Retarding the growth of scale is the most compelling reason to find alternatives to tap water. For this reason, or if white dust is a problem or you wish to minimize your exposure to minerals in the tap water as a matter of prudence, you should either:
Use bottled water labeled "distilled." While distilled water still contains some mineral content, it will likely contain lower mineral content than most tap water. Distillation is the most effective method for removing minerals from water.
Two additional demineralization processes, deionization and reverse osmosis, remove most of the minerals from water, but are generally less effective than distillation. Water demineralized by these two processes would, on the average, be expected to contain a higher mineral content than distilled waters. "Purified" water may be produced by any of these three or other similar processes.
Be aware, however, that not all bottled water is produced using demineralization processes. Bottled waters labeled "spring", "artesian" or "mineral" have not been treated to remove mineral content.
Consider using demineralization cartridges, cassettes, or filters if supplied or recommended for use with your humidifier.
Be aware, however, that the ability of these devices to remove minerals may vary widely. Further research is needed to determine how well, and how long, these devices work. Watch for the appearance of "white dust," which would indicate that minerals are not being removed.
Also, in areas of the country where the mineral content in the tap water is high, using distilled water may be less expensive than cartridges, cassettes, or filters.
Types of Humidifiers and Associated Pollutants Console humidifiers are encased in cabinets which are designed for floor use. Portable humidifiers are smaller and more readily moved. Central humidifiers are built into heating and air-conditioning systems, and humidify the whole house.
The two types of humidifiers which generally appear to produce the greatest dispersions of both microorganisms and minerals are:
Ultrasonic, which create a cool mist by means of ultrasonic sound vibrations.
Impeller, or "cool mist," which produce a cool mist by means of a high speed rotating disk.
Two additional types of humidifiers can allow for growth of micro-organisms if they are equipped with a tank that holds standing water, but generally disperse less, if any, of these pollutants into the air. These are:
Evaporative, which transmit moisture into the air invisibly by using a fan to blow air through a moistened absorbent material, such as a belt, wick, or filter.
Steam vaporizer, which create steam by heating water with an electrical heating element or electrodes. "Warm mist" humidifiers are a type of steam vaporizer humidifier in which the steam is cooled before exiting the machine.
Note: Steam vaporizer and evaporative humidifiers are not expected to disperse substantial amounts of minerals. A steam vaporizer tested by EPA did not disperse measurable amounts of minerals; evaporative humidifiers have not been tested by EPA for mineral dispersal.