Perhaps the greatest obstacle to the proper ventilation of the home is the lack of knowledge of the exact nature of the atmospheric conditions required to produce the desired effect. Health officers and physicians throughout the country have been very much interested in this subject and have conducted many investigations, especially in schoolroom ventilation. Once the desired conditions have been accurately determined by investigations or otherwise, the heating and ventilating engineer is capable of producing the desired results.

However, much has been learned during the last few years in the science of ventilation. The old popular idea that the chemical composition of the air was the all important factor has been disproved, and attention has been given to many other factors of great importance. Among these latter factors we have air supply, air temperature, relative humidity, air motion, and air purity.

The chemical composition of the air in the home rarely becomes changed enough to produce harmful effects to the occupants, since in general the oxygen content has to fall to 16 or 18 percent, and the carbon dioxide content rise to one to two per cent before harmful effects are produced. However, air which has been inhaled or made foul by breathing and by the combustion products from gas fires produces bad effects upon the body, both immediately and remotely. The immediate effects are dullness, oppressive breathing, headache, and general discomfort. The remote effects of foul air are a general lowering of bodily vigor and a vague weakness and lack of tone. Many authorities believe that these symptoms are not due to the changes in the chemical composition of the air (with the exception of the products of combustion from fires) but that they are due to excess temperatures, abnormal humidity, and lack of air movement, which affect the rate of liberation of heat from the body, for the symptoms just mentioned for foul air can be obtained with pure air heated to a temperature approaching that of the human body. Experiments have also shown that a group of subjects enclosed in an experimental chamber and suffering from the familiar effects of bad ventilation can in no way be relieved by permitting them to breathe fresh outside air admitted through a tube, but can be completely relieved by cooling the vitiated air of the chamber in which they are imprisoned.

1 Adapted from "Healthful Ventilation" (radio talk broadcast from University of Pittsburgh, 1929), Science for the Home Manager: A Series of Fourteen Radio Talks (University of Pittsburgh, 1929).

These foregoing facts indicate the relative importance of air temperature, air movement, and relative humidity.

You are no doubt all familiar with the meaning of the terms air temperature and air movement. A word of explanation, however, on the meaning of relative humidity may be of interest to some of you.

It is customary to express the conditions of the atmosphere with respect to moisture in the form of a ratio, termed the hygrometric state of relative humidity, which is defined as the ratio of the amount of moisture present in a given volume of air to the amount required to saturate this volume at the existing temperature. The relative humidity of the atmosphere in the home can be readily determined by the use of a wet and dry-bulb hygrometer. This instrument consists of two similar thermometers, hanging side by side, the bulb of one thermometer being dry and recording the atmospheric temperature, while the other bulb is kept wet by surrounding it with a piece of muslin connected to a wick immersed at its end in water. Owing to the evaporation constantly taking place on the surface of the wet bulb, heat is extracted from the mercury, and consequently the wet-bulb thermometer shows a lower reading than the other, which is exposed to the atmosphere. The relative humidity is readily determined from the readings of the two thermometers and a simple table which is supplied with the instrument. Relative humidity plays a very important part in the home in helping to regulate the comfort of the occupants. A person's feeling of warmth is not due alone to the temperature of the surrounding air as registered by a dry-bulb thermometer, for dry air at a relatively high temperature may feel cooler than air of considerably lower temperature with a high moisture content. In the average home, the relative humidity of the atmosphere is usually 25 per cent or less in the wintertime. This low moisture content of the air results in rapid evaporation from the body and the individual feels cold. A greater feeling of warmth is obtained with a dry-bulb temperature of 750 and a relative humidity of 60 per cent than with a dry-bulb temperature of 8o° with a relative humidity of 15 per cent.

You can readily understand that it is therefore more economical to lower the temperature of the house and raise the relative humidity, the lower temperature requiring much less fuel.

The economy in fuel consumption resulting from higher relative humidities, however, is not the only benefit to be derived. Doctors now trace common colds, grippe, influenza, bronchitis, pneumonia, and tuberculosis to the breathing of hot dry air in the home. The constant breathing of this hot dry air causes the mucous membranes of our noses and throats to become dry and irritated, thus lowering our resistance and making us supersensitive to dust and bacteria.

Ordinary attempts to increase the humidity, such as the placing of pans of water on radiators or the usual water tanks in the furnace casing, are generally of little benefit since they are not capable of evaporating enough water to do much good. To maintain a relative humidity of 35 per cent in a six-room house in zero weather, it is necessary to evaporate twenty gallons of water a day. This allows for one change of air an hour. To maintain the same humidity in the same house, with a 35 to 400 F. temperature outdoors, requires seven gallons of water a day, allowing for the same number of air changes an hour. To evaporate this much water per day usually requires a special form of evaporator or humidifier. Portable humidifiers which can be removed from room to room have been on the market for some time.

Just recently there has been placed on the market a heating and ventilating system for residences which automatically controls the relative humidity to any degree desired. The other essentials of good heating and ventilating of the home, such as air temperature, air motion, and air purity, are also taken care of by this new equipment. Since this equipment represents the latest advance in the heating and ventilating of the home, it may be of interest briefly to describe it here.

In this equipment, the warm air heating system is used. In this case the air is heated directly in the furnace by means of gas burners which automatically heat it to any desired temperature, the thermostat which controls the temperature being placed in the living room or any room desired.

Moisture is added to the warmed air by means of a humidifier. The quantity of water evaporated is controlled by a float valve which regulates the level of the water surface, insuring a constant relative humidity without the necessity of filling any pans by hand. The float valve can be set to give any desired relative humidity. For a temperature of 70 degrees F. a relative humidity of 35 to 45 per cent has proved to be very satisfactory.

Before the air is heated it is drawn through a filter, which consists of a mass of fine wire, like steel wool, pressed into a two-inch thickness. This filter removes the dirt and dust in the air, thus making it much more suitable for breathing and at the same time eliminating considerable dusting and cleaning of furniture, draperies, and so on.

The warmed, humidified, and cleaned air is then carried by ducts to every room and is distributed under pressure, without drafts and without hot or cold zones. This positive circulation of warm air insures a uniform heat in every room in the house. In the old hot-air systems there is usually one room or more which is very hard to heat owing to the inability of the gravity system to get sufficient warm air into the room.

Because of the fact that this equipment has been on the market for a very short time, it is probable that few of you have seen it in operation. I believe, however, that in the very near future many of the architects and building contractors will recommend such systems for heating and ventilating the home. While these systems at present are used only in the winter for heating, they will, undoubtedly, be used eventually to cool the home in the summer by substituting a cooling unit in place of the present heating unit.

[Note 1 . - Humidity and death rate: Professor Ellsworth Huntington of Yale University has shown in his analysis of weather reports in relation to 60,000,000 deaths that a relative humidity of 80 per cent is associated with minimum death rates and that a higher or lower humidity has shown an increase in death rates. See Ellsworth Huntington, Weather and Health (National Research Council, 1930).]

[Note 2. - Air-cooling systems: There are on the market several systems and devices by which air may be cooled - a centralized system used when there is a centralized duct arrangement in heating and a unit system for the purpose of cooling individual rooms. This latter device is guaranteed to reduce temperature 10° in a room with 600 sq. ft. or less of floor space. For information on air cooling devices see Howard T. Fisher, "The Country House," Architectural Record, November, 1930, pp. 379-82.]