During the 1980s, North American archivists became very interested in pollutants; it was also at this time that central ventilation systems began to offer better regulation of air quality.³ The list of controlled pollutants increased and even included substances produced within museums, such as acetic acid and formaldehyde. Standards based on technical performance proliferated: these standards prescribed the pollutant levels to be maintained by filtration systems, called for using the best available technology, or stipulated minimal pollutant levels. This encouraged a race to achieve the lowest pollutant levels. The lower the maximum allowable concentration of pollutants, the more prestigious the institution (to the great profit of the filtration industry!). This approach became increasingly removed from any correlation between pollutant levels and their impact on collections. In other words, the cost-benefit ratio became difficult to justify, indeed even to define.

In the meantime, a number of research projects throughout the 1980s and 1990s sought to better characterize the effects of pollutants on materials. All that remained was to analyse, standardize, and compile the data, and make use of them for the purpose of preserving collections. The approach that has been employed at CCI is similar to that used in risk management. It is based mainly on the concept of dose (the concentration of the pollutant multiplied by the duration of exposure) at which the first signs of deterioration caused by a pollutant are measurable in a material. In the jargon of risk management, this dose is called the “lowest observed adverse effect dose” (LOAED). In accordance with the principle of reciprocity, for a given dose of a pollutant on an object, it is possible to calculate the exposure time required before signs of damage appear. For example, basic fuchsin (a green dye) first begins to turn a lighter colour at a dose of 10 micrograms of sulphur dioxide per cubic metre for one year (10 µg m^-3 yr). Therefore, if we want to prevent this slight colour change for a period of 10 years, the average concentration of sulphur dioxide must be reduced to less than 1 µg m^-3 (1 µg m^-3 x 10 years = 10 µg m^-3 yr).

Determining an acceptable rate of deterioration due to pollutants is the responsibility of the collection manager. This is not a matter of adhering to strict standards, but rather using a set of guidelines to choose a suitable preservation level based on the objectives and resources of the institution. Each institution can then take into account the benefits and costs of a control strategy along with their other preservation priorities. A detailed list of acceptable doses for various pollutants and materials is provided in Airborne Pollutants in Museums, Galleries, and Archives: Risk Assessment, Control Strategies, and Preservation Management.⁴ Although it is possible to determine the acceptable concentration of each pollutant for each type of object, this approach can be time consuming and expensive. Therefore, many museums will prefer to establish the maximum concentrations of the most harmful pollutants for a collection of average sensitivity. Table 1 presents the maximum concentrations of key pollutants for various preservation targets (a similar version of this table will appear in the 2003 ASHRAE Handbook⁵). However, it must be noted that these guidelines are not appropriate for hypersensitive materials [e.g. cellulose acetate, cellulose nitrate, certain dyes (such as alizarin crimson, turmeric yellow and basic fuchsin), lead, natural rubbers, silver, and polyurethane magnetic tapes], which are very sensitive to certain pollutants and represent special cases.

Table 1. Air quality targets for museum, gallery, library, and archival collections

To assist in setting reasonable preservation targets, Table 2 provides targets that are attainable in various locations and under various conditions. This table is particularly useful when it is not possible to accurately measure pollutant levels, which often is the case. One of the greatest challenges in controlling pollutants is the high cost of pollutant concentration analyses; a complete program of analyses is often unaffordable for small or medium-sized institutions. Most of the published data dealing with pollutant levels in museums and archives were obtained with the help of government subsidies or with the collaboration of a conservation institute or a university with a scientific interest in the subject.

Table 2. Potential preservation targets for most collections

To attain a given preservation target — e.g. a 10-year period without evidence of damage for most of the items in a collection in a given location — it will be necessary to devise a strategy aimed at reducing the concentration of each pollutant and maintaining it at the designated level. Often, the museum will have to develop a series of specifications, i.e. a list of specific descriptions in terms of the performance requirements of the building’s structural elements, equipment, materials, and steps to be taken. It will always be easier to comply with a set of specifications than to meet either a preservation target for a collection or a maximum pollutant level. With respect to enclosures (such as display cases and storage cabinets), the specifications are generally easy to apply and effective; for example, “Substances used for the manufacture of the display case must be sulphur-free (according to the lead acetate test)” or “No oil-based paint (oxidative polymerization).” For rooms, the specifications may include stipulations for such things as the type of filter with which the ventilation system should be equipped. However, with rooms it is much more difficult to predict the effectiveness of the specifications. This is due to the fact that pollutant levels in a room are affected by so many different parameters (such as the activities in the room, various sources of pollutants in the room, and infiltration of outdoor pollutants).

It is nearly 40 years since Garry Thomson first voiced his concern, but a better overall understanding of the effect of pollutants in museums is gradually becoming a reality.

1. Thomson, G. “Air Pollution: A Review for Conservation Chemists.” Studies in Conservation 10 (1965), pp. 147–167.
   
2. Thomson, G. The Museum Environment. Second edition. London: Butterworths, 1986, pp. 268–269.
   
3. Wilhelm, H. The Permanence and Care of Color Photographs: Traditional and Digital Color Prints, Color Negatives, Slides, and Motion Pictures. Grinnell: Preservation Publishing Company, 1993, p. 563.
   
4. Tétreault, J. Airborne Pollutants in Museums, Galleries, and Archives: Risk Assessment, Control Strategies and Preservation Management. Ottawa: Canadian Conservation Institute, 2003. [This book covers in detail the challenges of pollution control in museums.]
   
5. American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE). “Museums, Libraries, and Archives.” Chapter 21 in Heating, Ventilating, and Air-Conditioning: Applications. Atlanta: ASHRAE, 2003. [This handbook is a reference for heating, ventilation, and air-conditioning engineers in North America and Europe.]