I’m am working on a remodel that involves the removal of the kitchen floor including the subfloor. The house was built in the 1970s, and the building code of those days allowed for the cold air returns to use wall and floor cavities. The current code requires using an actual duct for air returns.
Since we were reconfiguring the kitchen we had to reroute the return ducts, which allowed us to peer inside. The attached photos show what we found. You can see an accumulation of dust, hair, lint and other materials. All the things that can make an allergy sufferer worse.
The interesting fact is that the owner recently had his ductwork cleaned. Unfortunately, these types of ducts can be very difficult to clean due to the way that they are constructed. If you are doing a total remodel you may consider upgrading your ducts to the current code, if you are an allergy sufferer.