An air purifier's or humidifier's noise level can differ depending on the model, manufacturer, or settings. While working, certain humidifiers and air purifiers may make noise, while others are built to run silently.  Typically, air purifiers use fans to move air through the filters, which might produce noise from the fan motor and airflow. Depending on the fan motor quality and speed settings, air purifier noise levels can range from quiet (less than 30 decibels) to loud (above 60 decibels). While lower fan speeds may produce less noise, higher fan speeds may be necessary to move sufficient air through. Additionally, noise levels can vary depending on the type of humidifier. For instance, evaporative humidifiers may make noise from the fan that forces air through a moistened wick. In contrast, ultrasonic humidifiers often function silently because they use high-frequency vibrations to create a thin mist of water droplets. The heating element in steam humidifiers may make some noise as well. The CDC states that prolonged exposure to noise exceeding 70 dbA can irritate or gradually harm your hearing – the level of an average dishwasher or washing machine. Therefore, acquiring a humidifier/air purifier that emits noise levels below this value is advisable.