Maintaining humidity at a constant level is an attainable goal. Pockets of dead air should be eliminated, as these make the regulation of relative humidity and temperature levels more difficult and promotes the growth of biological agents, mold, and insects. Both gaseous and particulate air pollutants are harmful to collections. Gases such as sulfur dioxide, hydrogen sulfide, nitrogen oxides, and ozone are absorbed from the air into the paper. These gases react with the moisture in paper and in the atmosphere to produce acids, which increase the rate of paper deterioration, break down the fiber structure of paper, and cause embrittlement.
Particulate pollution is made up of dust, ash, smoke, dirt, and mold spores.
Particulate pollution abrades, disfigures, and obscures text and can also increase acidity and the presence of mold spores. The most common method of safely removing gaseous pollutants uses filters of activated charcoal, or aluminum impregnated with potassium permanganate. One vendor for these products is Purafil. The aluminum pellets initially cost more than activated charcoal, but they last longer. Established standards for gaseous pollutants specify levels of ten micrograms per cubic centimeter for sulfur dioxide and two micrograms per cubic centimeter for ozone.