In the mid-1990s, the International Organization for Standardization (ISO), the Comite Europeen de Normalisation (CEN), and the American Conference of Governmental Industrial Hygienists (ACGIH ) agreed on common definitions for health-based sampling conventions for the inhalable, thoracic, and respirable aerosol fractions.(1-3) The physiological data on the relevant knowledge of howlarge a fraction of particles (of different sizes) penetrate into and/or deposit in various compartments of the human respiratory tract particle were summarized by Soderholm. (4) These sampling conventions are plotted in Figure 1. The sampling convention for inhalable dust was based on research on mannequins equipped with breathing machines and filters that collected the aerosol aspirated through a "mouth." The experiments were carried out in wind tunnels for wind speeds in the range 1-9 m/sec, and the mannequins were rotated during the experiment.(5,6) The range of wind speeds was based on what was of interest to the mining industries. The aspiration efficiency of the rotated mannequin with respect to particle size can be called the "inhalability," but this term is not used in any of the three standards referred to above. Based on the experiments, "inhalability" was defined as direction averaged and was to be independent of wind speed in the range 1-4 m/sec. The convention was truncated at 100 Ám, as the underlying experiments for larger particle sizes had turned out to be exceedingly difficult to perform. Neither the "Inhalable" curve in Figure 1, nor "Thoracic" or "Respirable" depicts the spread of the underlying data. Most data on inhalability are found within a band of +/- 0.10.