Thoracic particulate mass fraction criteria (relative to total airborne particles) for size-selective sampling. Individual data points are observed human head penetration efficiency during oral inhalation for an inspiratory flow rate of 43.5 L/min, i.e., light exercise [5–7]. As stated by ACGIH , the sampling criterion is offset to the right of experimental data to overestimate the amount of exposure to the lower respiratory tract, i.e., the lungs, and correspondingly to provide a greater level of protection for exposed workers. From ACGIH®, Particle Size-Selective Sampling in the Workplace, Report of the ACGIH® Technical Committee on Air Sampling Procedures. Copyright 1985. Reprinted with permission. Courtesy: Dr. Otto G. Raabe.