"Protective Athletic Mouthguards: Do they Cause Harm?- Glass, Conrad et al ; Sports Health, 09-10; 09.
Soft tissue lesions about the mouth increased more than threefold during a D1 (American)football season. The authors concluded over the counter mouth guards "may have a significant influence in producing oral disease". When cultured, the mouth pieces themselves "yielded 339 bacterial isolates, 20 yeast isolates, & 108 fungal/mold isolates".
The authors recommend replacing a mouth piece every 14 days, and daily cleaning with a denture cleansing solution.
I'm not going to get mad at the kids for losing them any more.