I didn't see any study, just a reference talking about GI problems and a little mention of sand. I'm a long time owner [ almost 25 years ] + have bred beardies. Kept them on no substrate as babies, older dragons on both newspaper as well as sand + sand/soil mix. I've never seen one problem at all. One of the most reknowned breeders in the world with dragons sales in more than 10 countries [ Tammy from Draggintails ] has noted the same. And she does this for a living, is a true expert and her
care sheet notes that her dragons are all on children's playsand with not one impaction. This is with 10's of 1,000's of dragons. As for scale rot + fungus, that doesn't happen on sand either. It's a dirty tank, whether loose or solid substrate that can cause scale problems. I've seen yellow + black fungus cases on this forum for the about 10 years + I don't think a single one was on sand.
My tanks never smelled, as a matter of fact a dragon " home alone " on tile that poos in his tank will have a nasty tank + smear it everywhere [ one owner right now on the forum considering rehoming because of the mess + smell ] so the dragon on tile is exposed to as much mess , probably more, than on sand . Yes, you do have to scoop the poo out of sand it but it's not an impossible task. A dragon that is bathed + drinks the water can ingest the same bacteria in it's system this way just as easily as a little lick of sand. As for heavy ingestion of sand, this may occur with the calcium sand [ which I would not recommend ] because the dragon likes the taste + possibly be calcium deficient. Most cases of impaction ARE caused by inadequate
hydration , heat + diet. Aside from being unnatural, tile is what would cause joint damage being hard like cement. They walk on hard packed as well as loose sand + soil in the wild and don't live on rocks which would be more like tile. Beardies love to dig, it's one type of enrichment that sand/soil gives them in their tank and is a 100 % natural behavior.
The bottom line, I wouldn't pressure or insist people set up dragons with sand but it's not really accurate to say that sand is a health threat.
Bathing can be a health threat for them, just Google " bearded dragon drowned " and you'll see a number of incidents. So there are risks in many different aspects of care, no one risk should be blown out of proportion.