Industrial humidifiers add moisture to warm, circulated air in order to protect furnishings and reduce static electricity. They use deionized, demineralized, softened, or untreated water. There are several basic types of industrial humidifiers. Conventional steam humidifiers use an electrical or gas-fired heater to boil water and release steam into the air. Direct or live steam humidifiers inject steam directly into an air handling system or ductwork. By contrast, liquid-to-steam systems transfer energy from a hot liquid, usually water, through a heat exchanger inside a water-filled chamber. Steam-to-steam systems that use a heat exchanger inside a water-filled chamber are also available. Unlike other types of industrial humidifiers, ultrasonic devices vibrate a piezoelectric transducer at a very high frequency to create tiny water droplets instead of steam. In some systems, specialized nozzles are used to discharge a mist of atomized water and compressed air.