less mucus post-race days on the synthetics vs dirt (mucus is the body bringing irritants up and out of the larger airways), and less stuff in the larger airways. Maybe Chuck can comment on what his track vets have seen.
Obviously any dry, powdery surface (think dust) is a greater threat for stuff getting deeper into the lungs than a damper, heavier, larger particle (which gets caught upon inhalation in the sinuses and upper airways as it should be).
Sand isn't going to go very far generally, it's a big particle as far as the airways are concerned.
I, too, would be more worried about the dried manure, organic matter, fungal spores, etc in dirt than in a synthetic, if inhaled deeply.
I removed an intact set of pantyhose from a Labrador's stomach and intestine once
