The Long Term Drift of LogTag's Humidity Loggers, UHADO-16, HAXO-8, HASO-8) is typically less than 0.25% RH and less than 0.02°C per year when operating in the normal relative humidity and temperature range (0 - 65°C, 10-80%RH).
The long term drift value may be higher in environments with vaporized solvents, out-gassing tapes, adhesives, packaging materials, etc.
Where long term high accuracy is required, an appropriate calibration schedule should be set up and the unit calibrated by a laboratory for humidity measurement.