Boundary Layer Climates.

Multimodal Integration of Carbon Dioxide and Other Sensory Cues Drives Mosquito Attraction to Humans

Airborne spread of infectious agents in the indoor environment

Various infectious agents are known to be transmitted naturally via respiratory aerosols produced by infected patients. Such aerosols may be produced during normal activities by breathing, talking, coughing and sneezing. The schlieren optical method, previously applied mostly in engineering and physics, can be effectively used here to visualize airflows around human subjects in such indoor situations, non-intrusively and without the need for either tracer gas or airborne particles. It accomplishes this by rendering visible the optical phase gradients owing to real-time changes in air temperature. In this study, schlieren video records are obtained of human volunteers coughing with and without wearing standard surgical and N95 masks. The object is to characterize the exhaled airflows and evaluate the effect of these commonly used masks on the fluid-dynamic mechanisms that spread infection by coughing. Further, a high-speed schlieren video of a single cough is analysed by a computerized method of tracking individual turbulent eddies, demonstrating the non-intrusive velocimetry of the expelled airflow. Results show that human coughing projects a rapid turbulent jet into the surrounding air, but that wearing a surgical or N95 mask thwarts this natural mechanism of transmitting airborne infection, either by blocking the formation of the jet (N95 mask), or by redirecting it in a less harmful direction (surgical mask).

A schlieren optical study of the human cough with and without wearing masks for aerosol infection control.

Skin as a source of organisms The average human has a skin area of about 175 m^ divided approximately as follows: legs 18% each, trunk 37%, arms 9% each and head 9%; specialized areas such as the perineum and axillae account for less than 2% of the total skin area. The skin scales which compose the surface are about 30 X 30 X 3-5 |(m and consequently some 10® are needed to complete the skin surface. Females have larger scales on average than do males (Plewig, 1970). A complete layer of cells is lost and replaced on average about every 4 days (Halprin, 1972; Jansen, Hojyo-Tomoko & Kligman, 1974), hence we all disseminate more than 10^ particles of skin every day. Although clearly bathing or showering will remove very many scales by mechanical means, Sciple, Riemensnider & Schleyer (1967) found that natural walking movements released about io* scales per minute. Scales are also released even when standing naked, presumably because the flakes tend to curl up and become detached when dried on one side only. They are then carried up in the airstream so elegantly demonstrated by Lewis et al. (1969). Unpublished studies have shown that about 10 mg of skin is deposited in the clothing in 2 hours. Microbial skin colonizers are more numerous in some areas than others, for example the head and thorax carry more microbes than the limbs; males are more heavily colonized with aerobes than are females, though anaerobes appear to be more equally distributed between the sexes (Table i). A further complicating factor, however, is that the microorganisms are not spread out equally over their habitat but live in discrete microcolonies which may be small, ofthe order of 10^ cells, or large, ofthe order of 10' viable cells. These colonies may be relatively far apart on the skin and this accounts for the fact that only about 10% of squames carry viable microorganisms (Noble & Davies, 1965). In disease, especially eczema, psoriasis and pityriasis rosea, the skin may be densely colonized by Staphylococcus aureus and/or Streptococcus pyogenes. These organisms are then dispersed on skin scales and such persons may contaminate their environment with these pathogens (Noble & Davies, 1965; Selwyn & Chalmers, 1965; Noble, 1971). Any organism found on the skin surface wi'l be dispersed including fluorescent corynebacteria (Somerville-Millar, personal communication), fungi (Clayton & Noble, 1963; Noble, Lidwell & Kingston, 1963) and presumably viruses. There is some evidence that smaller skin particles may be involved in dispersal by dermatology patients despite the large fragments of desquamated skin which are a feature of some skin diseases, though it is not clear whether this is a reflection ofthe disease process or ofthe greater degree of colonization ofthe skin. Some individuals may be a potent source of organisms, such as the psoriatic anaesthetist recorded by Payne (1967) and the eczematous porter (Ayliffe & Collins, 1967). It must be remembered, however,

B.J. Mullan

Papers

Aerodynamics of the human microenvironment.