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A Quick Biology Lesson

In general, mold and mildew are different names for the same thing — a very simple life form that lacks the ability to photosynthesize. Mold uses enzymes to digest nutrients from other organic materials — such your collections. Essentially mold consists of fungi that grow in filamentous forms. Many fluoresce under UV light — often helping to verify their presence.

The two most common "problem" molds for preservations are the Ascomycetes and Fungi Imperfecti. The Ascomycetes include over 29,000 species, many of which are disease causing. The Fungi Imperfecti is the second largest subdivision, containing over 17,000 species and they are also very aggressive agents of biodeterioration.

The molds that we are most commonly concerned with reproduce through asexual means, forming what are called conidia, although they are often called "spores." Conidia are typically 5 to 50 m in diameter (Aspergillus fumigatus conidia are among the smallest, measuring about 2.5 m) and are very easily air borne.

Each vegetative filament of fungus is called a hypha and a large mass of hyphae is called a mycelium. These hyphae are the actively growing, assimilative phase of the fungi and new growth typically occurs as a linear elongation of the hyphae.

Molds often appear as circular spots, resulting from the outward growth of hyphae. As the mold spreads, it begins to look more like a solid mat across objects. It is important to understand that just as mold grows outward, it also grows down, into the substrate, of the books, bindings, paper, and leather.

Molds are everywhere and should not be viewed as a sudden "infection." The majority of fungal conidia in the indoor environmental come from the outdoors. Consequently, the most common molds are Cladosporium and Alternaria, with lesser quantities of Aspergillus and Penicillium. The number of conidia may reach one million per cubic meter under favorable conditions, although levels of 10,000 to 100,000 per mģ are more common.

Aureobasidium are stain fungi, commonly causing staining of wood finishes. Mucor and Penicillium are examples frequently associated with stone, producing acids which result in solubilization of silicates and the weathering of stone. Aspergillus, Penicillium, Cladosporium, Fusarium, and Paecilomyces are often associated with the discoloration of paint. Fungi such as Aspergillus and Penicillium are often found colonizing adhesives and caulks, with Penicillium in particular found associated with PVC, plastic, and rubber. Aspergillus is associated with surface discolorations of polyesters. Carpets are frequently attacked by Penicillium, Aspergillus, Cladosporium, and Mucor. Library collections under damp conditions are frequently infested with Penicillium, Fusarium, Aspergillus, and Cladosporium.

What Favors Mold?

Molds thrive in a wide range of environmental conditions. Often there is a reference to the need for relative humidities to be at or over 65% for mold to live. Some sources even claim that levels over 70% are necessary. In spite of this, mold can be found aggressively growing at much lower relative humidities.

Sometimes this can be explained by the concept of "water activity," abbreviated aw. This is the surface equilibrium relative humidity of objects — which can be dramatically higher or lower than the ambient relative humidities. Other times mold can be explained by inappropriate HVAC design, inadequate dehumidification, poor building design, a failure to ventilate, unsuitable mixing of the supply air, or a failure to adequately maintain the building envelope.

Molds also thrive on our failure to practice good housekeeping. Typical vacuum cleaning results in a 17-fold increase in mold conidia numbers and leaves these numbers elevated for at least an hour afterwards. One study found that disturbing a surface mold can cause a 3,300 times increase in airborne fungi with 0.3 meter of the source. In libraries molds often gain a foothold in the dust that accumulates on headcaps.

Health Effects of Mold

Inhalation of airborne microorganisms — and their metabolites — may cause a wide range of respiratory problems and symptoms. Mucous membrane irritation, coupled with a dry cough and eye irritation, are common responses to continuous exposure. Bronchitis and chronic pulmonary disease, while typically associated with smoking, may also be related to such allergic reactions, although the role of airborne microorganisms hasn't been determined. Allergic rhinitis and asthma, particularly among those who are constitutionally predisposed to allergies, is perhaps one of the most severe responses to mold exposure.

Even more potentially toxic are mold glucans — constituents of fungal cell walls — which can cause immune reactions, resulting in mucous membrane irritation, headache, muscular pain, cough, and chest tightness. Mycotoxins are poisonous secondary metabolites of fungi, chiefly produced by Fusarium, Penicillium, and Aspergillus — all of which are common to buildings and collections. Toxicity by inhalation can be 40 times greater than by ingestion.

Although health experts, HVAC engineers, and industrial hygienists all agree that mold isn't a good thing to have around, there aren't clear indoor limits. John Lacey sums up the current issues regarding air quality standards:

There are many problems to setting standards for air quality in buildings: not all organisms are equally allergenic or harmful to health; constitutional predisposition to allergy in the exposed person may be more important than the concentration of spores to which they are exposed; numbers out of doors frequently exceed those indoors although the species differ; occurrence of specific microorganisms indoors may indicate a constructional problem but not necessarily a health hazard; the hazards of some microbial products are, at present, little understood.

He notes considerable range in the guidelines proposed in the literature — one author, for example, suggests the recommended indoor limit should be set at one-third of the outdoor level, with the same species spectrum — but none are directly applicable to a museum, library, or archive setting. The accompanying table reveals comparative mold levels for indoor and outdoor air.

Microbe Lower Limit CFU/mē Average Range Low Average Range High Upper Limit Inside/Outside Ratio - High Inside/Outside Ration - Low References
Indoor Fungal Spores 50 10 500 150 0.1 0.33 American Conference of Government Industrial Hygienists
Indoor Fungal Spores       500     Canadian Guidelines
Outdoor Fungal Spores 100 100 1000       California Healthy Buildings Study

The First Step in Dealing With Mold

The first step in dealing with mold is to identify and correct its cause. There is never any sense in cleaning up the mold until you have dealt with the root cause.

Sometimes the mold is associated with disaster situations — a leaking roof or exploding toilet, for example. Dealing with such cases is easy: you repair the roof or fix the plumbing. Sometimes the problem is chronic. For example, a roof that always leaks, but no one can find the cause. Usually this means no one wants to invest the time or money in finding the cause. You will need to convince them of the value of the collection, the risk to the collection, their responsibility to safeguard the materials.

Until the problem is resolved, you may need to drape collections (ranges of books, banks of map cases, display cases) with clear poly sheeting, to deflect the water leaks. If this is necessary, you must be especially careful since the microenvironment under these drapes can be perfect for mold, even if there is no "free" water. One device that helps collect and channel water is made by Croton Products (732-560-9223). It is important to increase ventilation, which will usually mean using supplemental fans in order to keep the air moving. Be sure such fans meet local fire codes and are designed for continuous operation. Good choices are industrial quality high volume fans.

Sometimes, however, the cause of the mold is more complex. Often the HVAC system for the building was never designed to maintain a steady relative humidity. The system was designed to deal with what is called sensible (temperature related) heat, but not latent (moisture content) heat. Such systems may adequately cool your building, but they do little or nothing to reduce the quantity of water in the air (the absolute relative humidity). Without this capability such systems fail to dehumidify and often allow relative humidities to reach levels of 65% or higher — well into the region where mold growth is very likely.

In such cases, you must reduce the relative humidity levels before worrying about any cleaning operations. Otherwise, the mold will return in short course.

Often this will mean that your institution will need to contract for temporary dehumidification and cooling services and also that funds must be found for improvements to the building HVAC system. This is expensive — but its cost must be compared to the value of your collection and your responsibility to safeguard it.

Temporary Dehumidification Services

There are a number of companies that offer temporary dehumidification services intended to quickly dry out buildings. How quickly you dry the building and collections, of course, depends on the nature of both. A preservation expert can help you determine this.

Most companies use desiccant dehumidification. There are several types, but all can quickly reduce the relative humidity of a building to the 30% range by removing large quantities of water from the air, the building materials, and hygroscopic collections.

This process of dehumidification, however, will also increase the temperature — often by as much as 20 or 30° F. Such temperature increases can be devastating for many collections, causing premature aging and deterioration. What this means is that desiccant dehumidification must be coupled with cooling. Often this is accomplished by what are called "in-line DX units." This means that cooling units, operating off either electricity or chilled water, are placed in the same air stream as the dehumidified air. This way, the air being circulated through your building is not only at the appropriate humidity level (maybe 35% to 40%), but it is also cooled to 65 to 70° F.

When you are contracting for temporary dehumidification, don't accept contractors that provide you with "rules of thumb" or "guestimates" of how much dehumidification you need. You are paying for an expensive service and you should expect to receive detailed information. And you should examine bids on the basis on installation cost, operating cost, demonstrated operational reliability, and design assumptions.

Installation cost will include rental and set-up costs. It should include not only the desiccant dehumidification, but also the cooling necessary to maintain the appropriate set point temperature.

Operating cost should be calculated on the basis of prevailing energy costs projected over the anticipated period of operation (which will be until the building's system is adequate to provide appropriate levels of protection).

Demonstrated operational reliability should involve the contractor's understanding of what your institution is seeking to accomplish, as well as an understanding of equipment behavior in the specified application. In particular, you should require some means of accountability, such as docking the contractor for periods when the equipment fails to met the set point range.

Design assumptions can be quite complex — far more complex than the brief overview presented here. The design process itself involves four steps: establish the set points and tolerances; calculate the heat and moisture loads; select, size, and position the components; and select and locate controls. Of these, the calculation of the moisture loads will likely be the most difficult, since they will involve estimating the grains of water vapor per hour resulting from such things as permeation, people, fresh air intake, and door activity. Oversizing the equipment is likely to result in large fluctuations on a hourly or daily basis which are inappropriate for library, museum, and archival materials. The contractors should be asked to provide their design calculations and the evaluation of these calculations by an outside expert should be a criteria at least as important as price upon when institution selects a contractor.

Cleaning Up

Once mold appears people begin immediately asking, "How do we kill it?" That's the wrong question to ask. In fact, be skeptical of anyone or any firm that suggests treatments based on "disinfecting" for mold or totally eliminating it from your institution.

Remember, mold is everywhere and you can't totally eliminate it from museums, libraries, and archives. That's why it is essential to control temperature and relative humidity — creating conditions that don't allow mold conidia to transform into vegetative forms.

Remember also that even if you kill the mold (and the conidia are far harder to kill than the vegetative stage), it is still respirable and still an allergen. It can still have health consequences, even after it is dead.

So, what you need to do is clean it up — not worry about killing it. This means somehow physically removing it from collections, storage equipment, and building fixtures. Once removed, if we pay attention to the building envelope, ensure proper temperature and humidity levels, and provide adequate ventilation, we shouldn't have any recurring problems.

You may occasionally see advice to use a wet-dry vacuum filled with water and a chemical fungicide (or some special brand of water vacuum) to clean up mold. The theory is that the liquid traps the mold and the conidia. This is bad advice.

These vacuums, whether brand name or put together in your shop, aren't very effective at this task. They trap relatively little and allow a great deal to be expelled. When they are used to vacuum mold, you create a bioaerosol that is dispersed throughout the immediate area. Even if the chemical solution kills the conidia, it is still dispersed, it is still respirable, and now it is also coated with a toxic chemical.

The only vacuum appropriate for mold is a HEPA vac. Relatively small, light-weight models are available for around $300 to $600. They can be used with soft brushes to gather up mold on collections. Once vacuumed up, be sure to dispose of the debris outdoors.

HEPA respirators (identified as N95 or N100) may also be necessary when dealing with mold — you should consult your industrial hygienist. The common paper masks with a single elastic band are unacceptable — they offer a poor fit and don't filter out mold particles. Even those respirators with NIOSH approval and two elastic bands won't necessarily provide the protection you need.

Remember, also, that the use of respirators is governed by OSHA's Respiratory Protection Standard (29 CFR 1910.134). Failure to comply can result in substantial fines. Furthermore, NIOSH has recently revised their regulations for certifying air-purifying particulate respirators (42 CFR 84). You should ensure that you are providing the training and protection required by these laws.

If You Hire Outside Contractors to Clean

Many institutions don't have the staff, don't have the time, or don't want to deal with the health and safety issues involved in cleaning up mold. They hire outside recovery firms to provide this service. While this may be a good solution for your situation, be aware of the pit falls (beyond the cost, which is likely to be high).

First of all, be aware that most recovery firms don't maintain full-time cleaning staffs. They will bring in a supervisors and hire the rest of the cleaning crew from the locally available labor pool. These individuals will have had little or no exposure to your kinds of collections and will likely have no commercial cleaning experience.

You need to ask recovery firms some tough questions: What will the ratio be of full-time supervisors to labor pool employees? What kind of training program will they provide prior to introducing these employees to your collection? What is the level of their insurance and are their employees (including labor pool employees) bonded? Are they providing the legally required levels of Workers Compensation? And even if they are, what health and safety training do they provide their temporary employees? What training, specific to your type of collections, have their supervisors had?

You should also insist that they provide a demonstration of their cleaning skills. This will provide you with an opportunity to examine how they handle your collections and how they perform the cleaning operations.

Second, outside contractors may want to push the use of chemicals. Do not allow them to use chemicals on collections or collection storage equipment. Insist on HEPA vacuuming, or in the case of stubborn mold, perhaps the use of a dry chemical sponge (if appropriate for the material).

And third, be very skeptical of firms that push duct cleaning. Remember, mold is everywhere. You can't totally eliminate it. Even if the duct work itself has no mold, fungal materials will still be quickly introduced back into the building within days or weeks. The EPA has questioned the long-term impact of duct cleaning. It may be better to spend your limited funds on improving temperature and humidity controls than cleaning duct work. The only exception to this is if the ducts are harboring dangerous molds and cleaning is recommended by your industrial hygienist.

Cleaning Various Materials

At times the mold will have left stains, especially on textiles. There is very little you can do to remove these stains yourself. Household bleaches are far too strong for most historic textiles. Sometimes the stains can be removed by a textile conservator, but this is a very complex procedure. The mold may also have grown down, into the substrate of your collection. A conservator may also be able to remove this mold, but it will require picking it out, strand-by-strand under magnification.

Removing mold from leather objects is equally difficult. Vacuuming is again the best approach, but you must be careful not to press the nozzle of the vacuum cleaner onto the object, or use its brush attachment with too much vigor. Either may cause the mold to smear, staining the surface, or abrade the object. The best approach is to hold the nozzle very close to the artifact, and allow the suction to pull the mold off. You may use a moderately stiff hog hair or hake hair brush to "sweep" the mold toward the nozzle. Be especially careful if there are beadwork, delicate attachments, or loose fragments. You will want to cover the nozzle with nylon mesh or use mesh over the object, to provide additional protection.

If the mold has spread to painted or plaster walls and ceilings, they can be either vacuumed or treated with a fungicide such as a solution of ū cup of household bleach to 1 gallon of water. Of course, always wear rubber gloves and protect your skin and eyes, as well as nearby surfaces. Begin at the bottom of the wall and work up to the ceiling. Rinse well with clean water afterwards and ensure the surfaces dry completely, using plenty of ventilation. In addition, do not mix bleach with ammonia, or any other detergents or cleansers containing ammonia. Mixed together the two are a lethal combination producing chlorine gas. Many household cleaners contain ammonia, so be careful. In addition, consult an architectural conservator before undertaking such cleaning on historic plaster or paint (such as in a historic house museum).

For wood walls you may be able to clean mold off by using a mixture of 4 to 6 tablespoons of washing soda or trisodium phosphate to a gallon of water. Test in an inconspicuous area first. If this is ineffective, try 4 to 6 tablespoons of trisodium phosphate and 2 tablespoons of ammonia to a gallon of water. Do not allow either solution to remain on the wood for more than a minute or so. Work in small areas that can be rinsed and dried before moving on. Again, do not use this cleaning approach on historic wood surfaces without consulting an architectural or furniture conservator.

Modern carpet, a storehouse of not only moisture but also mold, should be discarded, and replaced. Carpet cleaning techniques are generally not effective on carpets with heavy mold levels.

Ceiling tiles are also good reservoirs of mold. Those stained with mold (even if the tiles are dry) should be carefully removed, bagged, and discarded. New tiles should be installed in their place. Do not attempt to clean or paint over the mold — it won't work.

For More Information

If you would like more information about mold, mold clean-ups, and water problems, check out these sources:

Environmental Protection Agency. 1991. Building Air Quality: A Guide for Building Owners and Facility Managers. Washington, D.C.: Government Printing Office.

Harriman, Lewis G., III, editor. 1990. The Dehumidification Handbook. Second Edition. Amesbury, Maine: Munters Cargocaire.

Kowalski, W.K. and William Bahnfleth. 1998. Airborne Respiratory Diseases and Mechanical Systems for Control of Microbes. Heating/Piping/Air-Conditioning. July, pp. 34-48.

Singh, Jagjit, editor. 1994. Building Mycology. London: E & FN Spon.

Trechsel, Heinz R., editor. 1994. Moisture Control in Buildings. Philadelphia: American Society for Testing and Materials.

A Couple of On-Line Sources

There are a lot of on-line sources concerning mold -- some good, some bad. Here are two that we believe are especially helpful for museums, libraries, and archives.

The Canadian Conservation Institute's Mould Outbreak: An Immediate Response provides an excellent summary of mold remediation efforts.

Indoor Mold: A General Guide to Health Effects, Prevention, and Remediation is a detailed report prepared by the California State Library's California Research Bureau that will answer many of your questions concerning the health effects of mold. Note that it is a pdf file.


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