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 The NIOSH REL is for TWA diisocyanate exposures up to 10-hours per workday, and the ACGIH TLV is an 8-hour TWA exposure. The NIOSH RELs-ceiling limits and ACGIH short term exposure limits (STELs) are based on 10- and 15-minute TWA exposures, respectively; and should not be exceeded during the work shift. The OSHA ceiling limit is a concentration that should never be exceeded during a workday. OSHA does not have a full shift, TWA PEL for any of the diisocyanate species. The State of Oregon OSHA has promulgated occupational exposure standards for HDI-based polyisocyanates of 0.5 mg/m3 as an 8-hour TWA, and a ceiling limit of 1.0 mg/m.46

5. AIR SAMPLING AND ANALYTICAL ISSUES The measurement of isocyanates in air is a challenging sampling and analytical problem for several reasons. Isocyanates can exist in air as vapor, or as aerosol having a wide range of particle sizes. Isocyanates are very reactive, hence, unstable. There are many different chemical species, even in the same air sample, that need to be quantified. Pure analytical standards are not available for the vast majority of isocyanate species, and qualitative standards (bulk products) do not account for isocyanate species generated during polyurethane formation or breakdown. Finally, to measure isocyanates at levels corresponding to current monomer exposure limits, analytical methods must be very sensitive. Because of the complex problems associated with accurate sampling and analysis of total isocyanate group in air, existing methods have limitations. To assess these limitations and to make rational decisions in choosing methodologies or making improvements to existing methodologies, it is useful to break down the sampling and analysis process, chronologically, into discrete steps. Each of these steps can be examined individually for the likelihood of isocyanate losses or the introduction of errors. The sampling and analysis of isocyanates can be logically divided into six steps: collection, derivatization, sample preparation, separation, identification, and quantification. Each of these can result in losses of isocyanate or the introduction of other errors. In addition to the issue of accuracy, options for these steps differ in terms of convenience, simplicity, and speed. All of these have to be considered in choosing sampling and analytical methodology. Table II summarizes current OSHA and NIOSH sampling and analytical methods and lists the factors involved in selecting the most appropriate method for a given workplace environment. a. Collection Collection is the removal of the isocyanate species from the air sample into a portion of the sampler amenable to subsequent analysis. A generalized isocyanate sampler would have to be able to collect both vapor and aerosol of widely varying particle sizes. Mechanisms of collection of isocyanate vapors include dissolution into a solvent (e.g., impingers) or adsorption onto a sorbent. In some cases, successful collection of isocyanate vapors may depend on reaction with a derivatizing reagent to create a nonvolatile derivative. The aspects of the sampler governing collection efficiency of particulate isocyanates are, of course, independent of the fact that these particles contain isocyanate groups. Collection errors can be divided into three types: (1) Aspiration errors: The efficiency with which particles enter the sampler inlet is the aspiration efficiency. The concentration of particles entering the sampler may be biased relative to the concentration outside the sampler or relative to human inhalation efficiency.

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