Similar to the fungi, the composition of bacterial aerosols depends on the abundance and strength of sources, on dissemination factors, and on factors that act directly on the aerosol such as mixing, dilution, and particle removal. Outdoor bacterial aerosols usually are dominated by gram-negative leaf surface bacteria such as Pseudomonas species (Nevalainen et al., 1990). Aerosols near such sources as cooling towers may contain more exotic organisms such as Legionella pneumophila , the agent of Legionnaires' disease. Indoors, where environmental sources are absent, the bacterial aerosol consists primarily of gram-positive cocci that inhabit human skin and respiratory tract secretions (Nevalainen et al., 1990). When gram-negative rods become dominant in indoor air, it can be assumed that they have been emitted from an environmental (rather than human) reservoir. Concentrations of bacteria that constitute a significant risk for sensitizing or provoking human allergic reactions are unknown.
Exposure to bacterial allergens has been associated with work-related asthma, hypersensitivity pneumonitis, humidifier fever, and a disease resembling allergic bronchopulmonary fungosis. Symptoms classified as humidifier fever have been attributed to gram-negative bacterial aerosols, although it is not clear whether the disease results from exposure to endotoxin alone or from exposure to the adjuvant characteristics of endotoxin acting in conjunction with other allergens (Hood, 1989; Polla et al., 1988). Endotoxin causes some of the symptoms related to infections (i.e., fever, chills) and can cause the same symptoms when large quantities of the organisms (or large amounts of toxin) are inhaled.
Like the fungi, bacteria secrete enzymes that can act as allergens, and these enzymes are being found in an increasingly broader range of products and locations. For example, Bacillus species are being used to produce proteases that are added to laundry detergents for stain removal. When initially introduced, aerosols of these enzymes were implicated in outbreaks of hypersensitivity pneumonitis among workers who manufactured these detergents. In this unusual case, a threshold limit value was established in relation to the risk of an allergic disease, and ventilation controls that now allow enzyme-containing products to be manufactured without apparent risk of hypersensitivity pneumonitis have been introduced. Enzymes and spores from gram-positive bacilli and the thermophilic actinomycetes have been implicated in epidemics of hypersensitivity pneumonitis and work-related asthma (I. L. Bernstein, 1972; Dolovich and Little, 1972; C. L. Johnson et al., 1980; Perelmutter et al., 1972; Wiberg et al., 1972).