fumes released by plastics, vinyl, paint, rubber, etc. If you seal off the cabinet 'air-tight' you'll probably cause more problems in the long run. I use IKEA Billy cabinets with the aluminum framed glass doors. I set the doors such that the hinge side has about 1/16th-1/8th inch air gap to the cabinet frame. The 'opening end' gets about the same by using a self stick 'bumper' that prevents the door from slamming into the cabinet frame.
SOME amount of air circulation is a good thing.Many of my models are approaching 18-20 years old and show no discernabile deterioration.