Most steam inhalers boil water within the device to make steam. Boiling kills organisms found in tap water, such as bacteria, viruses, and protozoans that cause illness. This makes the steam safe to inhale. This type of steamer doesn't require distilled or purified water, although there’s no harm in using this type of water.
Some steam inhalers emit cool steam mist. Others utilize technologies other than boiling to generate steam. If you use a non-boiling steam inhaler with your own water supply, always use distilled water or boiled tap water. Make sure the water comes to a rolling boil before using.
Another plus to using distilled, rather than tap, water is reduced mineral buildup in the steamer itself.