The subject says it all: Sometimes I bite my fingernails during Mass, due either to hunger or just some hangnails I feel the need to get rid of. I’m pretty sure I’m not breaking the fast (I’m not eating them for sustenance, that’s for sure) but figured I’d make sure.
Biting fingernails does not break the eucharistic fast because fingernails are not food. However, biting one’s nails at Mass is not proper church etiquette. Hunger and personal grooming should be taken care of outside of Mass. If you cannot entirely break yourself of this habit, I recommend that you at least try to refrain from doing it during Mass.