Some Posekim write (Yalkut Yosef Hilchot Mayim Aharonim and Mishna Berura 181, 4 and IIRC the Magen Avraham) that Mayim Aharonim has Ruah Ra'a and that once someone used the water for Mayim Aharonim we shouldn't walk over it. Why aren't we allowed to walk over Ruah Ra'a?
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The Mishan Brurah in Hilchos Netilas Yadyim 4:20,21 writes that they can be harmed from the ruach raah therefore they should pour water into soft earth or a slope.Even on twigs one shouldn't pour(18).