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 collecting a bioaerosol and culturing the collected particulate. Only culturable microorganisms are enumerated and identified, thus leading to an underestimation of bioaerosol concentration. Nonviable microorganisms are not living organisms; as such, they are not capable of reproduction. The bioaerosol is collected on a "greased" surface or a membrane filter. The microorganisms are then enumerated and identified using microscopy, classical microbiology, molecular biological, or immunochemical techniques. When sampling for culturable bacteria and fungi, the bioaerosol is generally collected by impaction onto the surface of a broad spectrum solid medium (agar), filtration through a membrane filter, or impingement into an isotonic liquid medium (water-based). Organisms collected by impaction onto an agar surface may be incubated for a short time, replica-plated (transferred) onto selective or differential media, and incubated at different temperatures for identification and enumeration of microorganisms [Tortora et al. 1989]. Impingement collection fluids are plated directly on agar, serially diluted and plated, or the entire volume of fluid is filtered through a membrane filter. The membrane filter is then placed on an agar surface and all colonies may be replicaplated. Culturable microorganisms may be identified or classified by using microscopy, classical microbiology, or molecular biology techniques such as restriction fragment length polymorphic (RFLP) analysis. Classical microbiology techniques include observation of growth characteristics; cellular or spore morphology; simple and differential staining; and biochemical, physiological, and nutritional testing for culturable bacteria. Analytical techniques which may be applied to both nonviable and viable microorganisms, but not distinguish between them, include polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA). Such methods may be used to identify specific microorganisms and to locate areas of contamination. Though these latter methods are generally qualitative, current research efforts involve modifying the methods to obtain semiquantitative or quantitative results. 2.

PRINCIPLES OF BIOAEROSOL COLLECTION a.

General Principles Most aerosol sampling devices involve techniques that separate particles from the air stream and collect them in or on a preselected medium. Impaction, filtration, and impingement are three common sampling techniques used to separate and collect the bioaerosol.

b.

Impaction Impaction is used to separate a particle from a gas stream based on the inertia of the particle. An impactor consists of a series of nozzles (circular- or slot-shaped) and a target. Perfect impactors have a "sharp cutoff" or step-function efficiency curve. Particles larger than a particular aerodynamic size will be impacted onto a collection surface while smaller particles proceed through the sampler [Hinds 1982]. Marple and Willeke [1976] have reported that high velocity, inlet losses, interstage losses, and particle reentrainment affect the performance characteristics of an impactor. The cut-diameter (d50) is a function of the Stk50. In other words, the mass of the particles smaller than the d50 that are collected equals the mass of the particles larger than the d50 that pass through the impactor. The collection efficiency of the

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NIOSH Manual of Analytical Methods