Pollutants

Much of the information contained in this section is summarised from lecture notes (2006) and a publication by Jean Tétreault (2003).

Pollutants are compounds, either natural or man-made, that have damaging effects on cultural materials. They may emanate from:

• the materials associated with a collection, including storage and display containers and construction materials;
• the objects in the collection themselves; and
• the surrounding air.

Depending on the type of pollutant and the susceptibility of material types, pollutants have a variety of detrimental effects on objects including increased corrosion of metals and degradation of organic materials. Acetic and formic acids from wood products actively corrode lead and produce efflorescence on calcareous materials such as sea shells, corals and calcium-rich fossils. Ammonia gas produced from alkaline-type silicone sealants, concrete and cleaning products, induces blemishes on ebonite, efflorescence on cellulose nitrate and discolouration of photographic prints and artists’ colourants.

Some pollution-sensitive materials and the airborne pollutants that affect them most seriously are listed below (after Tétreault 2006).

Sensitive Material	Most Harmful Pollutant
Lead	acetic acid
Silver	hydrogen sulphide
Polyurethane magnetic tapes	water vapour, particles
Cellulose acetate	water vapour, acetic acid
Cellulose nitrate	water vapour, nitrogen oxides
Natural rubber	ozone
Colourants (alizarin crimson, basic fuchsin curcumin etc)	nitrogen and sulphur dioxides, ozone
Photographic gelatin, colour photo prints	water vapour

Storage and Display Materials

The most common sources of pollutants in storage areas are the materials used to store and/or display objects. To gain the greatest buffering effect against external environmental changes, it is desirable for objects to be stored in well-sealed containers. This is problematic however, unless the materials used in these containers are stable, of archival quality and neither they nor the object to be placed in the container emit harmful pollutants. Otherwise pollutant levels will build up to damaging levels in the container. Materials which are known to release harmful vapours at room temperature (modified after Padfield et al 1982) include:

• wood (particularly hardwood and composite boards);
• protein-based glues (except photographic gelatin);
• cellulose nitrate and cellulose acetate;
• highly plasticised polyvinyl chloride;
• polyvinyl alcohol;
• RTV silicones;
• polyurethane;
• rubbers containing sulphur vulcanising agents;
• wool and felt;
• some dyes which contain labile sulphur; and
• concrete

Materials which are considered to be ‘safe’ (modified after Padfield et al 1982) include:

• metals;
• ceramics and glass;
• inorganic pigments;
• polyethylene and polypropylene;
• acrylic paints and varnishes;
• polycarbonates;
• polystyrene and modified polystyrene such as high impact polystyrene and acrylonitrile/butadiene/styrene (ABS);
• polyesters and fibres including polymethyl methacrylate (Perspex, plexiglass), polyethylene terephthalate; and
• unbleached and undyed cotton and linen.

Materials such as chipboard, wood, certain polymers and paints can contribute to the deterioration of objects in a collection. Oak and cedar for example give off high amounts of acetic acid, promoting the corrosion of nearby metal objects, with lead objects particularly susceptible (Figure 8). It has been demonstrated that carbonyl pollutants such as acetic acid from wood are more prevalent in internal museum environments than in the external air with concentrations increasing in the following order:

Galleries < storage areas < display cases < storage cabinets

Data of this sort provides clear evidence of the use of inappropriate materials and the build-up of pollutants in more confined spaces.

Figure 8: A lead painted model battleship showing corrosion induced by acetic acid vapours given off by wood used in display case construction. 

Objects in direct contact with certain polymer storage files may become contaminated as plasticisers migrate from these polymers. Artefacts stored in highly plasticised polyvinyl chloride are particularly at risk owing to the migration of plasticisers and the possible release of corrosive hydrogen chloride from this polymer. Staining is likely for objects in contact with acidic papers and some paints and adhesives while malleable materials like plasticine and blu tack will transfer sulphur and oils respectively to any objects with which they come into contact.

The placement of objects in freshly painted wooden cupboards or rooms is also not recommended as the solvent fumes from the drying paint may affect the organic components of some objects (glues, resins, plastics, lacquers and the like). It is recommended that a newly painted, internal surface be allowed to cure for at least four weeks before use (see Appropriate Materials later in this section for additional recommendations). Use paints that contain low levels of volatile organic components.

Artefacts

The objects themselves may prove to be pollutant sources as they degrade. Cellulose nitrate movie films for example, not only release acidic oxidising gases but also pose a fire risk as they deteriorate. These materials must be stored in their own well-ventilated and preferably fire-proof area.

This in-built, intrinsic form of pollution is often much harder to deal with than either airborne pollutants or pollutants that migrate from materials in contact. It is most important therefore to understand the nature of the materials from which collection objects are made so that precautions can be taken for those with a pre-disposition to deterioration.

Airborne Pollutants

Airborne pollutants including dust, oxides of nitrogen and sulphur, hydrogen sulphide, peroxides, ozone, cooking fumes and airborne salt may arise from vehicle emissions, industrial activity or decaying organic matter.

Dust can be made up of a variety of materials including fine dirt, salt particles, oily mists, textile fibres and wood and metal powders. The precise nature of the dust and the objects on which it settles will determine its effect. Dust can promote both chemical and biological attack on objects. It is particularly damaging for magnetic media such as polyurethane sound and visual recording tapes. Dust causes damage to objects by:

• physical abrasion and discolouration;
• attracting moisture; and
• attracting biological pests such as insects, mould and bacteria.

The deposition of small particles on objects often initiates deterioration processes. Metal corrosion often starts after particles are deposited on a surface, oily residues can absorb potentially damaging pollutants and many deposited salts are hygroscopic. Finer particles are generally more damaging than coarser particles. Finely divided dust is harder to clean, particularly from fragile or complex porous surfaces and increases the risk of damage to an object during cleaning. Note that unsealed concrete is a source of alkaline dust.

A further issue is the perception that dirty objects create. If objects on display are coated with dust it gives visitors the impression that the collections are not being well cared for.

The amount and type of dust can be monitored quite simply by using standardised ‘sticky’ surfaces such as double-sided tape stuck to a glass microscope slide. After exposure these dust traps should be covered with a microscope cover slip to stop further dust accumulation prior to optical microscopic examination. Classification of particles (e.g. salt, soil, clothing fibres, soot etc) and estimations of quantities will allow remedial action to be taken to reduce further dust deposition.

Note that for some objects, water vapour and oxygen in the air must also be considered to be pollutants as they can cause material deterioration in susceptible artefacts.

Preventive Measures

The significance of any air pollution problem will be determined largely by the particular collection environment and its location, be it in an industrial, tropical, coastal or desert site. Each particular environment will have its own set of problems. Pollutants may be transported by moving air or by direct contact between objects which allows the transfer of damaging materials from one object to another.

Although reducing pollutant levels is not easy, preventive measures will help to reduce damage to objects. Preventive action may involve combinations of the following:

• good housekeeping;
• exclusion of pollutants;
• sealing of exposed surfaces such as concrete and timber walls and floors;
• the use of archival quality storage and display materials;
• incorporation of absorbent materials in storage containers;
• appropriate use of ventilation and air exchange;
• reducing chemical reactions by reducing relative humidity, temperature, light and oxygen; and
• using filtered air conditioning.

The Building and Storage Enclosures

As with buffering against changes in relative humidity, the first line of defence against externally generated pollutants is the building itself. As a rough rule of thumb and, as long as there are no significant indoor pollution sources, if the pollutant level outside is 100, it will be approximately 10 inside a room and 1 inside a box (Tétreault, 2003).

A simple air-tight enclosure, containing no noxious substances, is therefore the simplest way of isolating an object from damaging pollutants. Valuable objects that deteriorate in the presence of oxygen can be sealed in an air-tight container filled with an inert gas like argon. Note that the argon should be refilled on a regular basis. If an object is to be stored in an airtight container it is critical that neither the object nor the container are potential sources of damaging emissions and that relative humidity fluctuations are controlled within the container. It would be most unwise for instance, to store cellulose acetate and cellulose nitrate objects or acidic paper in well-sealed enclosures.

If objects are stored in boxes, it is a good idea to make a mylar ‘window’ in the box so that the object can be viewed without having to open it. As monitoring of an object’s condition is an important part of its on-going preservation, the presence of ‘windows’ in stacked boxes is invaluable, as are regular inspections of objects.

When considering the use of airtight enclosures take relative humidity and pest control factors into account.

Appropriate Materials

Boxes and supporting materials for objects should usually be made of archival quality materials. This may not be necessary for all objects however. If acidic newspapers are to be stored in cardboard boxes for example, the use of acid-free boxes will be much more costly and of little long-term benefit with respect to minimising acid development in the paper. Slightly acidic cardboard boxes could be used as long as the storage environment has a stable relative humidity (approximately 50 %) and the boxes are not exposed to direct wetness. See other chapters in this book and consult a conservator if in doubt about the type of materials that are best suited for storage enclosures for particular material types.

Use enamelled, galvanised or powder coated metal, stainless steel, acrylic or glass cabinets and shelves in preference to wood or chipboard for storage and display, particularly for enclosed spaces.

Wood and wood products should be avoided for storage containers, cupboards and display cabinets, especially for acid-sensitive objects. Some timbers, plywood and composite board products may be used however, depending on the type of wood, the presence and type of surface coatings, the type of resin/glue used in manufacture and/or the presence and type of laminating surfaces. Some examples of wood products suitable for storage and display systems include (after Tétreault 2006):

• plywood with phenol formaldehyde impregnated paper overlays (high density overlaid);
• plywood with ABS plastic laminates;
• plywood with phenolic laminates (e.g. Arborie, Formica);
• exterior grade plywood; and
• particle board without urea formaldehyde resins.

If laminated wood products are used for storage systems it is critical that all surfaces are completely sealed, particularly the ends and edges. As a general principle exterior grade products emit less damaging pollutants and are of better quality than similar products designed for interior use.

It is strongly recommended that if there is no alternative other than to use products such as plywood, chipboard or medium density fibreboard then all surfaces should be either laminated (as described above), coated with plastic or aluminium barrier foils or primed and painted with at least 2 top coats. Although this latter method will minimise emissions, pollutants released by the wood products will eventually diffuse through the paint or varnish layers. Oxidative paints, including oil-based polyurethane, alkyds and epoxy esters (epoxy in one can) should not be used to paint wooden surfaces into which artefacts are to be placed. Other paints, including two-pack epoxies and water-based polyurethanes are acceptable, subject to appropriate drying periods before use. Varnishes and water-based paints are not as effective as polyurethane lacquers as a coating barrier. If coated wood products are used for open shelving then four days drying time is sufficient. In enclosed spaces however, four weeks drying time is essential.

Ventilation, Sorbents and Filters

If it is not possible to avoid pollutants either by using archival quality storage materials or airtight enclosures then alternative strategies must be adopted. These may include the use of appropriate ventilation, sorbents or filters.

Sorbents effectively ‘soak up’ pollutants from the environment. They are generally very good at mitigating the impacts of externally generated pollutants and will therefore protect objects in well-sealed containers. They are not as good at protecting objects against high levels of internally generated pollutants (such as off-gassing from wood) as the sorbent will quickly become saturated. Ventilation may be a better option in the latter case.

Sorbent materials include activated carbon, activated charcoal cloth, zeolites, silica gel, activated alumina and specifically impregnated composites. Each of these materials has specific absorption characteristics and none of them will control all possible pollutants. Activated charcoal cloth for instance, is very good at absorbing acetic acid vapours but has minimal effect on hydrogen sulphide or nitrogen oxides. Some sorbents and the pollutants that they are most effective against are summarised below (after Tétreault 2003).

Sorbent	Most effective against
Activated carbon	acetic acid, ozone, sulphur dioxide
Activated charcoal cloth	acetic acid, ozone
Activated alumina (with potassium permanganate)	acetic acid, hydrogen sulphide, ozone, sulphur dioxide
Molecular sieves (zeolite)	water vapour
Silica gel	water vapour
Zinc oxide	hydrogen sulphide

A variety of sorbents are available that incorporate different impregnating materials. These composite sorbents have the ability therefore to control more potential pollutants. For example, in addition to having good sorption properties for acetic acid, ozone and sulphur dioxide, activated carbon containing potassium carbonate is also good at absorbing hydrogen sulphide and nitrogen dioxide.

A sorbent may be incorporated as a layer or liner in storage and display areas or even as a wrapping material around a sensitive object (as long as the object does not have to be visible). Although expensive, activated charcoal cloth can last up to ten years, depending on the level of pollution. Commercially available Microchamber™ papers and boards incorporate zeolites and activate charcoal and are capable of providing protection against both acidic and oxidising pollutants such as ozone, oxides of nitrogen and sulphur dioxide. These products afford much greater protection to artefacts than alkaline buffered boards and papers.

An often overlooked factor in controlling pollutant effects is the importance of relative humidity control, both to reduce its impact on susceptible materials (for which it must be considered to be a pollutant) and also to lower the rates of other pollutant-induced chemical reactions. Metals which are more prone to corrosion at elevated relative humidity levels benefit from storage or display in low relative humidity environments. Implement strategies as outlined in the earlier Relative Humidity and Temperature section of this chapter for those objects that can tolerate lower relative humidity conditions.

The effects of people and their clothing should also be considered when reviewing pollution sources. Bio-effluents (colloquially the fart), off-gassing, particularly from wet clothing and the loss of skin and cloth fibres can contribute to deterioration in susceptible objects in poorly ventilated spaces. Having a separate area for visitors to leave coats and limiting the number of visitors will reduce potential pollutants and increase visitor comfort.

Dust Control

Clean areas regularly and maintain buildings to minimise dust problems. Practical steps that can be taken to reduce dust in collections include the following:

• clean storage and display areas with a good quality vacuum cleaner (HEPA filter recommended) rather than a broom. This reduces the amount of dust that may be spread and/or resuspended during cleaning;
• use a damp cloth rather than a feather duster for exposed surfaces;
• use dust covers, closed containers and/or covered shelving to protect artefacts from dust contamination; and
• maintain building surroundings to minimise dust generation.

For objects in storage, protective wrapping with materials such as washed cotton or linen and/or acid-free tissue will provide protection against dust and some pollutants.

For museums, keeping a one metre gap between visitors and objects will result in approximately 90 % of human-generated dust (cloth fibres, human dander etc) being deposited before it reaches the objects. This is because larger dust particles drop closer to their source. Finer particles remain suspended in the air for longer periods and can be transported further by air circulation in a building.

Air Conditioning

Larger institutions, museums and art galleries often rely on heating, ventilation and air conditioning (HVAC) systems to maintain internal conditions that are both comfortable for visitors and appropriate for preservation of objects on display. In addition to providing basic temperature and relative humidity control HVAC systems can also improve air quality by filtering out particles and gaseous pollutants. As there are many different HVAC systems, filters, components and designs, consultation with specialist engineers is essential in order to match a system to a particular environment, building and its contents.

Sophisticated HVAC systems are expensive to set up, operate and maintain. Less costly alternatives, while not as effective, include the use of using portable filtration units in rooms and the careful placement of objects to reduce exposure to pollutants (away from open windows etc). Increased ventilation may help to dilute internally generated pollutants while restricted ventilation will help to exclude externally produced pollutants. The strategy to be used must be determined by a careful examination of the situation in question.

While the majority of this section has concentrated on the effects and alleviation of airborne pollutants, the impact of pollutants that migrate via direct contact must also be taken into account. Avoid direct contact between objects and coatings for instance. Archival lining materials such as acid-free paper or board, polyethylene, polypropylene, polyethylene terephthalate and mylar can be used to separate objects from shelving or other materials from which contaminants may migrate. Such separation may even be as simple as sealing in a polyethylene bag.

Monitoring of collections is an important part of on-going collection care. While it is desirable to be able to monitor pollutant levels in storage and display areas, this requires specialised equipment and is often costly to undertake. In reality, what often happens is that the objects themselves become the deterioration monitors - when an object deteriorates, the nature of its deterioration gives an indication of the likely cause, providing a guide to appropriate follow-up remedial action. The key is to regularly inspect objects in collections to ensure that preventive action can be taken as soon as possible.