A humidifier is an appliance that uses water to produce vapor that is evaporated into the air. HEPA, a High-Efficiency Particulate Air filter, is a type of air filter that is designed to remove airborne air particles and pollutants from the air. HEPA filters are made of dense, fibrous materials that trap air pollutants as they pass through them. HEPA filters can effectively capture pollutants such as pollen, dust, mold, pet dander, and even some bacteria.