How big is the area that you want to humidify? Match the stated output of the humidifier to the area you want covered. Output is measured in gallons; for example, a 3-gallon humidifier can put out 3 gallons of moisture per day. The specs for the humidifier should also say how big an area it can cover.
How long is the time period you want to run it? Although a 3-gallon humidifier will humidify a large area, you'll have to refill it frequently if it has only a 1-gallon tank. Also, if you want to run it overnight, you want there to be enough water to do so.
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 and more 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 us 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.