The theory is that hard water is irritant to the scalp. Anything that irritates the scalp, anything that diminishes the chemical nature of the scalp can provoke seborrheic dermatitis because it does not respond well to non-specific irritation. Putting on caustic chemicals, over coloring the hair, rubbing the hair, harsh water, harsh soaps all non-specifically irritate . . . → Read More: Dr. Francis Iacobellis: What Are The Effects of Water on Dandruff?