Molds are members of the fungi kingdom. They feed by absorbing nutrients from living or dead organisms. Starches are among their favorite foods. Cellulose, which is the main component of paper money, is a long chain-starch. As such, molds love to eat paper. Molds can easily feed on your favorite currency notes by secreting enzymes that help them digest cellulose.

If left long enough, molds will weaken paper considerably or even devour it entirely. Mold staining is a far more common result that you will encounter. It can be a byproduct of the mold digesting the paper itself or come from pigments produced by the mold. Mold spores are ever-present and will grow when the environmental conditions are right.

The only way to prevent mold from growing is to deprive it of its appropriate environment. This means keeping the relative humidity below 70 percent. Below 70 percent, mold spores will be present, but they will not be active. So what should you do if you discover mold growing on your paper money collection?
Assuming you have not recently survived a flood, the presence of mold indicates that you are storing your collection in an inhospitable environment. Try to diagnose why you have mold growing in the first place. Mold spores need high humidity so the presence of mold could indicate a leaky roof or foundation or the presence of a “microclimate.”

In the winter, uninsulated exterior walls and floors tend to be more humid than the rest of the room. This is because the air by the wall is cooler. Since cooler air holds less moisture the relative humidity is higher. Since this humidity is high enough, condensation can occur. Any paper notes stored in the proximity of this high humidity will tend to absorb the moisture and provide mold spores with the critical environment they need to thrive in.
There are several approaches you can take if you discover moldy notes. You must first remove the items from the humid environment. Once removed , the notes will dry out and the mold will become dormant. You will need to be careful, not to spread the mold spores around while the notes are drying. Once the notes are dry, you can remove the mold.

Some experts recommend bagging moldy objects once they have dried so that the items can be cleaned up later. The problem is that once you bag an item with active mold and leave it at room temperature, you are practically guaranteed to hasten the mold along. Instead you can buy some time by bagging moldy notes and placing them in a freezer. Freezing puts the mold into statis, but does not kill it.

While mold spores are everywhere and a normal part of our environment, they can also have serious health effects. Reactions to molds depend on the species, its concentration, the duration of exposure, and your state of health. Molds can be much more than an irritant causing you to sneeze.
Over-exposure to certain molds can cause allergic reactions, which can range from hay-fever-like symptoms to asthma attacks and even anaphylactic shock. Some molds are toxic, causing serious neurological damage and even death in some cases.

Some molds can also take up residence in your lungs or other organs causing infections to grow. This usually occurs only to individuals who have a compromised immune system due to either disease or drug treatment. When dealing with mold, always err on the side of caution and protect yourself appropriately.