Cool moisture humidifiers require a replaceable wicking filter to operate. The filter captures minerals from the water, helping to prevent white dust in hard water areas. Humidifiers that require a wicking filter produce invisible cool moisture that has been filtered. Filters may need to be replaced more frequently in areas with hard water, but generally they will last 30-60 days, depending on water quality and frequency of use.
Cool mist and warm mist humidifiers don't require a filter. These types of humidifiers are convenient and don’t require replacement filters. However, whatever minerals are in the water may come out in the mist. This is known as white dust and is common in areas with hard water. Filter-free humidifiers require regular cleaning as the mineral build-up inside the humidifier may cause it to run less efficiently.