During humid weather, store a wa handle only after washing moisture has evaporated, in a stable location away from splash, steam and direct heat. The important distinction is between humid air and retained water: enclosing a still-damp knife can keep moisture at the handle and its junction.
Choose the local conditions, not a universal number
Wood responds to its surroundings. The USDA Forest Products Laboratory’s Wood Handbook chapter on moisture explains that wood exchanges moisture with the environment and that the process is affected by humidity, temperature and the wood’s existing moisture.
That does not provide a universal target humidity or a fixed safe drying time for an assembled handle. Instead, choose the most stable practical indoor location available. Avoid storage beside the sink, above a dishwasher, in cooking steam, against a heat source or where the knife experiences abrupt temperature changes.
Horn also has material-specific limits. Abbeyhorn warns that excess moisture can cause horn to swell and advises avoiding heat and rapid changes in temperature or humidity. Those limits do not prove that every visible change is moisture damage, but they support keeping a horn ferrule out of wet or rapidly changing conditions.
Check what surrounds the knife
Before putting the knife away, check the roll, sheath, block opening or other contact area for retained dampness. A towel-dried handle can still have moisture near the front junction, and a damp enclosure prevents that area from remaining exposed to air.
If the handle has just been washed, complete the handle-drying routine first. Keep the storage cover open or separate until both it and the knife are dry. For comparisons among storage devices themselves, use the separate Japanese knife storage guide.
Observe changes without forcing a diagnosis
During a humid spell, occasionally compare the handle under similar light. Note any new roughness, swelling, lifted finish, seam change or looseness at the blade junction. Do not remove a secure handle or stress-test it. If the fit has changed, stop using the knife until the assembly can be assessed.
Colour or odor alone cannot establish trapped moisture, contamination or internal decay. Likewise, humid weather is not a reason to apply oil on a calendar: oil is suitable only when the actual finish and its directions permit it. The wa handle care guide sets out the broader material-care boundaries.