Vol. I

ORIGINAL ARTICLES

THE PROBLEM OF HUMIDITY INDOORS.*

E. P. LYON, Professor of Physiology, University of Minnesota.

Minneapolis, Minn.

The air is usually described as a mixture of three gases. Really there are four, nitrogen, oxygen, carbon dioxide and water vapor. While all may vary, relatively or actually, the proportion of water vapor varies far more than any of the others.

When at any given temperature the air contains as much vapor as possible the air is "saturated." Table I shows the weight of water in saturated air of various temperatures. Note that at 70° F. the capacity of the air for water is sixteen times as great as at 0° F. and seven times as great as at the freezing point.

No. 12

added to each cubic foot in order to saturate the air under the new temperature. Or looking at the matter in another way, if air saturated at 0° were raised to 70° F. without addition of water this air would then be only 6% saturated.

Every intermediate degree between 0% and 100% of saturation is possible at each temperature. Hence the idea of relative humidity or percentage of saturation. This is the quantity obtained by the dew point or by the rate of evaporation test. evaporation test. It is valuable for certain considerations, particularly when temperature does not change. But the relative humidity changes with temperature, as has been indicated. I am therefore tentatively suggesting the term mass humidity to indicate the total quantity of water per cubic foot of air.

Since the absolute zero Fahrenheit is minus 459°, on the absolute scale our climatic fluctuations of temperature would be about 25%. The atmospheric pressure varies a few percent at any one point and perhaps 30% among the habitable localities. The variation of N, is negligible and of O, and CO, even including our closed houses is small. But the water vapor may be hundreds of times greater at some times and places than at others.

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The human mechanism stands these variations well and probably there is no optimum humidity per se. Probably, however, there is an optimum humidity for each temperature. No one knows exactly what it is. Our first problem if we are to modify humidity artificially is to decide upon some sort of standard.

The whole question is intimately related to the loss of heat from the body. We must lose heat as fast as we produce it; otherwise the temperature of our body would rise.

The various forms of heat loss are represented in table II.

Note that the greatest loss under usual conditions is through radiation and conduction. The rate of this loss depends on the difference in temperature between the surface of the body and its surroundings.

The second method of heat loss in degree of importance at moderate temperatures, is evaporation. Evaporation is conditioned from the outside by the temperature of the air in contact with the body and the amount of water vapor already in this air. From the body side, evaporation is conditioned by the area of moist surface exposed. Such surface is exposed (a) in the air passages and lungs and (b) on the skin where sweat glands are located. The area in the lungs is varied somewhat with the depth and frequency of respiration as well as by the character of the air breathed. The 24 hour water loss through the lungs (plus skin of face) in different activities is thus given by Rubner: at rest 408 grams: reading aloud 672 grams; singing 816 grams. However, the respiratory regulation of heat loss is by means as important in man as in the dog; (e. g., panting).

The automatic regulation of sweat is, on the contrary, an important part of our thermostatic control.

Now since the body obeys the thermodynamic laws, it cannot lose heat by radiation or conduction unless the surrounding air is cooler than the body. On a hot summer day radiation and conduction must sink into relative insignificance as factors in temperature regulation, although no exact data are available. In such weather we depend nearly wholly on evaporation for heat loss.

Similarly, in accord with inexorable physical law, we cannot evaporate water into air already saturated and of the temperature of the body. On a hot, humid day both our chief methods of

losing heat may largely fail. The body lowers heat production as much as possible. Nevertheless fever may result, with its discomforts and intoxications. You are likely to call the result "sun stroke".

It is plain therefore and supported by abundant physiological evidence that high temperature and high humidity together are unhygienic, even destructive.

At the other extreme, in our climate, conditions are quite different. At 0° F. it is impossible to have more than 1/2 grain of water vapor per cubic foot of air. The range of vapor content at 0° F. is so small that I do not believe we could differentiate between cold effects and humidity effects.

At intermediate temperatures conditions are still different. With a temperature of 40° F., say, there is a marked difference between the effect of saturated air and dry air. This is because the saturated air has now a considerable quantity of water, (see table I), and this increases markedly the rate of loss of heat by conduction from the body surface. This is by reason of the high specific heat of water.

At any temperature we evaporate and so lose heat faster into dry than into moist air, but around 40° or 50° F. the loss by conduction into moist air more than makes up for the decreased evaporation into such air. We see why humid days around 40° or 50° F. seem cold and why humidity can be either a heat preserving or a heat losing factor, depending on the temperature.

At higher temperatures the high rate of loss through conductivity does not prevail. On a May or June day around 70° F. and 70% relative humidity we take off some of our clothing and feel fine and comfortable.

The humidity standards in the hygienic and engineering textbooks are mere guesses and seem to be founded on moderate summer weather. 60% to 70% relative humidity is a frequent standard. These books were written by people living in more moderate climates than Minnesota in winter.

My experiments show that in zero weather it is not practicable to keep a 60% humidity in a building without double windows. Literally streams of water will condense on the glass and run down to the floor. Even with double windows one may get condensation on outside

walls and ruined wall paper. I have therefore come to the conclusion that 40% to 50% is as high a humidity as can satisfactorily be maintained in homes in this state in cold weather. If we accept this tentative standard, what does it mean as a practical problem?

Let us take a small house of say 10,000 cu. ft. capacity. From Table I it appears that each cubic foot of zero degree air will need 7.5 grains of water added to it to saturate it when it has been heated to 70° F., or about 4 grains to half saturate it. Calculation shows that about one gallon of water will be needed for the house of 10,000 cubic feet capacity and 50% saturation. This fact is perhaps startling to anyone when he first approaches this subject mathematically. Practically the problem would be of small significance if you could evaporate your one gallon of water into the air of your house and keep it there. But you can't. There is constant leakage and this is surprisingly large.

Experiments which I made three winters ago indicated that the air of my well built and wholly double-windowed house was renewed at least ten times a day in quiet weather. In engineering books twenty-four changes a day are usually assumed. Wind greatly increases the rate of exchange between outside and inside air. Suppose we say that the air in our 10,000 foot house is renewed ten to fifteen times a day. We arrive at the startling fact that ten to fifteen gallons of water must be evaporated every twenty-four hours if we care to maintain even the moderate humidity this paper advocates. Of course what most people are actually doing is nothing at all, with the result that our houses and offices in winter are often drier than any . desert on the face of the earth.

No quantitative physiological evidence as to the effects of dry air is at hand. I should like to get the opinion of those present from a clinical standpoint. Can you say, definitely, from clinical experience that the prevalence of respiratory troubles in winter is related to lack of humidity? Or might it be the high temperature maintained in American houses, itself, perhaps, partly attributable to lack of humidity?

We certainly live under artificial conditions. I believe they are bad. I believe (without I believe (without present scientific evidence) that artificial humidification of dwelling houses, offices and other places where relatively few people gather is de

sirable from the standpoint of comfort and perhaps is considerably protective from infection and from vaso motor strain.

If you accede even partially to this view, you will not begrudge a few moments devoted to the practical side of this question.

Do the ordinary home appliances meet the need? If you recall that 15 gallons of water are needed per day, you at once see that the little dish on the radiator is a delusion. The tea kettle on the kitchen stove may do pretty well for that room but has little effect on the house as a whole.

There are a good many devices on the market. Are these effective? Let us take up in succession the different types of heating.

For hot water radiators (also for steam) I know of the following humidifiers, all of which I have tested:1

TABLE III.

EVAPORATION PER LINEAR FOOT OF RADIATOR OCCUPIED.

Gms. per 24 hrs. "Speco," av. of 3 tests, zero weather...... 294 "Savo," av. of 3 tests, zero weather,...... 230 "Buddington," av. 3 tests, zero weather,..1,116 "Flobun," av. 2 tests, zero weather,. .1,248

TABLE IV.

WATER EVAPORATION FROM LUNGS AND SKIN. Gms. per 24 hrs. Resting man av. 13 exps. (Atwater)....... 939 Working man av. 6 exps. (Atwater). .1,912 70 students in laboratory, av. (Lyon)..... .1,200

Compare tables III and IV. Note that the best of the market humidifiers for use on radiators evaporates about as much water as one person from his lungs and skin.

Imagine father spending his good money for one of these devices and sitting, with his feet on the radiator, in deep content that he is now doing the right thing for his family, while at the same time he and each of his progeny are humidifying the air more effectively than the "humidifier" and without costing him a cent! The truth is all these devices were made to sell and not to humidify. It would take over thirty of the best type to evaporate ten gallons a day.

1See Science, N. S., Vol. XLVI, p. 262. Sept 14, 1917.

Moreover Moreover That is a

There wouldn't be room for them on the radiators of our 10,000 cu. ft. house. these devices must be filled by hand. bad defect in apparatus of this kind.

None of the manufacturers of these articles seems to have recognized the essential physical principles which are (a) large surface of water exposed to air, (b) rapid renewal of air over the surface of the water. The temperature within the limits set by radiator heating is of less importance in securing evaporation than either of the factors mentioned above. Depth of water is of no importance, yet several manufacturers have made their receptacles as deep as possible, (e. g. "Speco," "Savo").

Attacking the problem with the above principles in mind I have devised several types which are ten or more times as effective as anything on the market.

One of these shown in Fig. 1, consists of several trays, one above another, and so arranged that warm air rising from behind the radiator constantly passes between the trays

RADIATOR HUMIDIFIER MADE UP OF WATER TRAYS

-Wall line

Wall fastening (if desired)

and over the water surface therein. Twelve trays as indicated in figure give ten square feet of water surface per linear foor of radiator occupied.

One of these apparatuses thirty inches long evaporated 3.7 gallons daily in my house last January. Four or five such humidifiers would be sufficient, in our house of 10,000 cubic feet, to meet our tentative requirement of 40-50% humidity.

But the apparatus is subject to two criticisms. (a) It is bulky and the ladies don't think it is pretty. (b) It has to be filled by hand. I put 31⁄2 barrels of water into that one in my house during January last. Most people would balk at that. Of course it would be possible to pipe water and waste to each radiator and make the apparatus automatic.

I worked for a long time with drip systems. They can be made very efficient but require a large amount of supervision.

Last winter I experimented with wick systems. The troubles with those humidifiers on

2Lyon: Heating and Ventilating Mag., Aug., 1917.

BALL VALVE

EXPANSION TANK