The short answer for a blue tongue skink
Most blue tongue skinks thrive between 40 and 60 percent relative humidity, but that single range hides an important split. Australian subspecies — Northerns and Easterns — come from drier country and do well at 40 to 50 percent. Indonesian subspecies, collected from the steamy forests of islands like Halmahera, evolved in far wetter air and need 60 to 80 percent to shed cleanly and stay hydrated. Before you buy, find out exactly which subspecies you are getting, because setting an Indonesian skink up at Australian humidity is one of the most common beginner mistakes with this species.
Why humidity matters for a ground-dwelling skink
Blue tongues live close to the ground, nosing through leaf litter and soil where the air stays more humid than the room above it. Their thick scales lose moisture slowly, but the thin skin between them and around the toes, eyes, and tail tip dries out fast. When the enclosure air stays too dry, shed skin sticks in rings around the toes, circulation suffers, and repeated bad sheds can eventually cost a toe. Chronically dry air also nudges a skink toward low-grade dehydration, which shows up as sunken eyes and wrinkled, dull skin rather than obvious thirst.
Building the right humidity in the enclosure
Start with substrate: a deep layer of cypress mulch, orchid bark, or a soil-and-mulch mix holds moisture and lets the skink burrow into the damper layers when it wants to. Keep the substrate lightly moist below the surface, never soggy on top. A large, heavy water bowl adds steady evaporation and doubles as a soaking spot — many skinks will sit in it before a shed. For an Indonesian animal, daily misting of one end of the enclosure, or a fogger on a timer, usually closes the gap between room humidity and the 60 to 80 percent target.
Measuring honestly instead of guessing
Room humidity is not enclosure humidity, and the cheap dial gauges sold for reptile tanks can drift badly. Use a digital hygrometer with the probe placed at the skink's level, near the cool end where it spends much of its time, and check it morning and evening for the first few weeks. Expect a natural swing: a rise after misting or a water change, a fall under the basking lamp's heat. What you want is the average sitting in range, not a number pinned at one figure all day.
Signs the air is wrong, and when to get help
Too dry: retained shed on toes and tail, frequent soaking, dull grey skin that does not lift cleanly. Too wet: substrate that smells sour, mould on décor, or a skink with bubbly breathing and mucus around the nostrils. Wheezing, open-mouth breathing, or shed that will not release after warm soaks are reasons to book a reptile vet rather than keep adjusting the tank at home. Once your routine is settled, logging the misting and water-bowl duties in KinsLe Care Teams means a partner or sitter keeps the same rhythm, and your skink's humidity stays steady even when you are away.