Try turning the humidifier off and seeing if that makes a difference.
Clean the heck out of everything washable. with really hot water.
I think it's really unlikely for plastic fumes to be a real problem. There is a lot of air blowing through the machine to dilute any fumes. In order to contaminate the air, some sort of substance has to be evaporating from the plastic, and it should get used up pretty soon.
You might turn the machine on and let it run without being masked up for half a minute or so before you maskup.