Oh I’m fairly sure our high humidity is one of the factors. It’s usually around 70-80% all year long.
I don’t think this bubbling is caused by one single factor such as fumes or high humidity. If that was the case, then there would be many more people complaining about it.
If I had to take a guess, I would say that the plastic boxes created some kind of greenhouse effect together with the fumes and indirect daylight. Those boxes are made of PVC plastic, which are acidic themselves.