Document DvELd2Q6oma57ZdGokKg6b3e5

218 CHAPTER 12 1946 Guide properly conditioned air. While this point of view is usually held by persons not intimately acquainted with the complicated factors involved, they nevertheless carry some weight. It is apparent, however, to anyone 1 acquainted with the factors involved and the conditions effecting comfort, that in modern air conditioning, as in most other branches of engineering, modern science makes it possible to control the phenomena of nature for ` the service and comfort of man beyond any possibilities found in nature itself. When the requirements for optimum'comfort as determined by the atmospheric environment are known (and the comprehensive studies to date indicate that they are known at least to a high degree), the air conditioning engineer can supply these requirements indoors to the same perfection as may accidentally be found at times outdoors, and keep them under control. The freedom of movement, action and thought, together with the variability of stimulae experienced by persons under ideal conditions in the country, mountains or seashore, and the psychological effect of these wide open spaces undoubtedly have some stimulating effect, which when compared with the monotony of confinement indoors, even in the most favorable atmospheric environment, account for the contrast-. Various experimenters have attempted to duplicate the invigorating qualities of outdoor air by the use of ozone, ionization, or ultra-violet ' light, but results to date have been inconclusive dr negative 6. Ozone has been used with success for the destruction of micro-organisms (molds) in meat packing establishments and the like; and where con siderable amounts of organic effluvia are present it may be useful as a deodorant. For ordinary ventilation practice! however, neither of these purposes can be attained, since the concentration of ozone necessary for effectiveness would be likely to transcend the limit of comfort in ordinary occupied rooms. While ozone has been used in the treatment of certain diseases, there is no evidence that it has a tendency to increase comfort or to benefit health under conditions of normal human occupancy. The allowable concentrations in the breathing zone are very small, between 0.01 to 0.05 ppm parts of air. These, are much too small to influence; bacteria. Higher concentrations are associated with a pungent unpleasant .odor and considerable, discomfort to the occupants. One part per million causes respiratory discomfort, headaches, depression,' and a lowering of the metabolic rate, and may even lead to coma 6. v, ;' : r ' PHYSICAL IMPURITIES IN AIR Dust particles of various types, when present in- considerable con centrations, produce an irritating effect .upon the mucous membranes of nose and throat and may be associated with high prevalence of acute respiratory diseases such as bronchitis and! pneumonia. Dust which -contains free silica has special harmful effects, causing a primary disease of the-'lungs (silicosis) and predisposing the victim'in a high degree, to tuberculosis. These, however, are special problems of industrial hygiene which will not be discussed in detail in this chapter. (See Chapter 10.) A certain part of the dissemination of disease in confined spaces is caused by'the emission of pathogenic organisms from infected persons. Droplets sprayed into the air in talking, coughing, sneezing, etc.; do'not all fall immediately to the ground within a few feet from the source, as was formerly believed. The large droplets fall, but minute droplets less than 0.1 mm in diameter evaporate to dryness before they fall the height.of a.man. Nuclear residues from such sources, which, may contain infective organisms drift long distances with the air currents and the Physiological Principles . ________________ ;____________ 219 virus may remain-alive long enough,to; be transmitted to other persons in the same room or building. Droplet nuclei have been recovered from cultures of resistant micro-organisms-a week after introduction into a tight chamber of 3000-cu ft capacity, although' the majority of .disease germs died out within a few hours7.. Practical epidemiological-evidence, indicates that the danger of such atmospheric transmission is slight, with the bacterial diseases but may be appreciable with the diseases caused by the much smaller viruses. Avoidance of overcrowding is a major factor in avoiding such dangers. The microbic concentration in the atmosphere may be reduced by air. change, but. since the rate'of contamination, may be great at local points over short periods of time the hazardous concentra tion may not be eliminated quickly enough and may even be spread'over larger areas by local drafts. The possibility of sterilizing the air supply at the source, or destroying the micro-organisms at their-point of admis sion to the air by the use of ultra-violet light8 or glycol vapors9 is being' studied and'offers considerable promise. . .... .. While in some instances it may be possible to reduce the physical impurities of the air by dilution from a non-contaminated. source, such non-contaminated sources are rarely available. Frequently the outside air contains a higher concentration of physical impurities than that within an enclosure. Therefore, it is usually desirable to reduce the concentra tion of physical impurities by air cleaning methods (see Chapter 33). THERMAL INTERCHANGES BETWEEN THE BODY AND ITS ENVIRONMENT The importance of the thermal factors arises from-the profound in fluence which they exert upon body temperature, comfort and health; Body temperature depends upon the balance between heat production and heat loss. The heat resulting from the oxidation which occurs within the body (metabolism) maintains the body temperature well above that of 'the surrounding air in a cool or cold environment. At the same time, heat is constantly lost from the body by radiation, convection and. evaporation. Since, under ordinary conditions, the body temperature is maintained at its normal level of about 98.6 F, the heat production must be balanced by the. heat loss. . ..In.conditioning air for comfort and health it is necessary to know the rate of. sensible and latent heat liberation from the human body, which in conjunction with other heat loads-(see Chapters 6,. 14 and 15) determines the capacity required for proper conditioning. The data in common: use are those of the A.S.H.V.E. Research Laboratory10. ; The fundamental thermodynamic processes concerned in heat inter changes between the body and its environment may be described by the equation: M= 5+E R C (1) where ' '.- - M -- rate of metabolism. 5 = rate of storage. 1 ' E = fate of evaporative heat loss. R '= rate of radiative heat loss or gain. C = rate of convective heat loss Or gain.' Factor M, the rate of metabolism, is always positive. The storage, 5, may be either positive or negative, depending upon whether heat is being