Cool mist or warm mist? Some people prefer warm mist humidifiers, saying the heated vapor is easier to breathe (especially if you're congested) and that it gently raises room temperature. But experts say that any benefits that warm mist provide compared to cool mist humidifiers is minimal. In addition, warm mist humidifiers that boil water can present safety issues (see below), and warm mist humidification is less energy efficient.
How big is the area that you want to humidify? Match the stated output of the humidifier to the area you want covered.
What kind of water does your municipality have? If you have hard water, a demineralization filter that minimizes dust caused by the minerals left behind by tap water is a must with any ultrasonic humidifier. Some models come with a demineralization filter; in other cases, it's an optional add-on. Remember to replace the cartridge as scheduled.
Consumable costs can add up. Even among the best-rated humidifiers, complaints about the costs of filters are often seen. Demineralization cartridges and bacteria-killing products also add to the on-going cost. If you live in an area with hard water, some say that using distilled water can cut down on dust emitted by ultrasonic humidifiers and extend the life of your appliance.
Safety first. We didn't see many complaints regarding safety hazards with ultrasonic or evaporative humidifiers. However when it comes to warm mist humidifiers and vaporizers, caution flags aplenty are raised. These work by boiling water to create steam. That water can scald if innocent hands accidentally tip over the humidifier, making warm mist types a bad choice if children are about. Dual-mist ultrasonic humidifiers are a bit safer, as they don't heat water up as much to create a warm mist, making it warm to the touch, but not scalding. On the other hand, by raising temperatures to boiling, warm-mist humidifiers do a better job of killing any bacteria in the tank or the water. Because of that, it is important to follow maintenance recommendations when using a cold mist humidifier to avoid issues with bacteria or mold.