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 impactor approaches 100% when the aerodynamic diameter is greater than the d50 [Hinds 1982]. Aerodynamic diameter (dae) is defined as the diameter of a hypothetical sphere of unit density ( p = 1 g/cm3) that has the same settling velocity as the particle [Hinds 1982]. Impactors are selected so that the desired size particles will be collected. For the same aerosol sample, the mass and count particle distributions will have distinctive means and medians; however, they share the identical geometric standard deviation. The mass median aerodynamic diameter (MMAD) is descriptive of the mass distribution. In other words, the MMAD equals the diameter where particles larger than MMAD contribute half the collected mass; and those particles smaller than MMAD contribute the other half. The count median aerodynamic diameter (CMAD) is the median of the number of particles in a given particle distribution. Investigators are likely to employ stationary cascade impactors or individual impactors used in survey instruments, either as the primary collection mechanism, or as a preclassifier (for example to remove nonrespirable particles from the sampled air stream). Cascade impactors consist of a stack of impaction stages: each stage consists of one or more nozzles and a target or substrate. The nozzles may take the form of holes or slots. The target may consist of a greased plate, filter material, or growth media (agar) contained in petri dishes. Each succeeding stage collects smaller particles. A filter may be used as the final stage so that all particles not impacted on the previous stages are collected. The target may be weighed to determine the collected mass, or it may be washed and the wash solution analyzed. Filters may induce more particle bounce than greased or oiled plates. Although personal cascade impactors are available, these devices are not as widely used in personal sampling for bioaerosols as are filters [Macher and Hansson 1987]. Impactors used for the collection of airborne microorganisms may have range from a single slit to more than 400 holes per stage. The particles impact onto growth medium with one or more bacterial or fungal colonies forming at some impaction sites. Multiple particles, each containing one or more organisms, passing through a single hole may be inaccurately counted as a single colony. As the number of organism-containing particles deposited onto the growth medium increases, the probability that the next organism-containing particle will impact a "clean" hole decreases. The basic formula for the coincidence correction follows:

Pr

N

1 N

1 ( N 1)

1 ( N 2)

1 ( N r 1)

Andersen [1958] and Leopold [1988] stated that Pr is the estimated culturable particle count given r culturable particles are observed on the sample plate, and N is the total number of holes per impactor stage. For example, if 75% of the holes have received one particle, the chance that the next particle will impact a "clean" hole is one in four (25%) [Andersen 1958; Andersen 1984; Leopold 1988; Macher 1989]. c.

Filtration Collection of particles from a nonbiological aerosol sample is most commonly achieved by filtration. Filter media are available in both fibrous (typically glass) and membranous forms. Deposition occurs when particles impact and are intercepted by the fibers or surface of filter

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NIOSH Manual of Analytical Methods