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Airborne cross-infection risk between two people standing in surroundings with a vertical temperature gradient

The transmission of exhaled small particles from one person to another in an indoor environment can take place both directly (in the microenvironment around the persons) and via the room air distribution. The impact of these transmission routes for two persons is investigated in detail by evaluating the exposure to gaseous substances (simulating particles <5 μm) in a room with a vertical temperature gradient obtained by displacement ventilation. Experiments are conducted with two breathing thermal manikins—one the source and the other the target. In the experiments, the distance between the two manikins varies from 1.1 to 0.35 m (43 to 14 in.). A tracer gas N2O is used to represent the gaseous substances exhaled by the source manikin. The concentration of N2O is measured to study the impact on the exposure of the distance between manikins and manikin positions (face to face, face to the side of the target manikin, face to the back of the target manikin, and a seated source manikin). The exposure increases with decreasing distance between the manikins, and the highest values are obtained in the face-to-face position. Face to the side also creates some exposure of the target manikin, while face toward the target manikin's back does not give any direct exposure through the microenvironment. The thermal stratification in the room supports a significant exposure of the target manikin when the source manikin is seated and breathing toward the chest of a standing manikin.