Particulate pollutants

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Particulate pollutants, such as soot, dirt, and dust abrade, soil, and disfigure materials. Dust and dirt that have absorbed gaseous pollutants from the air become sites for harmful chemical reactions when they settle on library material. Particulate pollutants can also aid mould growth. Modern library material, such as magnetic and optical media, are very sensitive to dust and dirt.

Dust is commonly a mixture of fragments of:

human skin
minute particles of mineral or plant material
textile fibres
industrial smoke
grease from fingerprints, and other organic and inorganic materials.

There are often salts such as sodium chloride (carried in from sea spray or on skin fragments) and sharp gritty silica crystals.

In this chemical mixture are the spores of countless moulds, fungi, and micro-organisms which live on the organic material in the dust (fingerprints, for example, serve as good culture media).

Much of the dirt is hygroscopic (water-attracting), and this tendency can encourage the growth of moulds, as well as increase the corrosiveness of salts, hydrolysis, and the release of acids.