The bottoms of my feet constantly turn black. I clean them off and it comes back. I have hard wood floors and thought that was the problem but I will mop them everyday and it still happens. It does not happen to anyone else in the family. There are some spots that should be dirty like on the balls of the feet. They will be the dirty black but some spots are clean. My house is not dirty. I run a daycare and have babies so I am always sure the floors are clean. I am 45 yrs old. Does anyone have any info on this. Could this be toxins releasing?