Hello,
Do we have a full understanding of how humidity plays a roll in psa/graded cards? Has anyone done any studies on this or has examples of what might happen to cards inside the case? Are the effects of humidity on cards so slow that we should not worry about them? What are the best tools to monitor, control and protect ones collection from humidity?
Thank you! 
This is a complicated question to answer because it really depends on how humid you’re talking about, and how long the humidity is going to last. If you live in a tropical region where it is constantly humid (like 90%), then you should be more cautious when handling your cards to ensure that your cards are not constantly in moisture.
HOWEVER, in regards to cards in PSA cases, I don’t think there’s much to worry about as the PSA slabs are water resistant to begin with, and there seems to be a lack of discussion about humidity damaging cards in PSA cases when I was researching on this topic. I think the best way to tell if there’s too much moisture is that the first thing that would be affected in a PSA case is the label itself, before the card. If you see the label becoming water damaged, that’s when you know there is too much moisture in the environment.
I don’t think there’s much to worry about though; worse case if you are really paranoid when storing your cards in a safe/box just put those silica pouches/dehumidifier boxes near your cards and those will absorb whatever moisture there is in the air. You need to replace those however, and the more humid the air, the more frequent those needs replacing.
Just to be safe I recommend keeping silica desiccants with your collection and change them every few weeks. Especially if you keep your cards in a safe. There is no place for moisture to escape in a safe. If your cards are in a air conditioned environment, you should be ok for the most part. Use silica packs just to be safe. I buy the silica packs that turn from orange to green when full of moisture, so you know when to change them. Just pop them in the oven at 280 degrees for a couple hours and they are good to go again! Hope this helps
Fantastic! thanks everyone.
The Hvac/R guy agrees with the above. You need proper air conditioning 24/7. For human comfort, about 70 degrees Fahrenheit and 50% humidity is ideal. What is not ideal, however, is the fact that there’s always a chance of a bad install, random hot or cold spots throughout the house, and temperature/ humidity fluctuations, and the fact that the real world indoor conditions are much more volatile than the books claim them to be. Common thing for all professions.
The dehumidifier and/or dessicant is a great idea too. Keep those cards safe!
Pokemon cards by themselves are not affected by humidity, source: I live in an extremely humid part of the world and cards exposed to this environment since 1999 have no effect.
High humidity does affect plastic (sleeves etc) even after a couple of months but although not ideal does not damage the card in any discernable way. The only thing you might need to worry about in high humidity environments is the plastic on booster boxes and other similar product wrapping.