The main choice is between warm and cool. The difference between the two is exactly as it sounds. Do you want warm moist air, or cool moist air? This decision is usually made on personal preference, but there are some fundamental differences in the way each type works that might also effect your decision.
In general, warm air humidifiers make less noise, but are only suitable for single rooms. Cool air humidifiers can potentially cover a larger area, but as they have moving parts they also tend to make more noise. Every humidifier is different. If you understand how they work, the choice of which one to buy should be much easier.
Warm
Cool
All warm air humidifiers use a heating element to heat the water. This does mean that they use more electricity than cool air humidifiers, but the cost is no more than boiling a kettle. Air can be made to feel 'muggy' if overused.
Steam/Vaporizer
Quite simply, the water is boiled and sent out as steam. The advantage of this is that the boiling process kills any mold and bacteria. The disadvantage is a risk of burns, which makes these units unsuitable for use with children. This is the simplest type of humidifier, and is therefore very cheap to buy.
Warm Mist
Similar to steam, the water is heated to make it evaporate, but most units cool the air slightly before it is released. This reduces the risk of burns.
These do not heat the water, so there is no burn risk and they can safely be used with children. They also use less electricity, can humidify a larger area, and do not make the air feel 'muggy'. The disadvantage is that they generally make more noise, and some require regular filter replacement.
Evaporation Wick
A wick or filter is used to absorb water, which is then evaporated by a fan. This is a more natural form of evaporation, which is a big plus. As the humidity of the air increases, the evaporation rate will naturally slow, making these units, to a large extent, self regulating. The persistent noise of a fan blowing puts a lot of people off this design. A few people actually prefer it though! They say the sound is relaxing, and even that it helps them sleep. Again, this is down to personal preference.
Impeller
With these models, it is a spinning disk in the water that creates the mist. This process is generally a lot quieter than a fan.
Ultrasonic
A more modern design, these use ultrasonic sound waves to vibrate the water and create a mist. They are very efficient, and almost silent to the human ear.
All humidifiers require regular cleaning. Failure to clean your humidifier on a regular basis will lead to the buildup of mold, mildew, fungus and bacteria. Ideally, water should be changed on a daily basis.
