Gaseous Pollutants in Exhibits
- Assess the objects for their sensitivity and specific exhibition needs. Design the exhibition based on this preservation assessment, including appropriate use of space that includes factors such as building environmental and exhibition conditions. This may also determine case requirements such as how airtight to make the case (see TechNotes section: Sealing of Exhibit Cases).
- Select stable materials and products for case construction and finish layers. Based on the above assessment, avoid materials known to interact with the collection items due to emitted hazardous gases, becoming acidic, and/or losing physical or chemical stability with age. (see: Choice of Appropriate Materials for Exhibition, Storage and Transport) Testing may be required during the iterative material selection process. (see related section??)
- Use selected materials in ways that minimize the potential for gaseous emissions. This includes applying appropriate coatings, sealants, or separating layers, and allowing appropriate drying and aeration for materials such as paint and adhesives. Ideally a drying period of 4 weeks is recommended before using exhibit cases, and 1 or 2 weeks of aeration for the exhibit gallery prior to installation. Note that although there is little data for aeration of exhibit spaces (such as individual galleries or rooms), an appropriate drying and aeration period is recommended to evacuate volatile organic compounds (VOCs) from paints, coatings, sealants, or even materials listed as low volatility compounds.
- Monitor pollutants inside and outside the case. Assess the air quality within the museum and inside exhibition cases to ensure that the conservation criteria for the exhibit are fulfilled throughout the lifetime of the exhibition/display cases [especially if cases are re-used regularly]. (see TechNote: Monitoring Pollutants Inside and Exhibit Case).
Gaseous pollutants in a museum originate from both external and internal sources. Volatile organic compounds (VOCs) such as sulfur and nitrogen oxides, hydrogen sulfide, chlorine compounds, and ozone are some potentially harmful pollutants commonly found in the exterior environment. Construction and design materials may also emit harmful vapors such as organic acids and sulfur compounds. Organic materials (such as textiles, paper, and animal skin products) and inorganic materials (such as silver metals, silver-based photographic images, shell, and some minerals) are susceptible to deterioration by corrosive airborne pollutants.
Control of gaseous pollutants becomes a very serious conservation concern for materials that are particularly sensitive to low concentrations of volatile organic compounds (VOCs). These materials include those that are highly vulnerable to acids; metals such as lead, silver, and their alloys; solder; and shell or other materials containing calcium carbonate. Sometimes, well-sealed exhibit cases pose dangers; even a small amount of continuous offgassing can raise concentrations to hazardous levels in confined spaces.
The U.S. Environmental Protection Agency provides information on the concentrations of outdoor pollutants in a city or region. Gaining access to air quality information is helpful because it is the easiest method to identify outdoor pollutants that may be present in an exhibit area. The concentration of gaseous pollutants can be measured by quantity (in parts per million or parts per billion) or by weight (such as micrograms per cubic meter). Recommended permissible levels of pollutants within a museum environment vary, but generally they fall below 1 part per billion (ppb).
Because dust attracts and holds pollutants, particulate filtration automatically captures a certain amount of gaseous pollution. For this reason, upgrading HVAC filtration in a museum or exhibit space is the first defense against gaseous pollutants. For sensitive collections or in areas that are highly polluted, the additional step of incorporating activated charcoal or potassium permanganate filters into the environmental system is often required. Such filters should reduce sulfur and nitrogen dioxides to a level below 10 micrograms per cubic meter of air. Air from areas within the museum known to generate pollutants-including exhibition preparation areas such as spray booths and printing facilities-should be prefiltered or exhausted directly out of the building.
Good air circulation usually reduces the tendency for pollutant concentration to occur. A high rate of unfiltered airflow across the surface of an object is not recommended because it increases the total exposure to potential pollutants. Because elevated temperature and relative humidity accelerate the evolution of gaseous pollutants, maintaining a moderate climate is beneficial. Wide fluctuations in relative humidity can cause localized condensation that results in elevated pollution levels and reaction with object surfaces.
The exhibit design and production process can control the exposure of objects to dust and pollutants. As with other conservation issues, the location and placement of objects can be an important design tool for limiting the effects of pollutants. Follow these general guidelines:
- Isolate objects from direct contact with exhibit construction material with a neutral layer or film, fabric, or cushioning foam, or elevate objects with mounts.
- Avoid enclosing objects inside cases with chemically unstable materials.
- Locate sensitive objects away from doorways, air return ducts, walls that experience a temperature gradient, and other locations that have rapid airflow.
- Provide air space between collections and known pollutant sources (concentrations of pollutants are highest near a contaminant).
- Avoid creating pockets of still air in which pollutant levels can concentrate (for example, long runs of cases that extend from floor to ceiling).
Some objects are particularly sensitive to the chemical processes initiated by airborne pollutants. To provide the highest level of protection, enclose these objects in well- sealed cases or frames that include a pollutant absorbent. The option of a ventilated case with filtered vents, as well as a sealed case, is discussed in more detail in the section on Exhibit Case Design.
Choice of Exhibit Construction Materials
Some materials used to construct or decorate exhibit cabinetry have proven to be chemically unstable. Damage can occur when exhibit objects are in direct contact with unsuitable and unstable substances, such as wood, fugitive paints, soluble dyes, and oils. Damage also results from the offgassing of vapors from volatile organic substances, including acids, formaldehyde, and solvents. Many materials contain additives and unspecified chemical reagents. (see: Choosing Materials for Storage, Exhibition & Transport and Exhibit Case Construction Materials)
Offgassing often occurs when a material is new and it may continue throughout the life of the material, some- times increasing over time (see Jean Tetrault Tech Bulletin 32 and Guideline 13). Problematic additives and parent materials include:
- Acidity from low-quality mat boards, many paper and wood products, fabrics, paints, and adhesives
- Chemically unstable plastics
- Unreacted monomers and solvents from otherwise stable compounds
- Additives that, over time, migrate out of a plastic and deposit onto the objects
- Surface finishes such as fire-retardant products in textiles and ultraviolet radiation absorbers in plastics
When selecting construction and finishing materials, consider their stability. Because manufacturers may change the composition of commercial products, be aware that a product used in a previous application may no longer be acceptable. Seek technical information before use and note that each batch may be different (test as necessary).
Base the initial selection of materials on information gathered from:
- Be sure to check product name, manufacturer, and chemical components to confirm when you are investigating or purchasing a specific material (Note that generic materials can have very different properties depending upon how they were manufactured)
- Results of previously tested and accepted exhibit materials (add link to AIC Materials Testing Wiki)
- Manufacturer's product literature
- Safety data sheet (SDS), which lists basic components and safety precautions for the product's use
- Information from the manufacturer's technical department
Commercial testing services are available, and conservation laboratories can perform some less complex tests. Many materials can be eliminated from consideration before the testing phase.
Problem materials can sometimes be used safely in small quantities in large, open exhibit areas with good air circulation and filtration. Material stability is particularly critical, however, when large quantities of pollutants are used throughout the exhibit space or within a sealed case.
Before objects are installed, all surfaces and construction materials must be absolutely dry and fully cured; paints, adhesives, and caulks generally require a minimum of three weeks to dry. Aerating the overall exhibition space and the exhibit cases before object installation allows some of the highest levels of pollutants to dissipate. Newly painted surfaces should be allowed to cure totally before objects are installed. Heaters can be used to speed the curing process. For highly sensitive collections or after major construction, it may be advisable to purge the entire exhibit or the cases using a portable air filtration unit, which can generally be procured locally.
Sensitive objects can also be enclosed in a ventilated display case that incorporates a pollutant sorbent such as activated charcoal or potassium permanganate. Such a case provides a low risk environment for sensitive collections. Pollutant sorbents also prevent tarnishing of polished metal surfaces.
Guideline: Mitigating Pollutant Hazards
Appropriate measures [design, hardware and policies] must protect objects from exposure to pollutants
OR: The exhibit design must provide exhibit objects the required level of protection from exposure to pollutants
Pollutants come in the form of either particulates or chemical contaminants. These can corrode or abrade museum objects and even harbor insects and mold.
Particulate pollution—the “dust” in the air around us—can be generated by industrial emissions, vehicle exhaust, construction soil, mold, wear and tear of fabrics, etc. It is drawn into a building through natural ventilation and the environmental control systems. It is also generated by the day-to-day activities of the museum, such as visitor traffic and exhibit construction. Particulate matter is therefore a complex mixture, often including airborne soil, carbon soot, textile fibers, microorganisms, and protein materials. This combination can be very damaging to collection objects since it is abrasive, attracts moisture, and encourages insects, fungi, and mold.
Chemical contaminants in a museum also originate from both external and internal sources. Sulfur and nitrogen oxides, hydrogen sulfide, chlorine compounds, and ozone are some of the potentially harmful chemical pollutants commonly found in the exterior environment. Within the museum, construction and design materials may emit chemical vapors such as formaldehyde and other volatile organic compounds. These chemical vapors can react with collection objects, causing corrosion…[Any problems from chemicals used for cleaning or pesticides?]
Strategies to reduce pollutants must prevent pollutants entering the museum from the outside environment and eliminate sources of pollutants within the museum itself. A variety of tools is available: hardware and controls that filter the air; design strategies that prevent the build-up of pollutants and avoid use of polluting materials; and museum policies that reduce pollutants within the museum environment.
Elevated or fluctuating relative humidity and temperature can interact with pollutants to exacerbate their ill effect on object health. Therefore, efforts to control pollutants must include management of relative humidity and temperature. Utilizing the appropriate hardware in the design of the exhibit space is an important strategy for moderating pollutants, RH and temperature.
Best Practice: Hardware and controls that reduce particulates and chemical pollutants are utilized
How can the air handling system be used to reduce particulates and chemical pollutants?
The following strategies work most effectively if the exhibit space is properly sealed:
- Filter the air. Air filtration can provide protection from particulates at a variety of levels, depending on the severity of the pollution problem and the sensitivity of the collections. At a minimum, the air entering the museum’s heating and cooling systems should be filtered to remove particulate matter larger than 1 micron. Recirculated air should also be filtered.
- Upgrade the HVAC System. Dust attracts and holds pollutants. Therefore, particulate filtration will also capture a certain amount of chemical pollution.
- Incorporate activated carbon or potassium permanganate filters into the system in areas that are highly polluted or to protect sensitive collections. Such filters should reduce sulfur and nitrogen dioxides to a level below 10 micrograms per cubic meter of air.
- Prefilter or exhaust polluted air directly from the building. Air from areas within the museum known to generate pollutants—including exhibition preparation areas such as spray booths and printing facilities—should be prefiltered or exhausted directly out of the building.
- Provide good air circulation. [How is good air circulation provided?] This helps prevent the concentration of pollutants. However, avoid a high rate of unfiltered airflow across the surface of an object because it increases the total exposure to potential pollutants.
- Adjust the air handling system to create a positive room pressure to further limit the influx of potentially polluted external air.
- Reduce the amount of air taken in from outdoors to improve the efficiency of the filtration. The system must be operated within the minimum requirements set by the American Society of heating, refrigeration, and Air-conditioning Engineers (ASHRAE).
- Maintain a moderate climate. Elevated temperature and relative humidity accelerate the evolution of chemical pollutants; therefore maintaining a moderate climate helps with pollutants control. Additionally, wide fluctuations in temperature and RH can cause localized condensation that results in elevated pollution levels and reaction with object surfaces. (See Standard 19 for regulating temperature and RH).
- Use localized filtration equipment as needed. If improving filtration throughout the museum is not feasible, consider using room-sized units in a confined space such as an exhibit preparation area, an exhibit space during construction, or a finished exhibit. Some commercial units remove more than 99% of particulates down to a size of 0.3 microns; many also incorporate an activated carbon prefilter that removes chemical pollutants.
What air filters are appropriate for use in exhibits?
|Pollutant||Particle size (in microns)||Filter Efficiency|
|Soil dust||100 - 1||N/A|
|Coal dust||100 - 1||N/A|
|Household dust||5 - 0.01||Panel filters (10)|
|Insecticide dust||9 - 1||N/A|
|Tobacco smoke||5 - 0.01||Media filters (0.5)|
|Carbon particulate||1 - 0.001||HEPA filters (0.01)|