The American Conference of Governmental Industrial Hygienists (ACGIH) , the International Organization for Standardization (ISO), and the European Standardization Organization (ComitÚ EuropeÚn de Normalisation, CEN) have adopted identical particle size-selective sampling conventions for inhalable, thoracic, and respirable aerosols. The purpose of these conventions is to provide a scientific basis for a new generation of particle size selective occupational exposure limits (OELs) for aerosols. Such OELs can therefore be matched to the relevant sites of aerosol deposition after inhalation into the respiratory tract, and in turn to the health effects of interest in a given exposure assessment. These sampling conventions are used throughout this manual unless otherwise specified. The criteria presented in this manual are used to determine the most appropriate aerosol sampling equipment. The type of sampling to be performed determines which criteria are important for estimating the adequacy of the sampler and determining aerosol concentration levels. For example, "total dust" samplers generally do not have a size selective particle classifier preceding the filter media and fall under the inhalable sampling convention. Alternatively, sampling for regulatory or voluntary compliance with aerosol exposure standards usually requires greater accuracy, increased efficiency, size-specific selectivity, and good analytical precision. Furthermore, regulations may require the use of a specific sampler and sampling conditions to standardize sampling results (eliminate bias) and reduce uncertainty among laboratory reports. See Chapter P, Measurement Uncertainty and NIOSH Method Accuracy http://www.cdc.gov/niosh/docs/2003-154/pdfs/chapter-p.pdf
for further discussion on standardization and aerosol measurement error. Over the past two decades, researchers have pointed out strengths and weaknesses with several types of aerosol samplers. Some of these samplers were adapted from 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.