A woman should not soak bran to use it as a cleaning agent when going to the baths. She may, however, rub it on the dry skin. Similarly, one should not chew grains of wheat and place them on his wound for a cure - because this creates chametz.
If so, how is one to behave? Here are the rules that tell us when leavening can no longer occur: If one baked, cooked, or scalded grain in hot water, then this is completely safe on Passover.
Can one add flour to a pot that's cooking already? Some say that one must then immediately add vinegar, which stops the leavening process, but some completely allow this. (Today, a moot question; many later customs made it much more strict.)
Art: Flat Landscape with Harvested Grain Field by Paul Baum