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The subject of vaporizers and humidifiers comes up fairly often in cold and flu season, since there is not a lot we can do to make kids more comfortable outside of fever medication and ensuring adequate fluid intake.
The rationale for a humidifier or vaporizer is to make the air the child breathes a bit less drying to the nasal and bronchial membranes by adding moisture to the air. Which is a good theory, but in actual proactice, moisture measurements of nasal and lung air show that the temperature and moisture content of the air are adjusted naturally very quickly. While raising the moisture content of the room air may help a small bit towards relieving dry nasal passages, the effect is probably small. There is no reason to believe there is any benefit at all to the bronchial tubes, since the air has 100% relative humidity by the time it gets past the nasal membranes.
Humidifiers and vaporizers actually do nothing for chest congestion, and may aggravate it if there is any mold growing in the machine. Thorough cleansing with bleach is required every week or two of use to prevent that.
As for cool vs warm vaporizers, there is no documented difference in efficacy between the two (and there really is not much documented efficacy to speak of anyway). It is known that nasty burns can occur with some of the warm mist humidifiers - so I generally say don't use them, for safety reasons.
In general, humidifiers are best used with no additives - just water. The oils make the room smell "mediciny," which gives the parents a warm feeling but don't do much for the child.
If it helps you make a decision - we raised four sons without a vaporizer.