OK, I've done a lot of searching but most articles refer to damp ingress due to taking a lens from a warm to cold climate and vice versa but this is not my worry.
I intend to spend up to 8hrs a day in the rainforest where relative humidity can be as high as 100%. I'm wondering about the possibility of moisture ingress and subsequent mould due to the zoom action. I've already collected a bit of dust inside so I'm not convinced that the seals are as good as some claim them to be.
Opinions welcomed as to whether this is just an "occupational hazard" that one has to contend with in these environments?
Yes, mold can grow in these conditions. It doesn't matter much whether you zoom or whether it is a fixed-focus lens, the humidity will get into the lens and camera body.
If you store the gear in a low humidity environment for the other 16 hours of the day, you might be all right.
Photographers who live in very humid places have special storage cabinets for their gear to prevent harmful fungus growth.