Viable allergens in this post to provide the reader with an understanding of the potential sources and growth requirements needed for enhancement and amplification of allergenic microbes. The principal allergen in each case is thought to be airborne spores with inimal concern for the associated growth structures. The two major categories are fungi and thermophilic actinomycetes. The microbes that have been excluded from this list of allergenic microbes are either pathogenic or have not been reported as probable allergens.

Viable allergens, Molds and other fungal

Viable Microbial Allergens. Contrasted with the hardiest of microbes, molds not only can stay alive indefinitely on inanimate objects, or fomites, but they can grow into and destroy wood, cloth, fabrics, leather, twine, electrical insulation, and many other commercial products. They destroy lenses of microscopes, binoculars, and cameras. In places where humidity is high, fungi damage wood structures, telephone poles, railroad ties, and fence posts. Most of these problems are reduced by means of artificial preservatives. Sometimes the trouble starts in forests where fungi invade the heartwood and cause wood rot before the timber has had a chance to be cut down. The humid Amazon rainforest is one such example.

Thousands of products are treated to prevent decay, yet there are some types of fungi that thrive on preservative‐treated wood. One example is creosote‐treated railroad ties. Other fungi‐specific nutrients are vinyl wall covering adhesives, gypsum board, cellulose‐based ceiling tiles, dirt retained within carpeting, and surface paints. Some fungi feed on plywood. Others consume the glue used to laminate wood that is used in airplanes, furniture, and cars and will cause the layers to separate. Books and leather shoes are readily consumed by the microbes that are visible as mildew.

Aircraft electrical systems, operating in tropical climates, require protection against insulation‐consuming molds. Immune‐suppressed individuals (e.g., AIDS patients) can also be host to normally nonpathogenic molds. Exterior molds grow on decayed organic material, corn, wheat, barley, soybeans, cottonseed, flax, and sun‐dried fruits. They consume other plants, vegetable matter, and decayed organic material (e.g., dead animals).

Pathogenic molds parasitize and obtain nutrients from a host. The host may be plant or animal, and these molds vary slightly from the nonpathogenic molds both in environmental and sampling requirements. Pathogenic molds parasitize and obtain nutrients from a host. The host may be plant or animal, and these molds vary slightly from the nonpathogenic molds both in environmental and sampling requirements.

All molds require moisture activity in order to grow and reproduce. Most require a moisture activity in excess of 80 percent, but some do quite well at levels as low as 60 percent. The latter are referred to as xerophytic (dry‐loving) fungi. Whereas Stachybotrys requires considerable amount of moisture for growth and reproduction (91–94% relative moisture), Aspergillus restrictus does well where there is less water activity (65–75% relative moisture). Mold that requires low moisture activity is inhibited by high moisture activity.

Temperature preferences are variable as well. Although most molds do well at room temperature, some flourish at near freezing temperature (e.g., refrigeration), and some thrive at temperatures in excess of 100°F (e.g., hot tubs). Although most spores favor moderate temperatures, the investigator should be aware of the potential for growth and amplification of molds in just about any temperature setting.