There are two main types of humidifier: single-room and whole-house. We’ll focus here on the single-room variety, which is far more practical in the home.
Single-room models use one of a variety of techniques to increase the humidity of the room by adding water to the air. The simplest type is the evaporative humidifier, which uses an absorbent wick with one end in a tank of water. The wick increases the surface area for evaporation, and as the water evaporates the wick draws more up from the tank using a physical effect known as capillary action. The water vapor is distributed using a fan. Other models heat the water first and emit it as steam, while some use ultrasound to create a fine mist of water which is then released silently. These ultrasonic humidifiers also kill any microbes present in the water, and may therefore present the healthiest option. Since they’re quiet, they’re also more suitable for lived-in rooms.
Before you buy, there are several things to look out for. Similarly to emptying the tank in a dehumidifier, you’ll have to refill a humidifier’s tank periodically. The size of the tank will affect how often you need to do this; in other words, the bigger the better. The size of the unit should also match the room where the appliance will be used.
Try to buy a humidifier with a humidistat, a device that keeps the humidity in the room constant at a given level — this is analogous to the job of a thermostat in heating systems. Noise level is also a crucial consideration. In general, evaporative humidifiers are the loudest variety, but you should try before you buy wherever possible.