Letter to the Editor: Comments on a Recent Protection Factor Study

A recent article published in the Journal of Occupational and Environmental Hygiene (JOEH) reported on workplace protection factors (WPFs) for the 3M 8210 and 8110S filtering facepiece respirators (FFR) against dusts and biological particles. (1) The authors reported three results that are contrary to other research: (1) WPFs increased with increasing particle size, (2) WPFs were higher for dusts than for biological particles, and (3) the WPF results suggest the assigned protection factor (APF) of 10 is too high for FFR. A WPF is defined as a measure of the protection provided in the workplace, under the conditions of that workplace, by a properly selected, fit tested, and functioning respirator while it is correctly worn and used. (2) That is, the Co/Ci ratio is measured in the workplace. WPF studies should be conducted with analytical methods that are specific and with a Co concentration that are at least 10 times the mean blank value for half facepiece respirators to obtain meaningful results. (3) Briefly, in this study, Co and Ci samples were collected with identical sampling trains. A piece of tubing went from the sample inlet into a sampling chamber where the sample stream was split into two. An isoaxial sample line drew air to an optical particle counter to collect one sample; the remainder of the sample was collected on a filter for analysis. According to the authors, the sample chamber was designed for isokinetic sampling. The optical particle counter (OPC) samples measured both nonbiological and biological particles inside and outside the respirators. All the particles were classified by their optical size to calculate WPFs by particle size. The biological particles that were collected by the second sample stream were either cultured to identify specific organisms inside and outside the respirators or examined by microscopy. These results were used to calculate biological WPFs. The OPC is an inappropriate instrument for the measurement of WPFs. It is a nonspecific, direct-reading instrument. OPC readings are based on the amount of light scattering caused by the specific particles sampled. As the authors note, the shape and color of particles (in addition to their size) can affect the OPC response. In addition, the optical diameters the OPC measures differ from particle aerodynamic diameters, which more accurately characterize the way airborne particles behave. The accuracy of the sampling system depends on isokinetic sampling through the OPC sample line. The operation of the sampling system was described in a related article. (4) The total flow through the sampling chamber is reported to be 10 L/min. Based on a chamber diameter of 1.27 cm, the average sample velocity is 131 cm/sec. The OPC sample line flow rate is reported as 2.8 L/min. With a sampling line diameter of 0.32 cm, the average velocity through the OPC sample line is 580 cm/sec. Based on average velocities, the OPC sample line velocity is four times higher than the velocity through the sample chamber. If the flow is laminar then the flow will have a parabolic profile; the velocity at the wall is 0 and the velocity at the center is maximum. Flow velocity across the tubing is calculated by the equation V(r) = (p1 − p2)(R 2 − r2) 4ηL