OK - at the moment there’s a bit of a heatwave in the UK and the temperatures are 30c+.
The humidity in the room where my cards are stored is currently 30c with humidity of 50%. I’m so scared in the long run that my cards will disintegrate or get damaged due to this. I’ve run an air conditioner in the room before but it costs too much money to keep putting on 
Do you guys reckon my cards will be OK? I know this thread probably sounds a bit mad but I can’t stop worrying
Unfortunately you might get some warping. Unless they are in Graded cases. I went through this last spring, 'twas really humid and almost all of my reverse holos that I had out in my coldest room got warped. I noticed that this same room was more humid than usual, because around spring time I have my windows open.
Agreeing with this. Holo cards will probably get warped, I’d recommend either putting them in toploaders or moving your cards to a basement or dark closet or somewhere a bit cooler (although if you could have done that I assume you already would have).
all of my cards are in either toploaders, folders or still sealed. also in dark drawers and wardrobe. I hope this helps plz plz M’s post is making me panic haha
sorry just being honest. When I saw that my cards warped I immediately put them in a clear perfect fit box that I have. I packed the box with cards as much as I could. To this day I have not touched them. Because I know deep down inside, that they are still warped.
Edit: They are inside another box which has a pack of silica gel in it. Just like Scott mentioned below.
Wherever you have them stored, I would recommend some silica gel packs. I would also recommend purchasing some a safe combined with silica packs. Most safes tend to come standard with silica packs.
Reading this made me scared but then the fact I already have silica gel packs and that my cards are in a drawer means it was all for nothing!
Honestly I do not think they are safe in a drawer. Try putting them in the cleanest part of your basement. I keep my A/C on at all times and never turn on the lights in my collection room now, plus in boxes within boxes with silica gel packs inside them. Not a single card has warped.
I do have a silica dehumidifier in my room - have had multiple in the past and they don’t pick up much moisture. All of my cards are in drawers and cupboards so perhaps I’m going overboard with my reactions. Thanks guys
Perhaps you are. Just turn on the A/C you will be fine.
You all could move out here to the wonderfully dry desert which is Vegas? 
50% humidity is mild and many libraries actually store precious paper/documents at around those levels. Any potential damage your Pokemon cards might endure is if temperatures and humidity levels fluctuate rapidly and over long periods of time.
Silica gel packs do little to none in terms of reducing relative humidity it might reduce by 5-10% if you have a ton of them but that is inconsequential when dealing with Pokemon cards as they are made from modern paper/cardboard.
As long as you keep your cards in stable environment you will be fine, no need to turn on the AC!
Source:
I live in Australia where it is winter right now and will be 22-29c this week with humidity levels around 60-80%, it is like this all year round and worse in the other seasons and my cards haven’t warped etc.
Do note that silica gel packs have a finite life, especially those dinky ones you see in some products and food stuffs. They can absorb a certain amount of moisture then stop doing anything. Especially in a drawer or a frequently opened safe they do not last too long before maxed out in high humidity. They do make some nice products that are re-usable and re"charge"able by baking them in the oven. I use one in my large safe that I try to open as infrequently as possible.
A color change lets you know it is time to bake it to regenerate.
Use one of those ideally in a safe that you don’t open often or in a relatively sealed atmosphere container with clear sides so every so often you can peak for the color indication.
Well we had a spare dehumidifier/air con in our loft so have bought that down and will now intermittently use it to make sure it doesn’t get too hot/humid in the room. All is well 
I’m just keeping my curtains closed with a fan on 24/7 and it seems to be doing the trick