Some air purifiers may contain a PM2.5 sensor, with the purpose of being able to measure the room's air pollution level, often with the intention to control the purifier's fan speed setting according to how much pollution in the space needs cleaning away.
However, there is a key reason why this is usually ineffective. By design, the internal PM2.5 sensor is located in the very cleanest part of the room - inside the air purifier. Naturally, the air it measures here will be cleaner than the rest of the room, since air diffuses throughout a space, and outside the purifier, cleaned air will mingle with air containing higher levels of PM2.5 and other pollutants, so the sensor readings cannot detect the air quality status in the wider room. Therefore, the sensitivity of the purifier based on the sensor readings is likely to respond to a lower level of air pollution, than may be present in the wider space.
To get an accurate representation of how the air is diffused across your space, and how to adjust your purifier setting accordingly, it is recommended to measure using an air monitor further away from the purifier.