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Aerosols are sampled if it is felt that exposure to the particles may pose a risk to health. There is wide international agreement that such sampling should be carried out based on health-related particle size-selective criteria. Three fractions have been identified: inhalable (particles inhaled during breathing), thoracic (inhaled particles that penetrate beyond the larynx), and respirable (inhaled particles that penetrate to the alveolar region). These are intended as bases for defining the performances of sampling instruments used for exposure assessment. Most occupational aerosol exposures are characterized by particles not only covering a wide size range but also containing different chemical species. The health risk depends on both. With the emergence of the new particle size-selective criteria, we now, for the first time, have a rational basis for assessing health-related dose for multi-species aerosols. One of the first effects of the new particle size-selective criteria has been to stimulate the development of new sampling instrumentation. Meanwhile, new workplace studies have been conducted in selected nickel producing and using industries in which, recognizing the need for speciation, the inhalable fraction has been analysed with respect to the nickel contents of four groups of species (broadly defined as water soluble, metallic, sulfidic and oxidic) that may be associated with health effects. Such research poses new questions about how the information obtained can be used in realistic health risk evaluation.

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