How To Deal With Dry Impure Substance ?
Can be ignored means we are following the principle of ease in the matters of worship as told by Deen. It is generally not possible to be 100 percent clean all the time. So minor filth is ignored. Our common experience tells that a stain is an impurity or not. If it is we wash it or remove it and even then the stain persists we ignore it if there is no smell.
What we call small quantity, that can be ignored. Such things are not quantified. You have to decide what small or ignorable amount of impurity is.