Mould Prevention and Collection Recovery: Guidelines for Heritage Collections – Technical Bulletin 26

By Sherry Guild and Maureen MacDonald
Revised by Tom Strang and Sherry Guild

CCI Technical Bulletins

Technical Bulletins are published at intervals by the Canadian Conservation Institute (CCI) in Ottawa as a means of disseminating information on current techniques and principles of conservation of use to curators and conservators of Canada's cultural objects and collection care professionals worldwide. The authors welcome comments.

Abstract

Mould infestation in heritage collections can damage objects and may pose a health risk to individuals who work with these collections. This Technical Bulletin presents information on mould morphology, prevention of mould growth, actions to take should mould occur and health effects relating to mould exposure. It informs the reader on how to remove mould growth from objects and it describes the appropriate personal protective equipment to wear when working in a mould-contaminated environment or when working with mould-infested objects.

Authors

Sherry Guild graduated from the Art Conservation Techniques program at Sir Sandford Fleming College, now Fleming College (Peterborough, Ontario). In 1984, she started at CCI as a conservator in the paper laboratory, specializing in the conservation of works of art on paper. During her career, she assisted many clients on the recovery of mould-contaminated objects. She retired from CCI as a Senior Conservator, Works of Art on Paper, in 2015.

Maureen MacDonald worked in the Preventive Conservation Services Division at CCI from 1981 to 2010. Her areas of expertise included environmental monitoring equipment, ultraviolet filtering materials and biological materials. During her career, she worked on projects involving the microscopic characterization of frozen archaeological skins and preparation techniques for natural history specimens, and she studied mould and mouldy materials. She was named an Honourary Life Member of the Canadian Association for Conservation of Cultural Property (CAC) in 1998.

Tom Strang earned a Ph.D. in Conservation from the University of Gothenburg, a Master's of Art Conservation (Artifacts) from Queen's University and a B.Sc. (Hons) in Biology from Carleton University. Since joining CCI in 1988, Tom has generated solutions to problems presented by organisms that can damage cultural property. He is an expert in integrated pest management (IPM) for all types of cultural collections facilities and has conducted IPM reviews for leading museums, galleries and archives in North America. He has established the efficacy of thermal control methods against insect pests and investigated the risk of adverse effects of pest control treatments on cultural heritage objects.

Disclaimer: The information provided here is based on the current understanding of the issues presented. The guidelines given in this Technical Bulletin will not necessarily provide complete protection in all situations or protection against every possible adverse effect caused by the use of products in museum contexts.

List of abbreviations
Introduction
1. Mould prevention
2. Collection recovery
Conclusion
Acknowledgements
Suppliers
Websites and other sources of information
Appendix: Removing mould
Bibliography
Endnotes

List of abbreviations

µm: micrometre
a_w: water activity
ASHRAE: American Society of Heating, Refrigerating and Air-Conditioning Engineers
CFU: colony forming unit
dB(A): decibel
EMC: equilibrium moisture content
ERH: equilibrium relative humidity
HEPA: high efficiency particulate air
MVOC: microbial volatile organic compounds
PAPR: powered air purification respiratory (system)
PPE: personal protective equipment
PVC: poly(vinyl chloride)

Introduction

This Technical Bulletin provides general information for the archive, library, museum and gallery community regarding fungal^{End Note 1} (or mould) infestation in heritage collections. It presents information on fungi morphology, prevention of fungal growth, actions to take should fungal growth occur and health effects connected to mould exposure. It informs the reader on how to remove mould growth from objects and describes the appropriate personal protective equipment (PPE) to wear when working in a mould-contaminated environment or when working with mould-infested objects.

The treatment of infested objects with chemicals (thymol, ethanol, ortho-phenylphenol, etc.) or by non-chemical methods (gamma radiation, ultraviolet light, microwaves, etc.) is not addressed in this Technical Bulletin. For information on these topics, consult the conservation literature.^{End Note 2} In general, using chemicals to treat mould is no longer recommended for heritage collections. Although chemical methods have been used in the past, their efficacy, their possible deleterious effects on the object and the considerations regarding the effects of these substances on humans have not, in some cases, been fully investigated. Also, it is important to note that killing fungal organisms usually does not destroy their antigenic or toxic properties. This means that dead fungal growth on the object remains a health concern and that no chemical treatment confers lasting or residual mould control. These factors have contributed to the shift away from using chemicals to treat mould infestation in heritage collections.^{End Note 3}

Any treatment to combat mould infestation should focus mainly on measures that keep the level of moisture in the air and the moisture content of the object below the level conducive for mould growth. In the event of a mould infestation, the conscientious removal of visible mould growth and the reduction of spores from objects should be undertaken.

This Technical Bulletin adopts information regarding levels of mould contamination and appropriate PPE from guidelines for assessment and remediation of mould in buildings and applies it to heritage collections. It does not address mould remediation in buildings, building envelopes or heating, ventilation and air conditioning systems. For information on these subjects, consult the appropriate professionals.

Note: It is strongly recommended to read the entire Technical Bulletin and to consider the information carefully before proceeding to treat mould-infested collections. Mould can be a serious health concern and medical research into its effect on humans is a rapidly developing area. If in doubt, consult professionals.

1. Mould prevention

1.1 Mould

The nature of fungi, how they grow, under what conditions, the viability of mould spores and triggers of activation are discussed in this section. Although mould remediation in buildings is not addressed in this Technical Bulletin, mould, or conditions conducive to its growth in buildings, presents a risk to the collection. For this reason, the Bulletin presents techniques for the detection of mould spores and the level of mould spores likely to be found in indoor environments, as well as a chart identifying potential sources of mould in buildings and the factors contributing to its growth.

What is mould?

Moulds are members of the kingdom Fungi. There are thousands of fungi species, ranging from yeasts, which are single-celled organisms, to the more complex multi-cellular mushrooms and toadstools. The majority of fungi lie somewhere in between. Fungi play an important role in the cycle of nature. Unlike green plants, fungi lack chlorophyll and cannot photosynthesize their own nutrients from carbon dioxide and water. Most fungi are saprophytic: they are organisms that live on and derive their energy from dead or decayed organic matter, such as plants, food, leaves, etc. They feed solely by digesting the substrate on which they grow. A few are parasitic and some live symbiotically with a host. Their growth is often called mould. Mildew, a term often used to describe mould in the home, is actually a parasitic fungi that grows only on living plants.

Under a microscope, mould looks like a network of thread-like filaments, referred to as hyphae, woven into a network, called the mycelium. Moulds spread by and grow from various types of microscopic spores, of which some are referred to as conidia.^{End Note 4} The conidia are produced during the asexual reproductive phase from a specialized structure or fruiting body called a conidiophore. The shape of a spore is determined by the species. A spore can be round, elongated, oblong, cylindrical, sickle-shaped, single-celled, multi-cellular, etc. In general, spores and fruiting structures must be present before most moulds can be positively identified. Depending on the species, spores range from 1 to 200 μm in length (a micrometer [μm] is one-millionth of a metre or 1/25,000 of an inch). Even the largest spores are buoyant enough to be carried long distances by air currents.

In the temperate regions of Canada, the most common species of moulds are Cladosporium, Alternaria, Aspergillus and Penicillium. The latter two species are mostly found indoors. These species are known as filamentous or conidial types. The spores (conidia) form at the ends of branched (tree-like) structures called conidiophores (Figure 1). In general, the species of mould found inside a building are the same as found outdoors, but they are found in much lesser concentrations.

© Government of Canada, Canadian Conservation Institute. CCI 127508-0001
Figure 1. Conidia and conidiophores of a Penicillium species.

Mould growth

Mould can grow under a very wide range of conditions. When the environment is suitable for germination, the spore swells and a germ tube extends outward. For most mould, this action is triggered by a significant change in temperature or in the elevation of moisture. A germ tube is a vegetative cell that elongates as it assesses the amount of moisture and nutrients in the substrate. If adequate moisture exists, the germ tube begins to branch as it continues to elongate. These branches multiply and become the hyphae. The hyphae grow and form an interwoven colony of fibres called the mycelium. Depending on the texture or porosity of the material on which the mould is growing, the hyphae will penetrate, to some degree, into the substrate (Figure 2). Sporulation, the production of fruiting bodies and spores, can result from a change in the growing conditions of the mycelium (Griffin 1981). Specialized hyphae form into conidiophores that mature and release spores into the air, which starts the cycle again. Conditions that have the potential to trigger sporulation include:

exhaustion of nutrients
production of chemical by-products
changes in light and temperature

Mould grows outwards from a central point in a circular pattern (Figure 3). The centre of the mycelium is the first to die out. It does so for two reasons:

the nutrients become exhausted
different chemicals appear during metabolism that inhibit mould from growing back on previously infested areas

These mechanisms ensure that the hyphae are always spreading outwards and seeking new nutrients. The newest growth, usually white, is on the leading edge. The centre of the mycelium can develop a colour, which results when spores are formed. Colour change can also indicate a pH change in the substrate. As the spores mature, the colour of the area can also change.

© Government of Canada, Canadian Conservation Institute. CCI 127508-0006
Figure 2. Mould hyphae and spores embedded into paper fibres.

© Government of Canada, Canadian Conservation Institute. CCI 74903-0018
Figure 3. Agar plate of mould.

Requirements for growth

Nutrients

The nutrients required for mould growth are very basic and are supplied by organic materials. Enzymes break down the organic substrate into the necessary nutrients that are absorbed through the hyphae walls. Nutrients are derived from simple sugars, starches, small peptides and complex carbon-containing substances such as amino acids.

In fact, fungi are able to secrete a tremendous number of enzymes capable of digesting any organic material, either plant or animal. However, non-organic materials, such as glass or metals, can also support growth if there are residues of organic materials on their surfaces.

Moisture

In order for a mould spore to grow, several conditions must be present. The first and most important is an adequate amount and supply of moisture. Each mould species requires a minimum amount of water in order for the spore to swell and begin germination. This water comes from the substrate material. Water vapour in the air, measured as the relative humidity (RH), influences the moisture in the substrate material. This source in the substrate is the only moisture available to the mould.

Biologists describe the moisture available in a substrate with the term water activity (a_w), expressed as a fraction of 1 (Ayerst 1969). Water activity is equivalent to the equilibrium RH (ERH) of air adjacent to the material or within its pores. ERH is measured by placing the material in a sealed container and then measuring the RH of the trapped air, after enough time for equilibration to have elapsed. Thus, a measurement of 90% RH indicates an ERH of 90% and, in turn, an a_w of 0.9. The majority of moulds will not grow until the a_w of the substrate material is 0.75 or higher (Onions et al. 1981). (This value is discussed in more detail in section 1.2 How to prevent mould growth in the collection.)

Temperature

Moulds are able to grow under a very wide range of temperatures, as exemplified by mouldy food in the refrigerator. Most mould spores will grow in temperatures from 4°C to 30°C (39°F to 86°F). The rate of mould growth can be regulated by temperature. Growth decreases with lower or higher than optimal temperatures. The majority of heritage collections are maintained between 15°C and 25°C (59°F and 77°F). These temperatures are ideal for mould growth. (It bears noting that Canadian collections kept in unheated winter conditions consistently below 0°C [32°F] are, therefore, not at risk of mould growth until spring.) Short-term exposure to temperatures slightly beyond those for optimal growth will bring about dormancy. A return to optimal conditions will activate growth. Freezing temperatures kill growth. However, some spores can also tolerate extended periods of extremely low or high temperatures. Spore viability is diminished by the alternating action of freezing and thawing.

pH

The pH (acidic, neutral or alkaline) of the substrate will affect the germination, colour and growth of mould. The pH range of the substrate for spore germination may extend from 2 to 9. The optimum is between pH 4 and 7. Many objects in heritage collections fall within this range. Actions, such as washing or deacidification of paper objects, to adjust pH do not deter mould. As with moisture, certain species of mould have different pH triggers. The pH of the substrate material is likely to be altered due to the chemicals released during metabolism, such as metabolites, enzymes and exudates.

Interior of the cover of the Martyrs Mirror book before mould remediation — © Government of Canada, Canadian Conservation Institute. CCI 123238-0088

Close up of a wall hanging showing mould damage — © Government of Canada, Canadian Conservation Institute. CCI 123238-0088

Air circulation

Good air circulation will help maintain an even level of RH and eliminate areas where a microclimate of high or low RH might develop. If water damage occurs, a good flow of dry air facilitates rapid evaporation and drying of the substrate material, making it less conducive to fungal growth. In some circumstances (for example, air-drying wet or damp items), air circulation may be a factor in determining whether or not mould grows. An adequate airflow, one that feels “drafty,” will help prevent fungal growth.

Light

The role that light plays in mould growth is not well defined. Some studies of certain moulds have shown that light affects moulds in the following ways: it may inhibit growth, affect the direction and rate of growth, and affect the production of certain compounds (toxins and volatile organic compounds). Light can also affect the reproductive processes of mould. For some species, light is essential, for others, it is not required.

Objects stored in the dark to reduce the rate of deterioration are not more susceptible to mould growth. Light is not a critical factor for controlling indoor fungal growth: nutrients, moisture and temperature are the critical factors (Shaughnessy et al. 1999). However, mould growth may go undetected for long periods of time if routine inspections of storage areas or areas where human occupation is minimal are not regularly carried out. Also, if there is less air circulation in dark storage areas, it may contribute to mould growth.

Viability

Dormant (inactive) spores wait for the right amount of water and nutrients before beginning to grow. As spores wait for optimal growth conditions, they age and their viability decreases. Some species of mould spores are capable of remaining viable for many years, while others are able to survive for only a few hours. Within the particular range of a certain species, environmental conditions play an important role in the viability of spores. Fluctuations in temperature and RH are factors of viability, as is the presence of many chemical agents, such as fungicides. Spores of the species Aspergillus and Penicillium are known to be viable for up to 10 years (Sussman and Halvorson 1996).

It is also believed that certain species of mould spores are activated by chemicals, such as detergents or organic solvents (for example, acetone and ethanol/water mixtures). This action is not well understood. Perhaps some of the chemicals acting as wetting agents cause the activation (Griffin 1981).

Levels of mould spores

All mould spores originate from the outdoor air. The species of mould and the concentration of spores in the air vary with the season, temperature, RH and geographical location. Spores fall on objects, regardless of the location from which the objects originate, or where they travel or are stored or displayed. Thus, it is important to remember that objects are never completely spore free.

Every mould species has preferences with regards to the environment and nutrients for optimal growth. In the temperate regions of North America and similar climates, the highest concentrations of mould spores outdoors are in the spring and fall. In other climate regions, such as the tropics, high concentrations may be found throughout the year or mould concentrations may have higher and lower periods depending if it is the rainy or dry season. In tropical regions, Aspergillus and Penicillium species are more dominant than in the temperate regions of the world (Mullins 2001).

The species of mould found indoors should be similar to those found outdoors. In a normal healthy indoor environment, concentrations of mould spores are much less than those found outdoors. It has been reported that at peak times during the year, outdoor fungal particles can reach 10⁴/m³ (Miller 2001). An indoor air-handling system filters out a portion of the spore material based on the filter's efficiency.

If indoor air quality tests identify a fungus that is not found outdoors at that time of the year, it may indicate an indoor source, referred to as a fungal amplifier, growing inside a building or collection. These higher levels may indicate a moisture problem or other conditions conducive to mould growth.

High concentrations of certain mould species pose a health risk for humans. Breathing air containing a normal array of mould spores does not usually affect healthy humans. However, if a mould problem exists in a collection (or a building), the concentration of spores can be many times the concentration normally found indoors. This could lead to adverse health issues in hypersensitive people. (Health issues are addressed further in section 1.5 Health effects.) Further investigation will be required to determine the source of the mould.

Health Canada does not have any numerical exposure limits for mould. Since people have different sensitivities, it is not possible to establish a “safe” limit for mould. Health Canada recommends removing any mould found growing indoors and fixing the underlying moisture problem (Health Canada 2014).

Detection

Mould growth on objects can be recognized by a fuzzy coloured appearance and sometimes, but not always, a mouldy or musty smell. The smell is the odour of microbial volatile organic compounds (MVOCs), such as alcohols, esters, terpenes and ketones, which are produced by the mould. The fuzzy growth is often black or white but can appear in other colours depending on the substrate it is growing on. The growth consists of thread-like filaments—the hyphae. When actively growing, the mould will be damp and smear if brushed.

Fungal growth on objects may also be encountered in a dormant state. When dry, it may appear as a coloured stain, smudge or dirt. It may smear when brushed. Objects that exhibit signs of previous contact with water may contain fungal growth. If this growth has had a recent contact with water and crucial growth factors are present, it may become active again. Given enough time without critical growth factors, the mould hyphae may die. Dormant fungal growth may not emit the characteristic mouldy smell. Although dormant mould does not pose an immediate risk to an object, it may be a health risk to humans. It is important to note that mould does not lose its allergenic or toxigenic properties when it is dormant or non-viable.

One of the most obvious places to encounter mould is in a basement or in a below-ground-level storage area. Basements are usually damp, dusty and dark locations with stagnant air. These four conditions can make a basement or any area a likely habitat for mould to grow. Most often these locations are below ground level and have earthen floors, crawl spaces and cement or masonry-type walls that transfer water vapour. Basements are also prone to lower temperatures, higher RH and poor air circulation. Crawl spaces are usually poorly ventilated, which creates areas with chronic high humidity. It is important that properly screened, adequate venting be installed and maintained throughout the year. Venting should be open in the warmer months and closed in the colder months. Calculations to determine venting requirements can be found in ASHRAE 2013.

Fungal growth can also be found where there has been a saturation of water. Accidents, such as a leak from a broken pipe or roof, backed-up sewers, the failure of a heating, ventilation or air conditioning (HVAC) system as well as fire and floods can all elevate the RH and may initiate mould growth. A significant temperature drop can also increase the RH. Outside corners of buildings at roof or floor lines are susceptible to cooler temperatures, generating higher humidity. If these conditions are not dealt with quickly (within 48 hours), mould growth can begin.

Dirt and dust are hygroscopic (water loving) and a source of nutrients for mould. Their presence in the collection may increase the potential for mould to grow. Mould spores and mould fragments, such as hyphae and mycelium, settle out of the air at a rate determined by the size of particles. Spores can fall at the rate of 0.5 cm/s to 2.8 cm/s onto surfaces and become part of the dust and dirt (Mullins 2001, Tétreault 2003). Figure 7 shows the comparative sizes of various airborne pollutants.

Some storage conditions may contribute to mould growth on objects. Cardboard, wood, adhesives and sizes provide a suitable nutritional base for mould. Boxes, especially cardboard or wood sitting on cold floors or against exterior cement walls, absorb moisture and may support mould growth. Mould growth on the container may contaminate the contents of the container as well as the surrounding environment, including any objects in the area. Other examples of susceptible items include wooden furniture or paintings stacked against an exterior cement wall. It is not recommended to store objects directly on floors or leaning against exterior masonry walls.

Objects stored in archival-quality matboard boxes and paper and plastic enclosures are somewhat more protected against fungal growth than those stored in the open. For short periods of time, a box or another container made from a hygroscopic material, such as paper, cardboard or wood, can buffer the object from increased ambient RH. However, a prolonged period of elevated RH is enough to raise the a_w of the object inside the container to a point where it can support mould growth. Contact of the container (or the object inside) with water is a significant risk. If an object is in a well-sealed impermeable container (polyethylene bag or container), it is protected from sudden increases in ambient RH and from direct contact with water. It is crucial for the object to be dry when placed inside the container and not to have an equilibrium moisture content (EMC) capable of elevating the ERH in the container above 65%. If periods of elevated RH are prolonged, staff should regularly examine the items stored in enclosures (paper or plastic) for mould growth.

It is important to note that, eventually, those objects within polyethylene containers will reach equilibrium with the average outside ambient RH (Strang 2001), as polyethylene is not impermeable to water vapour, although it greatly delays its transfer.

Water-damaged wall from window leak — © Government of Canada, Canadian Conservation Institute. CCI 128876-0061
Figure 6. Water-damaged building materials.

Graph showing the particle size in μm of various materials — © Government of Canada, Canadian Conservation Institute. CCI 127508-0008
Figure 7. Graph showing the particle size in µm of various materials.

Description of Figure 7

The graph shows the comparative size of various airborne pollutants in particle sizes between 0.0001 and 1000 µm. The size for gas molecules is the smallest, between 0.0003 and 0.003 µm. Petrol and diesel combustion molecules range in size from 0.003 to 0.3 µm. Carbon black (soot) particulates range from 0.01 to 0.5 µm. Tobacco smoke particulates range from 0.01 to 1 µm. Sea salt, chloride particles range from 0.01 to 2 µm. Nitrate and sulfate compounds range from 0.02 to 0.7 µm. Oil smoke particulates range from 0.03 to 1 µm. Metallurgical dust ranges from 0.5 to 100 µm. Fungus spores range from 1 to 200 µm. Animal danders range from 1 to 3 µm. Cement dust ranges from 3 to 100 µm. Fabric lints and human danders have the same range for particle sizes: from 10 to 1000 µm. Human hair ranges in size from 30 to 200 µm. Dust visible to the eye ranges between 40 and 700 µm.

The first two columns of Table 1 follow Macher (1999) and illustrate potential sources of bioaerosol (mould, bacteria, viruses, etc.) contamination in buildings and the factors contributing to mould growth. The building engineer or maintenance person in charge of the building and its air-handling system should be aware of these potential trouble spots. The third column suggests programs or measures to prevent the problems from happening.

Table 1: maintenance of building and engineering systems
Source	Contributing factors	Preventive measures
Building exterior	intrusion of water through cracks, etc., improper grade near a building, rain gutters (blocked or drained too close to building), damage to exterior envelope (wall) pest intrusion (the majority of moulds are located in dirt and organic material on the ground—pests living in these areas carry mould spores on their bodies)	immediately repair all sources of water leaks from the outside maintenance and inspection programs for exterior building surfaces, etc. (rain gutters, door seals, etc.)
Outside air	nearby agricultural area, construction sites, composting operations, water treatment facilities	place air intakes so that they face away from source of contamination close windows and doors in direction of source of contamination
HVAC systems, air intakes	sources of bioaerosol composed of dead plant material, pest infestations, pest excrement, moisture from standing water, evaporating condensers and cooling towers adjacent to a building intake supply	regular inspections of air intake area and regular cleaning program
Filters	damp, improper fitting, low efficiency	regular maintenance and replacement program
Heat exchanger	poorly maintained (dirty, excessive water in pans, improper drainage, damp acoustical linings, stagnant water in humidifiers)	regular maintenance and regular cleaning program
Plenums and duct work	dampness, inaccessible humidifiers, dirty surface deposits	regular maintenance and regular cleaning program
Air diffusers	surface deposits, rusty (indicates a moisture problem), microbial growth, poor air mixing	regular inspection and regular cleaning program
Occupied space (water damage)	history of roof leaks, spills, plumbing problems, musty odours carpets always requiring cleaning, excessive humidity (>65%)	fix any leaks, spills and plumbing problems immediately
Constant condensation	poor insulation, poor vapour barrier and hot humid air from the outside condensing on air conditioned (cooler) surfaces causing moisture on windows—outside walls and cool surfaces and the opposite—warm, moist, interior air condensing on cooler envelope (exterior wall) material	use dehumidifier in conjunction with air conditioner in summer, lower humidity levels during winter
Window air conditioners	poor maintenance, dirty grills, standing water	properly sized for space, regular inspection and cleaning program (drip pans)
Potted plants	overwatering, mould growth on leaves, soil and plant containers	eliminate potted plants where possible, do not overwater plants
Carpets	previously water-damaged, dirty, poor maintenance, frequently washed without sufficient ventilation to dry quickly (spores are trapped by the electrostatic effect)	maintain regular cleaning program, replace or professionally clean previously water-damaged carpets, provide adequate ventilation for faster drying
Fabric partitions, drapes and furniture	previously water-damaged items, poor maintenance, dirty (spores are trapped by the electrostatic effect)	maintain regular cleaning program, replace or professionally clean damage items
Portable humidifiers and dehumidifiers	poor maintenance, drip pans	properly sized for space, regular cleaning and maintenance program

1.2 How to prevent mould growth in the collection

Because mould spores are everywhere in the air, it is impossible to completely eliminate them from a building, a collection and storage or exhibition areas. They circulate in the air, moving with the air currents, some falling on the surface of objects, floors, walls, ceilings and furnishings. They drift indoors through the air-handling system, open doors or windows and drop off people and materials coming into the building.

The most effective strategy to prevent damage to objects and to prevent adverse health effects for humans is to ensure that the environment and other conditions inhibit mould growth. This section focuses on ways to prevent fungal growth in a collection and a building.

Critical factors to control

Reducing indoor spores

Mould spores can only be reduced, not eliminated. The following actions can be taken to limit spores entering the indoor environment (Flannigan 2011):

closing windows and doors reduces outdoor spores by 2%
using central air-conditioning reduces outdoor spores by 5%
using electrostatic filtration reduces outdoor spores by 3%

Nutrient sources

When they are broken down into simple sugars, amino acids and small peptides, many objects provide suitable food for mould growth. Objects made from organic material or that have organic components, such as paper (cellulose, sizes and coatings), some media, book cloth, leather, basketry, wood, upholstery, cotton, linen, wool and photographic materials, are definitely food sources. Some inorganic objects may also be susceptible to mould because of their age and surface accretions of dust, dirt, insect debris and oils from handling. Where possible, remove nutrient sources that encourage fungal growth by cleaning and maintaining the collection as well as storage and display areas. Dust is a major source of fungal spores and nutrients.