Why does the PM2.5 value change when the Xiaomi Smart Pet Care Air Purifier is running without a filter?

When the purifier operates without a filter, the PM2.5 level can either increase or decrease, depending on the environment. Without a filter, the device essentially functions as a fan, circulating the indoor air. This process mixes areas with high and low PM2.5 concentrations. The particle sensor detects these fluctuating levels, resulting in a random PM2.5 reading. Over time, as the air circulation continues, the concentration of PM2.5 in the room becomes more uniform, and the sensor's readings stabilize.
Additionally, indoor activities such as using a humidifier, smoking, or cooking can introduce moisture and oil particles into the air, which can skew the purifier's ability to accurately measure pollution levels. These factors can cause the PM2.5 value inside the room to be higher than outside.