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 existing devices used for other purposes, e.g., the 10-mm nylon cyclone and the 37-mm cassette, without the benefit of current testing technology and understanding of particle behavior. More recently developed aerosol samplers were designed to either maximize the information gained regarding aerosol concentration levels or to minimize any inherent losses associated with the sampler design. Thus, each sampler design may have been based on some of the following criteria: inlet efficiency, classifier accuracy, cassette assembly (bypass leakage), electrostatic losses, particle deposition uniformity, collection media stability, sampler surface losses, and sampler field comparisons. These criteria are discussed in the following sections of this chapter. 2. INLET EFFICIENCY OF THE SAMPLER An important review of sampling theory and practice was compiled in a book by Vincent [4]. The inlet efficiency of several samplers has been evaluated including thin-walled tubular inlets [5], a cyclone [6], an asbestos sampler [7], total aerosol sampling cassettes [8], and inhalable aerosol samplers [9,10]. All samplers have an inlet efficiency that varies as a function of particle aerodynamic diameter, inlet velocity, inlet shape and dimensions, dimensions of the body it is attached to, external wind velocity, and external wind direction. The aspiration efficiency of an aerosol sampler can be defined as

where cs is the concentration of particles passing directly through the plane of the sampling orifice and c0 is the ambient concentration. This is true for aerosol samplers for which the entire amount of aerosol that enters the plane of the sampling orifice is quantified, as is the case for the IOM sampler (SKC, Inc., Eighty Four, PA). However, it is important to note that many commercially available aerosol samplers use only the aerosol collected on the filter that is housed in the sampler, while any particles deposited on the inner surfaces upstream of the filter are disregarded. Performance of this type of sampler is therefore best characterized by its sampling efficiency, in which the aspiration efficiency is modified by the particle sizeselective losses prior to the filter. The aspiration or sampling efficiency of a particular aerosol sampler, whichever is the most relevant, expressed as a function of particle aerodynamic diameter, is the primary index of sampler performance and should match the appropriate health-based criterion (e.g., the inhalability criterion) so that it may be said to provide a valid health-based assessment of personal aerosol exposure. As an aerosol is being sampled, the large-particle trajectories are more affected by external flow fields than those of small particles. Thus, the shape, orientation, and inlet flow field will be most critical for inhalable aerosol sampler inlets; they will be less important for thoracic samplers and unlikely to be important for respirable samplers, except at very high wind velocities. The flow field near the inlet of a sampler is different when the sampler is mounted on a person (or in laboratory simulations using a mannequin) than when it is freestanding. Therefore, it has been recommended that measurement of inhalability be determined from mannequins in wind tunnels [11-14]. Flow field studies using mannequins in wind tunnels indicate that the air is slowed down by the body, resulting in an enrichment of large particles in the upstream side of the body [15]. When intended for use at low wind speeds (less than about 1 m/s [200 ft/min], representing most indoor workplaces), it may be possible to test the

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NIOSH Manual of Analytical Methods