Evaluation of N95 respirator use with a surgical mask cover: effects on breathing resistance and inhaled carbon dioxide

Ann Occup Hyg. 2013 Apr;57(3):384-98. doi: 10.1093/annhyg/mes068. Epub 2012 Oct 29.


Objective: For pandemic influenza outbreaks, the Institute of Medicine has recommended using a surgical mask cover (SM) over N95 filtering facepiece respirators (FFRs) among healthcare workers as one strategy to avoid surface contamination of the FFR which would extend its efficacy and reduce the threat of exhausting FFR supplies. The objective of this investigation was to measure breathing air quality and breathing resistance when using FFRs with US Food and Drug Administration-cleared SM and without SM.

Methods: Thirty National Institute for Occupational Safety and Health (NIOSH)-approved FFR models with and without SM were evaluated using the NIOSH Automated Breathing and Metabolic Simulator (ABMS) through six incremental work rates.

Results: Generally, concentrations of average inhaled CO(2) decreased and average inhaled O(2) increased with increasing O(2) consumption for FFR+SM and FFR-only. For most work rates, peak inhalation and exhalation pressures were statistically higher in FFR+SM as compared with FFR-only. The type of FFR and the presence of exhalation valves (EVs) had significant effects on average inhaled CO(2), average inhaled O(2), and breathing pressures. The evidence suggests that placement of an SM on one type of FFR improved inhaled breathing gas concentrations over the FFR without SM; the placement of an SM over an FFR+EV probably will prevent the EV from opening, regardless of activity intensity; and, at lower levels of energy expenditure, EVs in FFR do not open either with or without an SM.

Conclusions: The differences in inhaled gas concentrations in FFR+SM and FFR-only were significant, especially at lower levels of energy expenditure. The orientation of the SM on the FFR may have a significant effect on the inhaled breathing quality and breathing resistance, although the measurable inhalation and exhalation pressures caused by SM over FFR for healthcare users probably will be imperceptible at lower activity levels.

MeSH terms

  • Air Pollution, Indoor / analysis*
  • Carbon Dioxide / analysis
  • Filtration / instrumentation
  • Humans
  • Influenza, Human / prevention & control*
  • Inhalation / physiology*
  • Masks / statistics & numerical data*
  • Occupational Exposure / prevention & control
  • Oxygen / analysis
  • Pandemics
  • Particle Size
  • Respiration
  • Respiratory Protective Devices / statistics & numerical data*


  • Carbon Dioxide
  • Oxygen