Woft:
You are right about those masks. They will block something propelled at your mouth and nose but they offer almost no protection for inhaled contaminants. If you wear one of these in a very dusty environment you will notice streaks of dust deposited on either side of your nose and mouth as the air current is diverted around the mask. For the best protection you would need a proper form fitting mask with the appropriate filter type. This seems a little excessive though.
Personally, I think you would be better protected by wearing disposable gloves if you are in a high risk area.

Chris