The holmes ultra humidifier adds moisture to the air. Its small and compactly designed, so you can keep it on your desk or on the nightstand. It has a variable mist control option that lets you choose your most comfortable moisture levels. This electric humidifier doesnt have any filters for changing. I have two in my home that have provided so much comfort and fresh air its amazing. I had this third one for backup, but u haven't ran into any problems with my other two in the 14 months I've owned them, so I am passing this amazing machine on to do wonders for someone else's home as it is only taking up space here, due to the efficiency and satisfaction the other in my home are providing.