An air purifier is presented with a sensor controlling a motorized fan, wherein the sensor continuously and automatically adjusts the sensitivity setting of the sensor as the quality of air being purified changes. The average air quality readings of the air passing through an air purifier are monitored over a designated time period and, as the quality fluctuates, the sensitivity of the sensor is automatically adjusted. If the air quality is poor (i.e. a high particulate count), the sensitivity of the sensor will be adjusted to a less sensitive setting (i.e. allowing higher fan speeds so that more air can be moved through the filter) and, if the air quality is good (i.e. a low particulate count), the sensitivity of the sensor will be adjusted to a more sensitive setting (i.e. setting a lower maximum fan speed because not as much air needs to move through the filter).