The Mayo Clinic warns against the use of tap water in a humidifier:
Use distilled or demineralized water. Tap water contains minerals that can create deposits inside your humidifier that promote bacterial growth. And, when released into the air, these minerals often appear as white dust on your furniture. You may also breathe in some minerals that are dispersed into the air. Distilled or demineralized water has a much lower mineral content compared with tap water. In addition, use demineralization cartridges or filters if recommended by the manufacturer.
What are the dangers of breathing minerals that are dispersed into the air?