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Influenza Transmission and the Role of PPRE: An Assessment of the Evidence 19Table 1Behaviour of Droplets in Still Air (adapted from Evans, 2000)Diameter Settling velocity (cm/sec) Time to fall(µm) (Stoke’s Law) 3 meters100.0 25.0 10 sec30.0 2.7 70 sec10.0 0.31 17 min3.0 0.028 3 hours0.3 0.00042 8.4 days0.03 0.000022 159 daysEffect of EvaporationParticles, once ejected from the respiratory tract of an infected person, are subjected tothe effects of evaporation. The rate of evaporation of the water in the particles islargely determined by the amount of water present in the air (relative humidity), theprevailing temperature, and the surface area of the particles (Lighthart, 1991;Duguid, 1946; Lidwell, 1967; Yassi, 2004). For a unit volume of water in air, theratio of surface area to volume doubles as the droplet diametre decreases by one-half.For this reason, the evaporation rate is faster for smaller particles. The evaporationrate also increases with rising air temperature and/or falling relative humidity.In a study that involved spraying a microbe-laden aerosol (i.e., mist) into the airand measuring the course of droplet size and downwind behaviour, Lighthart et al.(1991) showed that, at 49 per cent relative humidity and 21˚C , droplets in the80 µm diametre range evaporated before the droplets reached the ground; and thewater in the particles with initial diametres less than 35 µm evaporated in less thanone second. Lidwell re-evaluated Duguid’s 1946 data and found that the watercontained in the 5 to 50 µm particle size range quickly evaporated, reducing theparticle sizes to the 1 to 10 µm range (Duguid, 1946; Lidwell, 1967). Therefore,because of evaporation, expelled particles do not remain the same size. Many of thelarger ones very quickly end up as smaller particles as evaporation proceeds.Effect of HumidityThe relative humidity affects not only how quickly evaporation occurs, but to whatextent it will occur. The rate of evaporation decreases as relative humidity increasesand is slowed by the presence of salts, proteins and other non-volatile componentspresent in an ejected respiratory tract particle. As described by Yassi et al.(2004),while a droplet of pure water will evaporate fully if relative humidity is less than 100per cent, a droplet that contains soluble material, such as sodium chloride, will reach
20 Influenza Transmission and the Role of PPRE: An Assessment of the Evidencean equilibrium state which depends jointly on the concentration of the sodiumchloride contained in the droplet and the relative humidity of the ambient air.Since respiratory secretions contain many different compounds as solutes orsuspended material, the water in particles that contain microorganisms, will notalways completely evaporate in ambient air. However, in an HVAC-equipped building, 11located in a temperate climate, a relative humidity of 40 per cent or lower can beexpected, particularly during winter months. Under these conditions, the water ina particle containing soluble material will evaporate completely, leaving behind adroplet nucleus that could contain any infectious agents that were originally present(Yassi, 2004). If these biological agents are not damaged by the drying process theycan potentially infect a susceptible host if inhaled.Effect of DistanceThe physics of breathing, coughing, talking or sneezing plays an important role indetermining how far and how quickly an ejected respiratory particle can travel.The average total volume of emitted material in a cough is approximately fourmicrolitres at the moment of exhalation. Almost all of this volume is composed oflarger particles that follow ballistic trajectories and land on “surfaces” withinroughly one metre of the source (Duguid, 1946; Nicas, 2005; Loudon, 1967). Theremaining volume remains in the air as an evaporating mist with the liquid particlesconverting to desiccated particles (droplet nuclei) at a rate inversely proportionalto their size.Figure 2Particle mist created upon sneezing. (Davidhazy, 2007)11 HVAC = heating, ventilation and air-conditioning
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