Many industrial environments contain corrosive contaminants that can destroy expensive computerized process control equipment. These contaminants, if not properly controlled, can bring production to a standstill, resulting in downtime causing immense monetary losses. However, electronic controls can be replaced. This cannot be said of materials and objects kept in museums, libraries and archives, some of which may be thousands of years old. Once destroyed, their original condition can never be duplicated.
In museums and other “preservation environments” there are a number of factors which can cause the degradation of materials and artifacts. Among these are temperature, humidity, particulates and gaseous contaminants. Of these, gaseous contaminants are the most destructive.
While automotive and/or industrial emissions are considered as the largest contributors of the three main contaminant gases found throughout the industrialized world – sulphur dioxide, ozone and nitrogen dioxide, there are also many significant sources of internally-generated contaminants. Materials and activities associated with restoration and conservation laboratories, many artifacts and archival materials, and employees and patrons themselves can contribute to the overall contaminant load in preservation environments.
Although gaseous contaminants are of environmental concern, sources of gaseous contaminants, their introduction and migration through museums, and their interactions with artifacts are the least studied and least understood area of concern within museum environments.
Control Specification for Preservation Environments
The most commonly cited gaseous contaminants and their recommended control levels are shown in the following table. Background concentrations and the peak urban levels for these contaminants are listed for comparison.
As can be seen, the recommended levels for several contaminants are below the normal background levels and all are below contamination levels we would expect to encounter in urban environments.
Since there has been little research done to determine what levels actually cause deterioration of historical artifacts and archival materials, guidelines call for interior concentrations of gaseous contaminants to be maintained as low as attainable by gas-phase air filtration. This can be accomplished through the use of various dry scrubbing air filtration media employing the processes of physical adsorption and/or chemisorption.
In terms of gaseous contaminants, it has been determined that at least two different dry-scrubbing media will be required. One should be a potassium permanganate-impregnated alumina media for removal of nitric oxide, ozone, sulphur dioxide, hydrogen sulphide and formaldehyde. The other should be a caustic-impregnated activated carbon/activated alumina media for the removal of nitrogen dioxide, organic acids, and nitrogen and sulphur oxides. Both of these media remove gaseous contaminants by adsorption and chemical reaction, which irreversibly binds contaminants to the media, preventing their release back into the environment.
Also, particulates are one of the main factors which can cause the degradation of archival materials and historical artifacts. This is particularly true where temperature and humidity are not properly controlled. Therefore, particulate filtration must also be part of any contaminant control system for preservation environments.
Air Purification Methods for "Preservation Environments"
The optimum filtration system for museums will address as many of the potentially offending materials as possible – gaseous and particulate. The recommended system would consist of (1) a 30% ASHRAE rated pre-filter ; (2) a bed of caustic impregnated media ; (3) a bed of potassium permanganate-impregnated alumina media ; (4) a 90-95% ASHRAE rated final filter.
For controlling gaseous contaminants infiltrating in the Preservation room from outside air
Provide the above mentioned purification module at the fresh air inlet of the air-conditioning circuit to cleanse the outside air of contaminants. Due considerations have to be incorporated in the existing air-conditioning system for matching the air flows and the additional pressure drops.
For controlling gaseous contaminants generated from within the Preservation room
Provide stand-alone powered re-circulation air scrubbers, of the above configuration designed for at least 6 air treatment cycles per hour, inside the room.