I live in an extremely hot and humid climate in south Louisiana. As the only serious photographer in the family I get to take care of ALL the family negatives going back to the 1920s. (If I didn't snap them up years ago they would be long gone. Everyone else went digital ten years ago.) Those negatives have endured non-climate controlled late 19th century homes until the family got modern AC in the early 1980s. Even after the installation of central air conditioning you can never really pump all the water out of the air in those old tall ceiling houses. During some times of the year you wake up in the morning and walk out to get the paper, there is water literally streaming down every window and wall of the house in rivulets. In the last three major hurricanes the wooden structure of the house has warped enough to wreck a lot of interior paint jobs, pop some trim planks out of position, jam doors that are ordinarly loose fitting, etc. We have water out the wazoo, but short of actually being submerged in flood water I have found that negatives can be more resiliant than you might expect.
I once reprinted a bunch of those '20s and '30s 6x9 negs as Christmas presents to the family. A few of them were a little vinegary or shrunken, but most were okay. Maybe 10-20% of the negatives sustained a degree of damage. When I found them they were mostly in yellowed paper sleeves from the original drugstore that developed them. The first thing I did was separate the negatives and file them in the acid free sleeves. I organized them by format and film type, and within those categories I subdivided them by date.
If you see that a negative or page of negatives is breaking down, segregate it from the rest. I have read that they can outgas and spread the rot.
My main strategy for preventing mold on the negatives and on all my lenses is regular examination in sunlight. Besides trying to keep the file binders in a cool, clean, relatively dry place, putting eyes on the material from time to time gives me confidence. Mark a date on your calender every year to sit in the sun and look at your oldest negatives. I'm sure you have too many to do them all in a day, but the oldest couple of volumes of the bunch. Maybe rotate volumes. A bit of sunlight and stirring of fresh air with a casual visual inspection. If something starts to go wrong you'll catch it early. A lot of pleasant memories too. A long time ago I read a National Geographic story about a native tribe that regularly recites the names of everyone in the family tree to maintain their spirits. I found that idea vaguely appealing. I'm also always wary of chemical treatments beyond the essentials of processing, it's a minimalist, inexpensive approach.
