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140 CHAPTER 7 1951 Guide has also been reported that the influenza virus loses much of its virulence '. when the relative humidity is 50 percent.16 Well controlled, large scale tests of the various methods of air sterilization conducted in barracks16,17 have confirmed the importance of dust control in minimizing the spread of airborne disease, a consideration which has guided the practices of ventilating engineers for a number of years. The importance of the dust factor has been emphasized by many engineers, and has been convincingly demonstrated by subsequent bacteriologic studies aboard ships. Treatment of floors and bedclothes with oil emulsions has proved effec tive in reducing bacterial dispersion by as much as 90 percent in Army barracks and station hospitals.17 The incidence of acute respiratory in fections was from 10 to 30 percent lower in barracks with oiled floors and bedclothes than it was in control barracks which received no special treat ment. . More recent studies, however, have yielded inconsistent results. Fig. 2. Monthly Incidence of Acute Respiratory Illness Among Naval Recruits and Ship's Company (Permanent Personnel)1 An emulsifying mixture, Fixanol C containing cetyl pyridinium bromide, when incorporated in the oil-in-water emulsion imparted a bactericidal action to the emulsion. Blankets -treated with tins substance and - oil became bactericidal and retained this property for as long as three months. The possibility of hypersensitivity of an occasional individual to bromide drugs should be borne in mind when exposing large groups to treated gar ments or blankets.17 ;. . No simple method for disinfecting droplet-nuclei has yet been devised. Under favorable laboratory conditions, propylene glycol in concentrations of 0.07 to 0.14 milligrams per liter, and triethylene glycol in a concentra tion of 0.0045 milligrams per liter were highly germicidal for most air borne bacteria in clean air when the relative humidity was between 40 and 60 percent.18,19,20 ` A humidity of 50 percent, without the use of glycol vapor, has been reported as destructive to some bacteria. However, maxi mum rates, of bactericidal action of triethylerie glycol vapor will be secured at humidities between 20 to 50 percent.71 ' In a recent report oh the. effect of trietliylene glycol vapor in air dis infection, it is pointed out that the rate of ventilation, as determined by Air Conditioning in Prevention and Treatment of Disease 141 the number of air changes per hour, is important and that continuous va porization js needed to maintain effective concentrations of glycol vapor.81 In the absence of an apparatus to measure the concentration of vapor in the room, a slight fog is an indication of adequate concentration. Ab sence of such a fog indicates a non-bactericidal concentration. The report stated: "However, under experimental conditions we have observed no apparent decomposition of triethylene glycol when temperatures up to 290 F were maintained at the site of vaporization, provided the vaporizing unit was designed so that heat was applied to.liquid glycol only at the point of vapor formation. On the other hand, decomposition was frequently observed when the vaporizing temperature was raised above 290 F, or when liquid glycol was kept above 120 F for long periods of time, The precise nature of the decomposition products is as yet unknown, but they may well be irritant or toxic and their formation should be avoided. For this reason, it is felt that a limit of 290 F should be applied to the tempera ture employed in vaporization, and furthermore, that liquid giycol should not be heated above 120 F for long periods of time." It was also recommended that a given vaporizer be capable of various outputs as measured by grams of liquid glycol vaporized per hour; for instance, from 0.5 g to at least 2.0 g per 1000 cu ft of volume treated, for small vaporizing units. Such high rates secure bactericidal concentra tion in less than an hour. Each room should be furnished with a single small vaporizer, and one vaporizer should not be expected to handle a volume greater than 4000 cu ft. Apparatus for the production of glycol vapor, and an independent duct system for carrying this vapor and dilut ing air for large rooms or spaces, and unit type vaporizers for small spaces, have been described recently.22 There is also available a device for the automatic regulation of glycol vapor in the air called the glycostat. This instrument has been calibrated to measure the degrees of saturation of the air with glycol vapor by direct reading of the variations in the intensity of light reflected from the glycol condensing surface of the wheel of the instrument.23 Under practical conditions, particularly in the presence of dust in the air, glycol effectiveness is much reduced. The use of other chemical aero sols that have been tried is limited by their toxicity, odor, or destructive. ness to fabrics and metals. A recently reported controlled experiment in the offices of the Metropolitan Life Insurance Company showed that under ordinary working conditions triethylene glycol vapor failed to reduce the number of airborne bacteria and the incidence rate of minor respira tory infections.24 Ultraviolet radiation of floors and upper air has been studied extensively at the Naval Training Center, Sampson, N. Y. In barracks housing naval recruits, hospital admissions for respiratory infections (mostly catarrhal fever) were 25 percent lower in a group of men exposed to ultraviolet radiation--(2537 Angstrom Units, 1 to 7 ergs per (cm2) (sec) at bed level)-- than they were in adjacent control barracks without ultraviolet radiation.26 A controlled study over a six-year period on the evaluation of ultraviolet radiation of sleeping quarters as a supplement of accepted methods of respiratory disease control was recently reported.26 The amount of ra diation over the last two years of the period was about five times that recommended commercially. No significant effect in the incidence of disease could be detected in about 400 inmates during the six-year period. The data imply that air layers were not sufficiently mixed. However, more efficient mixing would have been obtained at the expense of increased