Does the sensor provide an absolute or relative measurement?
Let me explain: we said that 25 indicates "the best air you have ever seen" and 250 "the worst air you have ever seen". So could it be that two sensors both indicate 250 even if one is in a disabled house and the other inserted in a painting plant? And if I take the sensor in the uninhabited house and take it to the painting plant, it might start to detect, for example, 400 until it discovers that the worst "new air" it has seen is that and then it returns to indicating 250 as the maximum? This would indicate that the 250 is relative to each environment, not to the absolute % of substances. Is that so? Thank you.
IAQ is relative. It gives an indication of the relative change in ambient TVOCs detected by BME680.
IAQ = 25 corresponds to typical good air and IAQ = 250 corresponds to typical polluted air.
In your use case you mentioned, IAQ value will drop from 400 to a lower value and spans from 25 to 250 after a long time in a stable environment, but it won't stay at 250.