AUTOMATED BREATHING AND METABOLIC SIMULATOR (ABMS) EVALUATION OF N95 RESPIRATOR USE WITH SURGICAL MASKS

Objective: To reduce the threat of exhausting N95 filtering face piece respirator (FFR) supplies during pandemic influenza outbreaks, the Institute of Medicine has recommended using surgical mask covers (SM) over FFR among healthcare workers as one strategy to avoid surface contamination of the FFR. The objective of this investigation was to measure and evaluate breathing air quality (average inhaled CO2 and O2 concentrations), peak inhalation (InPr) and exhalation (ExPr) breathing pressures, and average inhaled dry-bulb (Tdb) and wet-bulb (Twb) temperatures when using FFR with FDA-cleared SM and without SM. Methods: Thirty NIOSH-approved FFR models with and without SM were evaluated using the NIOSH Automated Breathing and Metabolic Simulator (ABMS). The ABMS protocol consisted of the following levels of O2 consumption, CO2 production, and minute ventilation performed consecutively for minimum of five min each (units in STPD): 0.5, 0.4, and 9.8 L∙min-1; 1.0, 0.8, and 25.3 L∙min-1; 1.5, 1.3, and 38 L∙min-1; 2.0, 1.9, and 62 L∙min-1; 2.5, 2.5, and 70 L∙min-1; and 3.0, 3.15, and 80 L∙min-1, respectively. Results: The mean across all FFR without SM (FFR-alone) for average inhaled CO2 and O2 ranged from 2.7% and 17.1%, respectively, for the lowest metabolic rate to 1.7% and 19.2%, respectively, for the greatest metabolic rate. The mean across all FFR with SM (FFR+SM) for average inhaled CO2 and O2 ranged from 3.0% and 16.7%, respectively, for the lowest metabolic rate to 1.9% and 18.9%, respectively, for the greatest metabolic rate. The mean across all FFR-alone for InPr and ExPr ranged from -5 and 7 mmH2O, respectively, for the lowest metabolic rate to -41 and 24 mmH2O, respectively, for the greatest metabolic rate. The mean across all FFR+SM for InPr and ExPr ranged from -7 and 8 mmH2O, respectively, for the lowest metabolic rate to -51 and 30 mmH2O, respectively, for the greatest metabolic rate. The mean across all FFR-alone for Tdb and Twb ranged from 29 to 27°C, respectively, for the lowest metabolic rate to 32 and 28°C for the greatest metabolic rate. The mean across all FFR+SM for Tdb and Twb ranged from 29 to 27°C, respectively, for the lowest metabolic rate to 33 and 30°C for the greatest metabolic rate. When grouped by respirator type and compared to FFR-alone, average inhaled CO2 concentration was significantly higher for cup FFR+SM and significantly lower for horizontal flat-fold FFR+SM. Reciprocal significant changes were observed for average inhaled O2 concentrations. ExPr was significantly higher for cup FFR+SM at V.O2 >1.0 L∙min-1. InPr was significantly higher for cup FFR+SM at all levels of energy expenditure, and higher for other flat-fold FFR+SM at V.O2 >1.5 L∙min-1. Tdb and Twb was significantly higher for cup FFR+SM at V.O2 >0.5 L∙min-1. Conclusions: The orientation of the SM on the FFR may have a significant effect on the inhaled breathing quality at lower levels of energy expenditure and breathing pressures at higher levels of energy expenditure. The measureable InPr and ExPr caused by SM on FFR for healthcare users likely will be imperceptible at lower activity levels. While statistically significant, the changes in Tdb and Twb for FFR+SM compared to FFR-alone were small.