One question that is often asked during the holiday season is if mold can die in winter elements. Active mold produces microscopic spores in enor­mous quantities which are spread by air currents—meaning they are almost always there waiting for the right conditions to reproduce. Extreme cold, freez­ing, and heat can deactivate spores but it does not kill them. They are resistant to desiccation. If temperatures go up after a cold spell, spores can reactivate and continue to grow.

The right conditions for mold growth vary by species. The relative amount of mold spores waiting for these conditions also vary – by geography, season of the year, and local weather conditions. There is a difference between indoor and outdoor conditions. But mold is tenacious; the spores lie dormant until favorable condi­tions occur.  Once it finds a host material, a spore needs only enough moisture and nutrients to germinate. With moisture, they will germinate if given the right combination of elevated temperature, poor air circulation, dim light, or accumulated dirt.

To sum it up, there are four critical requirements for mold growth – the presence of mold spores, available mold food, appropriate temperatures and considerable moisture. What are the best approaches to managing these elements to reduce or remove the potential for mold growth?

Mold spores are ubiquitous – they are literally everywhere. There is no reliable or cost-effective means of eliminating the spores from environments that humans inhabit. Therefore trying to control mold growth by eliminating mold spores is not feasible.

Mold needs organic materials to supply nutri­ents, therefore museum and library collection objects composed of organic materials are potentially at risk. Cellulose-based materials, such as cotton, linen, paper and wood, and proteinaceous materi­als such as leather, parchment, adhesives, and hair are particularly susceptible to direct attack by microorganisms. Even inhospitable materials, such as plastics, are not immune to fungal growth. How they sup­port growth is not fully understood. The ability to exist on almost any material illustrates why mold is considered a primary agent of deterioration.  Eliminating the host is not an option for controlling mold.

Most mold species grow very well at the same temperatures that humans prefer, which is why it isn’t uncommon to find mold growing in our own kitchens, bathrooms and basements. Unfortunately, collection storage temperatures dropped close to freezing are not cold enough to prevent mold. Temperatures below freezing won’t kill mold, but they do make it go dormant. Temperatures raised above human comfort levels are even more inviting to mold growth and combined with high humidity will cause abundant development of mold. Therefore, it is not feasible to control mold growth by controlling temperature alone.

Most mold species require the presence of considerable moisture for growth. When water or high relative humidity provides the necessary moisture, dormant mold spores will germinate, grow, and eventually release more spores. How much moisture is “considerable” in this case? Organic materials naturally contain a certain amount of water and they are hygroscopic, so when relative humidity goes up, they absorb water to achieve equilibrium with the environment. At 50% relative humidity, the moisture content of paper is approximately 7%; at 70% relative humidity, it is approximately 10%. Relative humidity levels at 70% or above can easily lead to mold growth. To be safe, it is generally recommended that relative humidity be maintained below 65% in collection storage areas.

Mold blooms occur in many colors and are sometimes confused with dust, dirt, foxing on paper surfaces, or cobwebs. Active mold in the early stages of a bloom has hair-like filaments in webs, which develop a fluffy or furry appearance as the bloom matures. Active mold may be soft and will smear when touched. It may also be slimy and damp. Inactive mold is dry and powdery and will appear to brush off materials readily. Both active and inactive mold can have a distinctive smell, which is often described as musty.

Mold can permanently damage the materials supporting it, and make them more suscep­tible to future mold contamination. It can produce stains and weaken structures of vulnerable collection objects. Mold and mildew excrete digestive enzymes that allow them to eat starches and cellulose, commonly found in book and paper collections. Cellulose in paper is difficult to digest; so many molds prefer the starch in cloth coverings on books and in paper sizing. This is why mold is often spotted on the bindings long before it grows on text blocks. Once mold has attacked the protein or starch sizing in paper materi­als, they will absorb water more easily. Mold growth can result in scattered spots, or foxing, on paper. Leather is particularly susceptible to mold and can be stained and weakened by it. Collection managers must be able to recognize signs of these problems and be prepared to take preventive actions.

The best way to prevent or control mold growth is to deny spores the moisture necessary for germination. Regu­lating the environment, especially the relative humidity, is essential for preventing collection damage from mold. Avoid storage of collections in or near damp areas such as attics, basements, sinks, windows, or directly on floors. Store all collection materials at least four inches above the floor. Avoid storing collection materials directly against outside walls where they are more susceptible to dampness, condensation, and leaks from upper floors. Correct any mechanical or structural problems that contribute to high humidity such as leaking pipes, poor drainage, cracked windows, roof leaks, deteriorated brick, etc. Check for malfunctioning or unclean humidifiers, standing water, or areas of previous water damage.

Maintain other environmental factors, such as adequate air circulation, to decrease the potential for mold germination and growth. A fan helps increase circulation. Proper ventilation can help keep materials dry, prevent mold spores from landing on objects, and reduce microclimates with high RH levels.