Modern Industrial Hygiene, Volume 2: Biological Aspects. Perkins JL ed., Cincinnati, OH: American Conference of Governmental Industrial Hygienists, Inc., 2003 Apr; 2:329-381
Microorganisms such as molds and bacteria are normal inhabitants of the environment; they exist in the soil we walk on, in the air we breathe, the water we drink, and even the food we eat. The saprophytic varieties (those utilizing the organic products of other organisms, living or dead, as a food source) generally predominate. Under the appropriate conditions (optimum temperature, pH, sufficient moisture, and available nutrients), saprophytic populations of bacteria and molds can increase in number, or amplify. In the air, they can exist as discrete particles (e.g., as individual fungal spores and bacteria, or their aggregates). These organisms can also aggregate, attach to other particles, form aerosols, deposit on surfaces or growth substrates. Many microbial species have been documented to tolerate various environmental extremes of temperature and pressure allowing them to acclimate even to the most harsh conditions. Certain bacteria, called extremophiles, have been identified in the lower depths of the ocean growing amidst intense volcanic activity. Mycobacteria have been found in mummified tissues. It is clear that microorganisms (fungi and bacteria) are ubiquitous occupants throughout all of the diverse environments of this world and are continual companions on the roads that we travel. Many species of microorganisms are beneficial to, if not a necessary part of, the environment and to our very existence. Saprophytic molds and bacteria are composters of organic material and are essential actors in the carbon and nitrogen cycles. Many essential nutrients are recycled through microbial actions. Microbial flora in our gastrointestinal tract aid metabolism, and help protect us from pathogens. Humans have made use of microbes over time. Yeasts have been used for millennia to produce alcoholic beverages and to leaven bread. More recently, the metabolic systems of specific microbial populations have been used to produce pharmacologically important products (e.g., penicillin and citric acid), food products (e.g., amylase), and a number of other chemical 330 Modem Industrial Hygiene agents. Unique uses of the microbial community include the application of bacterial species in the decontamination of hazardous substances (i.e., oil spills in the ocean) and as an insecticide (i.e., Bacillus thuringiensis) to control cabbage worm, cotton boll worm, and chicken louse. Our everyday encounters with most members of the saprophytic community generally do not pose a significant health risk to the immunocompetent worker population (healthy adults). However, select occupational environments present unique exposure concerns due to the nature of the microorganisms encountered, the microbial concentrations observed, and the susceptibility of the exposed population. Within the healthcare industry, attention has focused on human infections or infectious agents of which the obligate parasites and facultative saprophytes (including the primary pathogens and opportunistic pathogens) are the primary concern (Burge, 1989). In recent years, bioaerosols (the term given to microorganisms and/or their products entrained in/on airborne particles) have become prominent safety and health issues in agriculture, biotechnology, industrial settings, and more recently, the non-industrial indoor environments. Much of the concern for these types of exposures has focused on the ability of certain microbiological species to elicit allergic or inflammatory responses in susceptible individuals.