A field study was conducted at a range of worksites at a number of plants, for two companies, in the primary nickel production industry. The aim of the field study was to provide direct measurements of the distributions of both particle size and relevant nickel species groups (soluble, sulfidic, metallic and oxidic), and to use the data to characterize health-relevant aerosol exposures of workers at the worksites in question. Since there was no commercially-available sampler that could provide samples of sufficient quantity to enable chemical speciation for the species fractions of interest, as well as particle size distribution information, new instrumentation was developed, based on a modified version of the Andersen cascade impactor, incorporating a porous foam media top stage that produced particle classification over the upper end of the inhalable range (Kerr, Vincent and Ramachandran, Annals of Occupational Hygiene, 2001, in the press). This modified-Andersen sampler was used in extensive field studies, along with other sampling instruments, including, in particular, the IOM personal inhalable aerosol sampler. The results of the field study provided results that could be represented succinctly in terms of the distributions of the four nickel species groups and the three health-related particle-size fractions: inhalable, thoracic and respirable. They showed that, for practical purposes, the distributions of the four nickel species groups were consistently uniform across the full range of particle-size distribution. For the purpose of characterizing the distribution of nickel species for each worksite, this finding enabled incorporation of additional data taken at those same sites using the IOM personal inhalable aerosol sampler. This yielded a data set sufficient to permit the development of exposure 'fingerprints' for each worksite. Such 'fingerprints' make it possible for the occupational hygienists at the plants in question to estimate workers' exposure to individual nickel species fractions based on measurement only of overall inhalable nickel. In addition, by reference to the differing threshold limit values (TLV) for the various nickel species, the 'fingerprints' also allow the individual worksites to be characterized by hazard indices (in the form of 'equivalent sulfidic fractions', or ESFs) that reflect the weightings of the risks experienced by workers at each worksite.