

To begin with, any given environment wants to achieve equilibrium. When moisture is added to the environment, for example by placing a container of water in it, the moisture over time will disperse throughout the environment until the entire environment is at the same relative humidity. In other words the environment has equilibrated.

The next important consideration is that different materials react differently to moisture. Many materials have different moisture holding capacities, or the ability to absorb moisture, than others. For example, wood can absorb a significant amount of moisture while metal can absorb only negligible amounts, if any at all. Others, like papers, can be completely broken down by water.
What this all means is:

  • Metals cannot effectively absorb moisture. Instead, when a metal object is exposed to high humidity, the metal will rust through what is called a reduction-oxidation reaction.
  • Woods on the other hand absorb a significant amount of water causing them to swell in size. This can cause wooden valuables to warp and break. Since wood typically contains some levels of moisture, it can also lose it if the environment is too dry causing it to shrink and crack.
  • Most every material reacts to humidity in one way or another, so it's best to keep it in a range which is optimum for that material.