Earlier quoted context omitted.
It seems like one of the countries where masking while sick and during flu/cold season is common would have some studies on this.
You'd notice it in professional populations, I'd wager. I also assume it would be a confounding factor in any study that involved masks, if it was a significant effect. "Weird, the group that breathed in asbestos for 20 years without wearing a mask had 90% more lung cancer than the masked group, but 20% less XYZ. Does asbestos exposure prevent XYZ?"
As an added bonus they're usually (in the Before Times) wearing surgical masks as opposed to a [K]N-95, which is anecdotally a lot more reflective of what the general population is wearing (if they're wearing anything at all).
Edited to add:
As an added, added bonus, dental hygienist skews heavily female, whereas construction and remediation skews heavily male. Female populations also wouldn't have the confounding factor of facial hair, which interferes with the seal of a respirator.
Full disclosure: I type all this while I'm wearing a half-face respirator with P-100 cartridges on it because I'm doing a bunch of sanding today. I also have a goatee. Read into that what you will.