Boveda use natural salts and water. When salts are saturated, they can release or absorb moisture to maintain a stable relative humidity (RH) in their surroundings through osmosis. The reverse osmosis membrane used in Boveda would let pass only pure water vapour in and out. If the relative humidity (RH) in your humidor deviates from the indicated RH on the sachet, moisture will be emitted or absorbed until the indicated RH is reached. Then the vapour transfer stops. Thus, Boveda naturally evens out all humidity fluctuations inside your humidor.
These are ideal for long-term storage: