In an example, an air filter system includes an air filter and a thermally conductive particulate sensor (TCPS). The TCPS includes a temperature sensor to measure a first temperature of the TCPS at a first time, measure a second temperature of the TCPS at a second time, and provide the measured first temperature and the measured second temperature to a controller. The controller to determine an actual rate of temperature change based on the measured first temperature and the measured second temperature, compare the actual rate of temperature change to a target rate of temperature change, and provide a notification when a difference between the actual rate of temperature change and the target rate of temperature change is greater than an accumulation threshold indicative of a threshold amount of particulate accumulation on the TCPS.