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can see that the dryness of air is related to its temperature rather than its actual moisture contents, hence the term "relative humidity" which is expressed in percentage. If we say that the relative humidity is 50%, we mean that at the given temperature the air contains 50%, or one-half as much water vapor as it would contain if it were saturated. The relative humidity, therefore, varies as much with the temperature as with the actual quantity of water vapor mixed with the air.
It is quite generally agreed that humidities below 30 are unnatural and injurious physiologically, and tend to reduce the efficiency of the occupants of the building. This is due, first, to the fact that excessively dry air overworks the mucous membrane of the nose and other air passages, and second, because its effect upon the skin or upon the mucous membrane, has an irritating effect upon the nervous system. This has been the experience and conclusion of a great many observers who have investigated the subject. In addition to the bad effect physically, excessively low humidity is objectionable due to its effect upon the furniture, woodwork, pipe organs, etc., which is well known, but perhaps not properly appreciated.
The effect of low humidities is not so serious except where accompanied by comparatively high temperature, but it is necessary to carry a higher temperature in a building where the humidity is low in order to obtain what we might call the necessary sensible temperature which will be referred to a little later.
Another objection to a high temperature and low humidity in any building is that when a person enters the building his clothes are apt to be filled with moist air due to rain, snow or high outside humidity, and with this great amount of moisture in his clothes, he at once feels damp and cold due to evaporation, a very bad combination, the effect of which need not be commented upon further.
There is. a corresponding objection to an excessively high humidity. Humidities above 65% are objectionable when accompanied by a room temperature of 70° or more, due to the pronounced soothing effect to which the nervous system is subjected in contrast to the more or less stimulating effects produced by extremely low humidities. High humidities cause a tendency towards lack of energy and even sluggishness. The effect of excessively high humidities is drowsiness, and patrons are apt to enter dreamland during the most interesting part of a film story.
From' the foregoing the need of automatic humidity control to prevent either excessively low humidities or excessively high humidities is evident. The air washer as such produces a considerable degree of humidity for the reason that it partly saturates the air which is passed through it, but of itself it does not control the degree of saturation nor the temperature of saturation. In other words, the humidity may be either too high or too low, probably some of the greatest dangers being that the humidity will be too high ; the cost of operating excessive, and the conditions of ventila
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