A model for correcting Workplace Protection Factors for lung deposition and other effects

The Workplace Protection Factor (WPF) is a measure of the protection provided by an industrial respirator against a challenge agent. It is traditionally defined as the ratio of the ambient contaminant concentration (Co) in a worker's breathing zone to the in-facepiece contaminant concentration (Ci) that occurs during inhalation, and is determined by simultaneous concentration measurements during the time the respirator is worn. There are several sources of particulate loss that result in the overestimation of the true WPF. A model is presented to "estimate" these losses so that an adjusted or "unbiased" WPF can be calculated. This model requires three measurements: Co, Ci, and the ambient aerodynamic mass frequency particle size distribution (PSD). Both Co and Ci are expressed in units of "mass per unit volume." There are four steps to the calculation of the unbiased WPF. First, the in-facepiece PSD is estimated using the ambient PSD and a particle leak penetration curve. Second, the fraction of the in-facepiece PSD that will deposit in the lung is estimated using the in-facepiece PSD and a "reference worker" total lung deposition curve. Third, the fraction of the in-facepiece PSD that will deposit at the inlet of the sampling probe during both inhalation and exhalation is estimated using the in-facepiece PSD, the exhaled in-facepiece PSD, and published inlet deposition data. Last, the adjusted in-facepiece concentration is calculated using the estimates from steps two and three. The adjusted WPF, WPFa, is then calculated as the ratio of the measured ambient contaminant concentration and the adjusted in-facepiece concentration.