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Evaluation of rebreathed air in human nasal cavity with N95 respirator: a CFD study
The carbon dioxide level within N95 respirator is higher than without it, which needs a mechanical explanation. In the current study, we built a three-dimensional (3D) model of normal human nasal cavity to simulate the volume of fraction of both fresh air and respired air within the nasal cavity. The model consists of large rectangular domain outside the nasal cavity representing ambient air, human nasal cavity and partial of the pharynx. Two cases were simulated. Case I refers to a human face with a N95 respirator onto human face, and case II refers to a human face without a respirator. Respiration cycles of both low (7.5 L/min) and medium (15 L/min) flow rates were simulated using Low Reynolds Number k-ω turbulent model. All the air entered the external box during inspiration was assumed to be fresh air, and entered the pharynx during expiration was assumed to be respired air. The results show that above 60% of inspired air is respired air in case I compared to less than 1.2% in case II. During expiration, the volume of fraction (VOF) of respired air in both cases was above 95%. The streamlines at peak inspiration were relatively smooth while entering the cavity in both cases; while at peak expiration large vortex was observed within the air space between human face and respirator in case I. In conclusion, the N95 respirator trapped respired air within the respirator which increased the VOF of respired air during inspiration. This might be one of the major contributors to elevated carbon dioxide level while wearing N95 respirator.
https://doi.org/10.15761/TEC.1000106
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