Cool mist humidifiers can be placed on any surface, but because warm mist humidifiers dispense hot steam, they must be placed out of the reach of children.
To minimize the possibility of minerals or dust buildup in the air, always use distilled water in a humidifier. Fill the tank to the indicated maximum fill line. Do not overfill a humidifier. Refill as necessary to keep the humidifier running. If the tank gets too low, it automatically shuts off.
Once the humidifier is in place and filled with water, adjust the settings to get the desired mist output. Some humidifiers also allow you to direct the mist, so make sure the output is not pointed at something that can be damaged by water, such as furniture or a wall.
Some humidifiers have filters that should be changed once a buildup of dust or minerals develops. For filter-free machines, simply clean the tank and surrounding areas frequently with hot, soapy water.