Assessment of particle size distributions in workers' aerosol exposures

A full description of the regional dosimetry of inhaled workplace aerosols requires information about the particle aerodynamic size distributions of inhaled aerosols for appropriate health-related species. Therefore, since the late 1970s, interest has risen in how to assess workers' aerosol exposures in terms of particle fractions which may be inhaled and penetrate down to different parts of the human respiratory tract. The personal inhalable dust spectrometer (PIDS) was proposed as a versatile sampling instrument for assessing workers' exposures along these lines. Raw data from this cascade impactor-based instrument are obtained initially in terms of discrete particulate masses collected on the individual impactor stages. Recovery of the desired particle size distribution is difficult because the mathematical inversion problem is 'ill-posed' (i.e., there are many possible solutions that would be consistent with the observed masses). However, a routine has been developed that involves considerations of the detailed deposition characteristics of the impactor stages, measurement errors and a priori assumptions about the shape of the expected particle size distribution (e.g., log-normal). These differ from other such routines in that account is taken not only of aerosol deposited at the impactor stages but also that collected inside the entry system. Hence the final result covers the whole inhalable range, for particles up to and exceeding 100 microns in aerodynamic diameter. The PIDS has recently been used in studies in several industries to examine inhalable particle size distributions for individual exposed workers. This paper considers the results of such studies, and discusses how they may be used to provide information not only about the inhalable, thoracic and respirable fractions identified in the latest particle size-selective sampling criteria (upon which it is expected that future aerosol standards will be based), but also other health-related fractions that may be defined numerically (including, for example, alveolar deposition fractions for workers with different breathing patterns relevant to the type of work undertaken).