Critical examination on particle leakage through the exhalation valve on a face mask

We experimentally evaluate the particle penetration through the exhalation valve induced by the airflow simulating the human respiration. The bi-directional jet flow in the range of 20 - 80 L/min is applied on the two representative valve types (square and circular) in three different conditions: exhalation, inhalation, and periodic flows. To mimic hazardous airborne particulate matters, we used Silicon particles (6 um in size). For each case, the airflow is measured by adaptive PIV, and the particle distribution is obtained using planar nephelometry. During the exhalation, the high-speed jet is generated through the flap opening, inducing entrainment of surrounding air and trapping particles inside the recirculation region under the jet. In the inhalation, reasonable protection from external pollutants is achieved except for the circular valve in lower flowrate. For the periodic flow, particle penetrates instant the flap closes in the square valve owing to the discrepancy in the response time of the particles and the flap to the airflow and for the circular valve, particle penetrates steadily over time due to the imperfect sealing.