Exhaled air dispersion during coughing with and without wearing a surgical or N95 mask

Abstract

Objectives: We compared the expelled air dispersion distances during coughing from a human patient simulator (HPS) lying at 45° with and without wearing a surgical mask or N95 mask in a negative pressure isolation room.

Methods: Airflow was marked with intrapulmonary smoke. Coughing bouts were generated by short bursts of oxygen flow at 650, 320, and 220L/min to simulate normal, mild and poor coughing efforts, respectively. The coughing jet was revealed by laser light-sheet and images were captured by high definition video. Smoke concentration in the plume was estimated from the light scattered by smoke particles. Significant exposure was arbitrarily defined where there was ≥ 20% of normalized smoke concentration.

Results: During normal cough, expelled air dispersion distances were 68, 30 and 15 cm along the median sagittal plane when the HPS wore no mask, a surgical mask and a N95 mask, respectively. In moderate lung injury, the corresponding air dispersion distances for mild coughing efforts were reduced to 55, 27 and 14 cm, respectively, p < 0.001. The distances were reduced to 30, 24 and 12 cm, respectively during poor coughing effort as in severe lung injury. Lateral dispersion distances during normal cough were 0, 28 and 15 cm when the HPS wore no mask, a surgical mask and a N95 mask, respectively.

Conclusions: Normal cough produced a turbulent jet about 0.7 m towards the end of the bed from the recumbent subject. N95 mask was more effective than surgical mask in preventing expelled air leakage during coughing but there was still significant sideway leakage.

Figure 1. Room ventilation design and experimental set-up.

The isolation room is fitted with the downward ventilation systems. The design is to supply conditioned and clean air from the ceiling diffuser to sweep away contaminants, which would then be removed via the exhaust outlets at the floor level. The Human Patient Simulator was lying at 45° on the bed. The exhaled air plume was marked with intrapulmonary smoke, and was revealed by the laser light sheet. The images were captured by a high-definition camera positioned to the left side of the simulator when examining the cough propagation distances in the median sagittal plane. Smoke concentration in the plume was estimated from the light scattered by smoke particles.

Figure 2. Changes of cough propagation distances in the sagittal (a–c) and transverse plane (d–f) in normal lung condition, moderate and severe lung injury when the human patient simulator was wearing no mask, a surgical mask and a N95 mask respectively.

Box and whiskers plot: the upper and lower edges of the boxes indicate the interquartile ranges, the line through box is the median value, the whiskers are the 5% and 95% centiles and the closed circles are outliers.

Figure 3. Exhaled air dispersion distances along the median sagittal plane during coughing with and without wearing a surgical or N95 mask.

Normalized concentration in the plume was estimated from the light scattered by smoke particles by computer analysis. The white color code and the red color code represented regions consisting of 100% and 70% respectively of exhaled air whereas the background of the isolation room (deep blue code) was essentially free of exhaled air.

Figure 4. Exhaled air dispersion distances along the transverse plane during coughing with and without wearing a surgical or N95 mask.