AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Numerical Simulation of N95 FFR Efficiency Relative to Aerosol Size Distribution
PATRICK O'SHAUGHNESSY, Joel Ramirez, University of Iowa
Abstract Number: 51 Working Group: Control Technology
Abstract The “95” in the designation “N95” of a filtering facepiece respirator (FFR) refers to the lowest efficiency provided by the respirator occurring at any one airborne particle diameter. It therefore does not indicate the percent reduction in aerosol concentration; a quality of a respirator that is referred to as a protection factor. The objective of this study was to use numerical simulations to determine a protection factor that occurs without leaks for an N95 subjected to aerosols derived from a range of lognormal size distributions. The simulations were also conducted to determine those factors that influence FFR efficiency relative to breathing resistance. Using the best available equations that model the performance of an N95 FFR, including fiber charging effects, an overall model of N95 efficiency across a wide range of particle sizes (1 nm to 100 micrometers) was developed. This effort resulted in an efficiency curve relative to particle size for a typical N95 FFR with a minimum efficiency of exactly 95%. With that curve, the mass of all particles collected by the filter could be computed to determine a protection factor of the FFR. The protection factor was determined for 42 spherical, unit-density aerosols with mass median aerodynamic diameters (MMAD) ranging from a 2 – 14 and geometric standard deviations (GSD) from 1.6 - 3.6. The worst-case aerosol occurs when then the MMAD is low and the GSD is high. This results in a wide distribution of small particles that overlap the N95 efficiency curve in the region where it dips to 95%, which occurs between 0.01 – 0.3 micrometers. Regardless, this FFR has a PPF of 99.86% for all mass and 99.79% for respirable mass for the lowest MMAD (2) and highest GSD (14) analyzed. Dusts with MMAD’s > 4 micrometers had protection factor values above 99.96% regardless of GSD.