I have a Dyson pure humidify + cool. I am noticing that when I turn it on, the air quality readings are good. After running the machine for 30 min the VOC and NO2 quality moves very quickly to poor or very poor. If I turn off the machine and wait 20 min, the readings go back to normal. 
 

I completed a deep cleaning cycle even though it wasn’t prompted but this has not improved.