These units heat the water before releasing it into the room. Most units boil the water to produce steam - note: these steam producing units are not ultrasonic machines. Steam exits through a cooling chamber. This kills all possible bacteria, so airborne bacteria released with the water is less of an issue than with cool mist.
Units which offer dual ultrasonic cool mist or ultrasonic warm mist on the same machine (like Air O Swiss) heat the water to 176 degrees, which kill only some bacteria.
The warm mist feels soothing, especially if you have a cold or the flu. These are often preferred during the cold winter months. Warm mist does not lower the ambient temperature of the air in the room unlike cool mist or evaporative units.
Video on the Vick's warm mist humidifier and general info on warm mist humidifiers from Lisa, site owner. Enjoy!
Vaporizers are similar, as they heat the water to boiling and release steam, but don't have a cooling chamber. The steam released from a vaporizer can burn if it’s accessible to small children or pets. They are cheaper, but spending just ten or twenty dollars more will get you a much better warm mist unit. Warm mist units will sometimes have reservoirs which can hold essential oils or vicks inhalants to help with colds.
The drawback of warm mist units is they are more expensive to run because of the extra energy used heating the water. Also, the heating element will need to be cleaned of mineral build up depending on how much minerals are in your water. I find I need to clean my unit every few weeks and I use hard water out of the tap.
Overall, I like warm mist for winter months. I just use mine at night in the bedroom since it is more expensive to operate, and it is close to me in the room. I have my evaporative unit running day and night in the large living area when it’s really dry. The air coming out of the evaporative is cold, but I have it away from seating areas, so it is not a bother.