A natural cork is compressed and forced into the bottle as a 100% natural seal. The resilient and elastic cork now wants to expand and is continuously pushing out against the glass to form the seal. The air space between the bottom of the cork and the wine is called the ullage. The ullage will increase over time as the cork is far from a perfect seal. It will let some air and oxygen in while also absorbing some wine and allowing humidity from the wine to escape.
The greatest threat to the natural cork seal is having the wrong humidity level in the cellar: Low humidity in the cellar will cause a cork to shrink while too high will cause it to mold and rot. The lower or higher from optimum the relative humidity and the worse the cork, the faster this will happen to your precious wine. Once oxygen or mold breach the cork it’s not long before your wines starts turning into vinegar or worse.
Moderate humidity is the key to keeping the cork in good resilient condition and prevent it shrinking or molding. A relative humidity of 50-80% is an acceptable range with about 70% recommended. A standard commercial refrigeration or air conditioning system is designed to run at only about 20% humidity. This relative humidity is so low that the air outside the bottle will draw wine through the cork, even when the bottle is stored on it's side.
A very easy way to introduce humidity into a newly built cellar system is to leave out a pail of water with a cloth draped over the lip. This water will naturally evaporate and raise the relative humidity to the maximum for the given temperature.
Excessive humidity may not harm the wine directly but it will cause the labels and wood to mildew and corks to rot. Whether for your own personal enjoyment or as an investment for the future, any mold or mildew is unacceptable and should be controlled as quickly and thoroughly as possible.
GT Systems installs Environmental Control Units in Wine Cellars specifically used to reach and maintain the desired humidity in all conditions.
Summary: Anywhere between 50-80% relative humidity is acceptable, 70% being ideal.