A mathematical model for correcting Workplace Protection Factors (WPF) for lung contaminant deposition and other effects was developed. WPF was calculated as the ratio of ambient contaminant concentration (Co) that the worker would inhale when not wearing the respirator, to the in facepiece concentration (Ci) that existed during inhalation as a sole result of the equipment. The model for adjusting conventional WPF for lung and sample deposition losses was developed and adjusted WPF values (WPFa) calculated using the model were compared with conventional WPF values for various particle size distributions. Three measurements required were Co, Ci, and the ambient aerodynamic mass frequency particle size distribution. The proposed WPF calculation model was detailed with equations. The assumptions required in the calculations were discussed. Other aspects considered in detail included in facepiece particle size distribution, sampling probe losses, lung deposition losses, reference worker definition, and methods. Results showed that differences between the WPFa calculated using the model and conventional WPF were least biased when total lung deposition was least, at a geometric mean particle size of mass frequency distribution (GM) of about 0.5 micrometers. As GM varied about this GM value, lung deposition increased and WPFa decreased. The maximum differences between low and high particle penetration curves were - 4% and 15% relative to the reference worker. Results from adjustments for inlet deposition showed that these losses could be ignored. The effect of sampling biases of +/-50% showed that the greatest difference between conventional WPF and WPFa was for a mass distribution with a geometric mean of 10. Here, the WPFa was four times that of WPF. When a positive sampling bias was introduced, the WPFa exceeded 100 for geometric means less than 5 micrometers. The authors conclude that Ci is underestimated in conventional WPF calculations, and that the model presented provides an improved estimate of the true WPF.