The ISO and Acgih Standardized Size Fractions and their Relation to Human Deposition Data

Abstract

Abstract ISO and ACGIH have recently issued reports to establish particle-size selected exposure limit values. Both reports define an inspirable fraction, divided into sub-fractions corresponding to regions of the respiratory tract. Differences in basic approach are that all the ISO regional sub-fractions are constrained to add up to the inspirable fraction, so that one sampler can be used to collect all the fractions by sequential sub-division by aerodynamic diameter. The ACGIH sub-fractions in contrast are defined relative to the total ambient aerosol, and are not intended for sequential sub-division. The ISO report also considers the non-occupational environment, which is excluded from the ACGIH considerations. The ACGIH numerical recommendations for the inspirable fraction differ from the ISO above 30 μm, because the ACGIH used more recent data. Numerical differences for the other fractions occur mainly because the ACGIH values are defined relative to the total ambient aerosol. There are also differences of approach to determining whether an instrument matches the curves within acceptable error. As far as the fits to human deposition data are concerned, it is instructive to compare the recommended curves with the shapes of the deposition curves, as opposed to the absolute values. The chief conclusion is that the uncertainties in measured deposition curves are greater than the differences between the ACGIH and ISO recommendations.