A susceptible person experiences the highest exposure risk of respiratory infection when he or she is in close proximity with an infected person. The large droplet route has been commonly believed to be dominant for most respiratory infections since the early 20 th century, and the associated droplet precaution is widely known and practiced in hospitals and in the community. The mechanism of exposure to droplets expired at close contact, however, remains surprisingly unexplored. In this study, the exposure to exhaled droplets during close contact (< 2 m) via both the short-range airborne and large droplet sub-routes is studied using a simple mathematical model of expired flows and droplet dispersion/deposition/inhalation, which enables the calculation of exposure due to both deposition and inhalation. The shortrange airborne route is found to dominate at most distances studied during both talking and coughing. The large droplet route only dominates when the droplets are larger than 100 μm and when the subjects are within 0.2 m while talking or 0.5 m while coughing. The smaller the exhaled droplets, the more important the short-range airborne route. The large droplet route contributes less than 10% of exposure when the droplets are smaller than 50 μm and when the subjects are more than 0.3 m apart, even while coughing. Keywords: exposure, disease transmission, close contact, short-range airborne, large droplet Practical implications Our simple but novel analysis shows that conventional surgical masks are not effective if most infectious viruses are contained in fine droplets, and non-conventional intervention methods such as personalised ventilation should be considered as infection prevention strategies given the possible dominance of the short-range airborne route, although further clinical evidence is needed. Nomenclatureefficiency [-] Exposure due to large droplet route [μL] Exposure due to short-range airborne route [μL] Gravitational acceleration [m•s -2 ] Mass current [kg•s -1 ] IF Inhalation fraction [-] Constant (=0.3) [-] Thermal conductivity of air [W•m -1 •K -1 ] LS Exposure ratio between large droplet and short-range airborne [-] Latent heat of vaporization [J•kg -1 ] Droplet mass [kg] Mass of liquid in a droplet [kg] Mass of solid in a droplet [kg] 0 Jet initial momentum [m 4 •s -2 ] Molecular weight of H2O [kg•mol -1 ] MF Membrane fraction [-] n Number of droplets [n] 0 Number of droplets expelled immediately at mouth [n] Number of droplets entering the inhalation zone [n] Number of droplets potentially deposited on mucous membranes [n] Total number of released droplets [n] Nu Nusselt number [-] p Total pressure [Pa].

A susceptible person experiences the highest exposure risk of respiratory infection when he or she is in close proximity with an infected person. The large droplet route has been commonly believed to be dominant for most respiratory infections since the early 20th century, and the associated droplet precaution is widely known and practiced in hospitals and in the community. The mechanism of exposure to droplets expired at close contact, however, remains surprisingly unexplored. In this study, the exposure to exhaled droplets during close contact (< 2 m) via both the short-range airborne and large droplet sub-routes is studied using a simple mathematical model of expired flows and droplet dispersion/deposition/inhalation, which enables the calculation of exposure due to both deposition and inhalation. The short-range airborne route is found to dominate at most distances studied during both talking and coughing. The large droplet route only dominates when the droplets are larger than 100 μm and when the subjects are within 0.2 m while talking or 0.5 m while coughing. The smaller the exhaled droplets, the more important the short-range airborne route. The large droplet route contributes less than 10% of exposure when the droplets are smaller than 50 μm and when the subjects are more than 0.3 m apart, even while coughing.

Close contact was first identified as the primary route of transmission for most respiratory infections in the early 20th century. In this review, we synthesize the existing understanding of the mechanisms of close contact transmission. We focus on two issues: the mechanism of transmission in close contact, namely the transmission of the expired particles between two people, and the physical parameters of close contact that affect the exposure of particles from one individual to another, or how the nature of close contact plays a role in transmission. We propose the existence of three sub‐routes of transmission: short‐range airborne, large droplets, and immediate body‐surface contact. We also distinguish a “body contact,” which is defined with an interpersonal distance of zero, from a close contact. We demonstrate herein that the short‐range airborne sub‐route may be most common. The timescales over which data should be collected to assess the transmission risk during close contact events are much shorter than those required for the distant airborne or fomite routes. The current paucity of high‐resolution data over short distances and timescales makes it very difficult to assess the risk of infection in these circumstances.

The Changjiang Estuary is a large bifurcated estuary where different hydrodynamic processes influence its South Branch compared to its North Branch. The South Branch is the dominant pathway of Changjiang River discharge, while the shallower and narrower North Branch is dominated by salt water intrusion, especially in the dry season. Absorption and fluorescence spectroscopy were measured along with dissolved organic carbon (DOC) concentrations to characterize the properties of dissolved organic matter (DOM) collected in different seasons during an extreme drought year in 2011. The refractory DOM from the Changjiang River flowed mainly through the South Branch, whereas in the lower South Branch, the input from the polluted Huangpu River contributed a large amount of biolabile DOM, demonstrating an anthropogenic perturbation from megacities. The DOM properties in the North Branch showed conservative behavior in the wet season, while noticeable addition was observed in the dry season, accompanied by the reversed flux of DOM from the North Branch to the South Branch, emphasizing the regular seasonal oscillation of the DOM dynamics in this monsoon-controlled bifurcated estuary. The estuarine turbidity maximum zones played distinct roles on DOM dynamics in different estuarine environments. The DOC and chromophoric DOM (CDOM) abundance in the Changjiang River and other Chinese rivers were at lower levels compared to other world rivers, showing a characteristic of the regional CDOM-poor features for many East Asia rivers.