Low humidity causes eye irritation, trouble shedding, trouble shooting and catching food, digesting food, constipation or bowel and sometimes hemipene prolapse, eventually kidney failure, organ failure and death. A dehydrated cham will show sunken eyes, skin that looks wrinkled and papery, bits of old shed stuck on, the casque will look sunken, not full. Their tongue doesn't seem to stick to feeders. When you take a fold of skin between thumb and fingers and release it, it will not bounce back flat right away, but stay tented.
Overly high humidity can be OK as long as there is good air flow through the cage/tank. No air flow and constantly high humidity leads to respiratory infections due to molds and bacteria build up in the cage.