Inhalation of airborne microorganisms — and their metabolites — may cause a wide range of respiratory problems and symptoms. Mucous membrane irritation, coupled with a dry cough and eye irritation, are common responses to continuous exposure. Bronchitis and chronic pulmonary disease, while typically associated with smoking, may also be related to such allergic reactions, although the role of airborne microorganisms hasn't been determined. Allergic rhinitis and asthma, particularly among those who are constitutionally predisposed to allergies, is perhaps one of the most severe responses to mold exposure.

Even more potentially toxic are mold glucans — constituents of fungal cell walls — which can cause immune reactions, resulting in mucous membrane irritation, headache, muscular pain, cough, and chest tightness. Mycotoxins are poisonous secondary metabolites of fungi, chiefly produced by Fusarium, Penicillium, and Aspergillus — all of which are common to buildings and collections. Toxicity by inhalation can be 40 times greater than by ingestion.

Although health experts, HVAC engineers, and industrial hygienists all agree that mold isn't a good thing to have around, there aren't clear indoor limits. John Lacey sums up the current issues regarding air quality standards:

There are many problems to setting standards for air quality in buildings: not all organisms are equally allergenic or harmful to health; constitutional predisposition to allergy in the exposed person may be more important than the concentration of spores to which they are exposed; numbers out of doors frequently exceed those indoors although the species differ; occurrence of specific microorganisms indoors may indicate a constructional problem but not necessarily a health hazard; the hazards of some microbial products are, at present, little understood.

He notes considerable range in the guidelines proposed in the literature — one author, for example, suggests the recommended indoor limit should be set at one-third of the outdoor level, with the same species spectrum — but none are directly applicable to a museum, library, or archive setting. The accompanying table reveals comparative mold levels for indoor and outdoor air.