Aerodynamics of the human microenvironment.

Abstract The skin, the boundary between man and his environment, is separated from the ambient atmosphere by a layer of convecting air. This boundary layer has been photographed using colour schlieren cinephotography, a technique which makes use of the fact that this air is warmer, less dense, and has a refractive index different from that of the ambient air. Starting from the feet, there is a layer of air which passes up adjacent to the surface of the body, its thickness progressively increasing as it rises and accelerates. At times the outer part of the boundary layer becomes partly detached. When the layer passes over the face some of it enters the nose with each inspiration. The air flow reaches the orbit, passes over the eye, and clears the cornea by a millimetre or so. The maximum velocity of the boundary layer at head level is about 0·5 m. per second. Preliminary experiments reveal that the human microenvironment has a significantly higher content of microorganisms than the ambient air. The source of bacteria is either the skin and/or the ambient air. Since some of the air of the microenvironment is inhaled during the respiratory cycle, this may be one of the missing links in the natural history of airborne infection; it may also provide a connection between skin disease and respiratory disease. Particles, pollens, and microorganisms in the air, entrained and concentrated by the boundary layer, could be presented to the nasal orifice and inhaled. There is inevitable dissemination to the ambient air of microorganisms by the human boundary layer, and this may be a mechanism in hospital sepsis. In special circumstances (e.g., transplant surgery, treatment of burns, leukaemia), by isolating the surgeon or nurses in a pneumatic suit, it may be possible to reduce this serious complication.