Air pollutants fall into two categories, both of which can harm cultural heritage materials: particulate matter and gases.
Particulates include all kinds of organic and inorganic materials that find their way into the air - fibers, soot, dust, pollen, and even skin cells. These particulates can collect on materials and cause abrasions, attract insects, or create a substrate for mold growth. Some particulates are acidic, and further accelerate harmful chemical reactions.
Gases include sulfur dioxide, ozone, and nitrogen oxides, many of which come from the outside environment. Indoor pollutants include formaldehyde and volatile acids that off-gas from carpets, furniture, cleaning supplies, and paint. Gaseous pollutants can initiate chemical reactions that lead to deterioration, especially in conjunction with high relative humidity. For example, you may sometimes see books with pages that are discolored around the edges, where exposure to pollutants is higher.
A phenomenon known as silver mirroring can be seen in the upper left corner of this photograph, where oxidation-reduction has caused the movement of silver elements within the photograph, obscuring parts of the image. Atmospheric pollutants can be one cause of the oxidation part of this process.
While cellulose is the backbone of paper, collagen is the backbone of leather. Red rot, the characteristic breakdown of leather as seen in this photo, occurs when leather absorbs sulfur dioxide from the environment. The sulfur dioxide is oxidized and hydrolyzed to form sulfuric acid, which then sets hydrolysis of the tannins in motion.
When our new preservation storage environment was created, we installed high-grade air filters in the HVAC system. Our filters are rated MERV 14, which is one of the highest grade filters available, often used in medical facilities. These filters remove 75-85% of 0.1 to 3 micron diameter particles and greater than 90% of larger diameter particles, including pollen, smoke, mold spores, and dust. For comparison, the diameter of human hair ranges from 20-200 microns. We also designed the system so that it has minimal air exchanges with outside air, thereby reducing the number of pollutants introduced into the preservation environment.
LET’S BE GREEN
What’s good for cultural heritage materials is also good for the environment! All products and finishes in the preservation space, including flooring, paints, and shelving, were evaluated both for their preservation qualities as well as for environmental sustainability. Selecting materials with low emissions keeps harmful toxins out of the environment in addition to protecting the materials.