Maintain your TruSens Humidifier by changing the filter at regular intervals. The enhanced filter softens hard water to minimize mineral build up to deliver a cleaner mist. This also helps to prolong the life of the humidifier. One filter lasts approximately 4 months for a full season of humidified air. The filter can be used with TruSens medium or large humidifiers, models N-200 and N-300. Change indicator light is displayed on your humidifier when the filter needs to be replaced. Sold bv each.