Short-range airborne transmission of expiratory droplets between two people

Abstract

The occurrence of close proximity infection for many respiratory diseases is often cited as evidence of large droplet and/or close contact transmission. We explored interpersonal exposure of exhaled droplets and droplet nuclei of two standing thermal manikins as affected by distance, humidity, ventilation, and breathing mode. Under the specific set of conditions studied, we found a substantial increase in airborne exposure to droplet nuclei exhaled by the source manikin when a susceptible manikin is within about 1.5 m of the source manikin, referred to as the proximity effect. The threshold distance of about 1.5 m distinguishes the two basic transmission processes of droplets and droplet nuclei, that is, short-range modes and the long-range airborne route. The short-range modes include both the conventional large droplet route and the newly defined short-range airborne transmission. We thus reveal that transmission occurring in close proximity to the source patient includes both droplet-borne (large droplet) and short-range airborne routes, in addition to the direct deposition of large droplets on other body surfaces. The mechanisms of the droplet-borne and short-range airborne routes are different; their effective control methods also differ. Neither the current droplet precautions nor dilution ventilation prevents short-range airborne transmission, so new control methods are needed.

Influence of pulmonary ventilation rate and breathing cycle period on the risk of cross‐infection

“…This distance is in agreement with the threshold distance suggested in past studies10,52 when distinguishing between short-range and longrange cross-infection. There are many places where people might either stand or sit, such as hospital wards, buses, and trains.…”

“…A few studies using thermal manikins reported the velocity of the exhaled airflow for normal breathing: 3.0 m/s (at peak) from the nose, 23 2.0-3.0 m/s (at peak) from the mouth, 24 4.74 m/s (at peak) from the mouth, 6 and 2.67 m/s (averaged) from the mouth. 10,23,32 It is important to define and use the concept of a "standard person" in studies of cross-infection when comparing the results of different studies. [26][27][28][29] The differences in exhaled airflow velocity defined and reported in past studies suggest that there may be considerable variation between different groups of people.…”

“…25 These velocities were in general higher than those measured in human subject studies, where the peak flow velocity during normal breathing was reported to be approximately 1-2 m/s. In general, the pulmonary ventilation rate and breathing cycle period of "standard persons" were used in past studies [3][4][5][6][7][10][11][12][13][14][15][16][17][18][19][20][21][22]30,31,33 of airborne cross-infection. In previous studies using breathing thermal manikins, a breathing cycle period of 6 seconds was usually assumed for a pulmonary ventilation rate of 6.0 L/min and 4 s for 10.0 L/min; the breathing cycle of 2.5 second inhalation, 2.5 second exhalation, and 1 second pause was most widely used, 11,12,30,31 although many studies used a cycle without a pause between exhalation and inhalation.…”

“…A thinner and weaker convective boundary layer is more easily penetrated by the exhaled flow and thus provides a lower level of protection from exposure. Previous studies5,6,10,16,22,53,54 suggested different threshold distances ranging from 0.8 to 1.5 m under different boundary conditions. The findings of this study suggest that new control measures should be developed and implemented to either limit the spread of the air exhaled from supine or seated persons to the breathing level of standing persons or to provide clean air directly to the breathing zone of standing persons.The results of the present study also show that, under the experimental conditions examined, the influence of breathing pattern on the risk of cross-infection extended only up to 1.0 m in separation distance between the infected and the exposed person.…”

“…For the data under steady-state conditions, time-averaged mean values were calculated.The risk of cross-infection in steady-state conditions was indicated by the parameter exposure index (ε), which has been widely used in past studies [3][4][5][6]10,12,19,[45][46][47][48][49][50]. The FCM data were corrected for the time required for the N 2 O samples to travel in the sampling tube from the sampling location to the FCM monitors.…”