Dear Maria,

Yes, it is a problem when a particle of the host is dropped to the ground, even the smallest piece.  If it can be retrieved it should be given to the priest, he'll take care of it.  If the particle is too small then the priest still needs to be informed so he can take care of floor after the Mass, some priests may even want to clean up right away during the Mass.

It would have been entirely appropriate to stop the communion line to investigate whether or not an accident had occurred.  I don't believe the Church has any rules that actually oblige the laity in this area, but it should be done anyway out of respect for the Eucharist.

I know some people may not at first realize what has happened, or may be too shy to "make a scene", if this is the case then the priest should be informed immediately following Mass.  Once he's told, it becomes his problem and you wouldn't need to worry any more.

By the way you should tell your pastor about what happened.  Just in case it really was a particle of the host that you saw falling, he needs to know so he can be more careful in the future.

Hope this helps,

Mr. Slavek