For humidifiers using sponge, oasis material or clay, a 50-50 mixture of propylene glycol and distilled water should be used. This will automatically stabilize your humidity at 70%. The newer high tech humidifiers using acrylic beads or units such as the water pillow should be moistened with distilled water only. The mineral content in the tap water in my area is so high that it can virtually destroy a humidifier within a couple of months.