When it comes to choosing the best humidifier overall, Air-O-Swiss humidifiers stand out. They earn particularly high marks for their dual warm/cold-mist machines. These ultrasonic humidifiers can produce warm or cool mist and are suitable for rooms up to about 600 square feet. The water tanks on Air-O-Swiss humidifiers are detachable and transparent, which makes them easy to clean and allows you to keep an eye on the water level. A digital humidistat allows you to adjust the humidity in the room to your desired level. These humidifiers come with a demineralization cartridge (*Est. $20), which needs to be replaced every two to three months (or more often with hard water), and an antibacterial silver-ion stick (*Est. $30), which should be replaced annually.
AllergyBuyersClub.com names the Air-O-Swiss Ultrasonic 7144 (*Est. $ 200) its best humidifier. It is also named as a "splurge-worthy pick" in the comparison tests conducted by the Good Housekeeping Research Institute, and it earns a score of 8 out of a possible 10 from Wired. User reviews for this humidifier on Amazon.com are mixed (as they are for virtually all humidifier models, regardless of price). Although most of the 90 or so reviews we found praised the Air-O-Swiss 7144's quiet operation, attractive design and adjustable humidity level, it earns its share of complaints as well. Users' most common beef is that the 1.5-gallon tank is hard to refill.
One other humidifier in the Air-O-Swiss line, the Air-O-Swiss Ultrasonic 7135, gets similar reviews, but with this model we also saw several complaints about leakage.
For smaller rooms (up to 500 square feet), the Sunpentown SU-4010 Dual Mist Warm Cool Ultrasonic Humidifier (*Est. $80) earns excellent ratings across the board in one independent test and is also highly rated by AllergyBuyersClub.com. Like the Air-O-Swiss 7144 humidifier, it is designed to produce cool and warm mist, with variable humidity levels. However, this model does not include a humidistat. It does have a demineralization cartridge (called the Ion Exchange Filter), which should be replaced every six months. Other features include overheat protection and a split rotating nozzle to direct mist in two directions simultaneously. This model is energy efficient (using up to 43 watts on the cool-mist setting and 90 watts for warm mist) and very quiet.
However, we saw more reliability complaints for this humidifier than we did for the more expensive Air-O-Swiss models. We saw reports of humidifiers breaking down anywhere from a week to four months after purchase, and we also read several reports of leakage, often severe. A couple of users consider this humidifier to be a safety hazard, as water leaks could get into the wiring, though we read no actual accounts of shock or fire problems.
The Stadler Form William Ultrasonic Humidifier (Est. *$230), which also outputs either warm or cold mist, earns positive reviews as well, particular for its sleek design. It earns stellar marks for being whisper-quiet and easy to use. Stadler Form notes that this model is appropriate for use in spaces up to 1,000 square feet -- more than double than the Air-O-Swiss models.