There could be two factors contributing to this:
A blocked sensor: causing the reading to appear incorrect.
A source of off-gassing causing this severe reading.
Off gassing is where a pollutant source is producing a gas at a constant rate over a prolonged period.
Try and locate the source that is producing the gas.
Use the machine in ‘Auto mode’.
We update the MyDyson™ app regularly, so your machine always performs at its best. Every release includes improvements, from bug fixes to performance updates and increased reliability.