# Popular Science Monthly/Volume 84/April 1914/Fresh Air

Layout 4
THE
POPULAR SCIENCE
MONTHLY
APRIL, 1914
* FRESH AIR
By FREDERIC S. LEE
DALTON PROFESSOR OF PHYSIOLOGY, COLUMBIA UNIVERSITY
ON one of the hottest of the hot nights of British India, a little more than one hundred and fifty years ago, Siraj-Uddaula, a youthful merciless ruler of Bengal, caused to be confined within a small cell in Fort William, one hundred and forty-six Englishmen whom he had that day captured in a siege of the city of Calcutta. The room was large enough to house comfortably but two persons. Its heavy door was bolted; its walls were pierced by two windows barred with iron, through which little air could enter. The night slowly passed away, and with the advent of the morning death had come to all but a score of the luckless company. A survivor has left an account of horrible happenings within the dungeon, of terrible stragglings of a steaming mass of sentient human bodies for the insufficient air. Within a few minutes after entrance every man was bathed in a wet perspiration and was searching for ways to escape from the stifling heat. Clothing was soon stripped off. Breathing became difficult. There were vain onslaughts on the windows; there were vain efforts to force the door. Thirst grew intolerable, and there were ravings for the water which the guards passed in between the bars, not from feelings of mercy, but only to witness in ghoulish glee the added struggles for impossible relief. Ungovernable confusion and turmoil and riot soon reigned. Men became delirious. If any found sufficient room to fall to the floor, it was only to fall to their death, for they were trampled upon, crushed and buried beneath the fiercely desperate wave of frenzied humanity above. The strongest sought death—some by praying for the hastening of the end; some by heaping insults upon the guards to try to induce them to shoot. But all efforts for relief were in vain, until at last bodily and mental agony was followed by stupor. This tragedy of the Black Hole of Calcutta will ​ever remain as the most drastic demonstration in human history of the bondage of man to the air that surrounds him.
What is this thing upon which the life of the body is so dependent? As history goes, it is only comparatively recently that we have learned what air is. "To tell the story of the development of men's ideas regarding the nature of atmospheric air/' says Sir William Ramsay, "is in great part to write a history of chemistry and physics." Believed at first to be a single substance, by the middle of the seventeenth century men began seriously to try to learn by means of experiment whether air is not compound. It would take us too far from our immediate subject to wander through the mazes of more than a hundred years of those early efforts, of the rise of the belief that air contains some ingredient that is necessary to both combustion and respiration, of attempts to identify this substance, of the contest between the phlogistic and the antiphlogistic theories, and finally of the rather rapid crystallizing out of the air's constituents. The credit of solving the problem belongs almost wholly to Englishmen. In 1755 Joseph Black isolated carbon dioxide, the first constituent of the air to be definitely recognized. Nitrogen was next to appear, the discovery of Daniel Rutherford in 1772; and two years afterward Joseph Priestley published the first description of oxygen. Here the matter rested for more than a century, when in 1895 Lord Rayleigh and Sir William Ramsay aroused the world by the announcement that they had found in air a new gaseous element in minute quantity, which they proposed to christen argon, the inert. To this Ramsay subsequently added the still more rare helium, krypton, neon and zenon, and he says:
It would be rash to predict that no other elements still remain to be discovered among its [the air's] constituents; but if there are they must be present in still more infinitesimal amount than the rarer non-active gases.
We may, therefore, doubtless rest content with the thought that the problem of the air's constituents is practically solved and that, when pure, air is simply a mixture of gases, mostly elementary.
Air of ideal purity never exists outside the chemist's test tubes. The gases of atmospheric air are usually present in the following approximate proportions by volume:
| | Per Cent. |
| Oxygen | 20.94 |
| Carbon dioxide | 0.03 |
| Nitrogen | 78.09 |
| Argon | 0.94 |
| Helium, krypton, neon, zenon, hydrogen, hydrogen peroxide, ammonia | traces. |
As we know it and breathe it, air always contains other constituents derived partly from the inorganic earth, partly from plants and ani ​animals and largely from man and his works. Its oxygen is often diminished in quantity, its carbon dioxide often increased; it always contains the vapor of water in appreciable amounts, traces of nitrous and nitric acids, radio-active constituents, dust and usually bacteria; its composition may be altered by the presence of sea salts, by the respiration of man and other animals and plants, by the combustion of illuminating gas and its products, and by a host of industrial processes. Of these various alterations those produced by the respiration of man is of chief interest to us here. The gases of air as it comes out of man's lungs are present in the following approximate proportions by volume:
| | Per Cent. | Per Cent. |
| Oxygen | 16 | .4 |
| Carbon dioxide | 4 | .1 |
| Nitrogen | 78 | .09 |
| Argon | 0 | .94 |
| Helium, krypton, etc. | traces. | traces. |
Of the air's various gaseous constituents, nitrogen, argon and helium and its companions are what the chemists call inert substances, i. e., they are slow and backward about entering into chemical alliances. However important nitrogen is in the life of living things, neither is it, nor are these other inert gases, known to exert as atmospheric components specific actions on living human beings. They may, therefore, be eliminated from consideration, as may also the minute traces of hydrogen and ammonia, and our attention may be focused at once upon oxygen and carbon dioxide.
The writer's shortness of breath became more and more distressing as he rose; all were more affected than at any time before, but none of the others in this acute way. The fits of panting became more frequent and more violent; at such times everything would turn black before his eyes and he would choke and seem unable to recover his breath at all. Yet a few moments' rest recovered him as completely as ever, to struggle on another twenty or thirty paces, and to sink gasping on the snow again. . . . With keen excitement we pushed on. . . . The last man on the rope, in his enthusiasm and excitement somewhat overpassing his narrow wind margin, had almost to be hauled up the few final feet, and lost consciousnes for a moment as he fell upon the floor of the little basin that occupies the summit.
Various experimental researches, and especially the latest and very careful investigation by Haldane, Douglas, Henderson and Schneider on Pike's Peak, have proved beyond a doubt that that formerly mysterious disease, mountain sickness, is due solely to the greatly diminished pressure of oxygen existing at all considerable heights. That wonderful power of adaptation to unusual conditions, however, of which the human body is so generously possessed, is here demonstrated in the fact that on reaching the unusual height the quantity of hemoglobin, which gives the red color to the blood and enables it to carry oxygen to our tissues, begins to increase, and a few days of life at the high altitude renders us capable of continuing to live there under the diminished oxygen pressure without further danger to life.
Within a crowded assembly the proportion of oxygen may fall to one twentieth of its usual amount in the outdoor air, probably never more except in the most extreme experimental conditions. Experimentation has apparently shown that the evil effects of such indoor air are not due in any respect to this slightly lesened quantity of the gas. It has even been diminished to less than seventeen per cent, in experimental chambers without apparent detriment to persons confined therein. Hill says of a group of his students whom he confined in a narrow air-tight room: "We have watched them trying to light a cigarette (to relieve the monotony of the experiment) and puzzled by their matches going out, borrowing others, only in vain. They had not sensed the percentage of the diminution of oxygen, which fell below seventeen." The ventilation of coal mines by air containing only seventeen per cent, of oxygen has indeed been suggested as a preventive of explosions. On the other hand, a "sand hog" working in a caisson at a depth of one hundred feet must be subjected to a pressure of oxygen four times that found in the usual atmosphere. Here he can work for several hours with impunity; a longer time would give an opportunity for the excess of this gas to manifest its toxic action on his tissues. Because of this poisonous action too, a man can breathe pure oxygen when in excess for a limited period only. The administration of oxygen in extreme illnesses thus has its limitations.
​Ozone is a form of oxygen in which three, instead of the usual two, atoms are united in the molecule. It is present in minute quantity in the atmosphere, usually not of cities, but of the country and the sea. Its powerful oxidizing properties and its intemperate advocacy by enthusiastic but unscientific persons have caused it to be hailed popularly as highly beneficial to the human body, not only in ordinary respiration, but in the purification of the air of living rooms, the destruction of bacteria and other organic matters, and the cure of disease. On crisp cool mornings we are fain to enlarge our chests as we step into the open, and breathe in deep draughts of this supposedly health-giving gas; to mountain tops and forests we go in search of its renovating properties; and our mail is fat with circulars descriptive of the marvelous benefits of ozone machines, of ozonizers and ozonators. In many offices and homes we find these machines, busily at work discharging into the atmosphere their peculiarly odoriferous product. Very recent investigations, however, seem to make it clear that the supposed beneficial powers of ozone as a home companion are creations of the imagination. Two groups of American investigators, Jordan and Carlson in Chicago and Sawyer, Beckwith and Skolfield, in Berkeley, have independently carried out each a series of careful experiments on the action of ozone on bacteria, animals and human beings. They find that ozone will indeed kill bacteria exposed in a room, but only when in such concentration that it will kill guinea-pigs first.
There is no evidence for supposing that a quantity of ozone that can be tolerated by man has the least germicidal action.
When present in any considerable quantity in the air ozone is irritating and probably corrosive to the lining membrane of the air passages of the nose, throat and lungs, causing the blood-vessels of this membrane to be excessively dilated and to present the customary symptoms of "sore throat." It causes headache and drowsiness. The heart, at first accelerated, is later slowed and weakened, and the pressure of the blood in the arteries is unduly lowered. The case for ozone thus seems to narrow down to a supposed beneficial action in destroying or modifying unpleasant odors in the air of a room. When in not too great concentration such odors are, it is true, overcome, though it is quite probable that their disappearance is due, not to an actual destruction of the odoriferous substance, but partly to a replacement of the disagreeable odor by the odor of ozone and partly to fatigue or anesthesia of the olfactory membrane of the nose. It is very questionable whether this is wise, and Jordan and Carlson well say:
It seems to us that this is wrong in principle, and that ozone is being used and will be used as a crutch to bolster up poor ventilating systems. Ozone does not make pure air any more than strong spices make pure food.
​Perhaps the last word has not yet been said on this subject; nevertheless I strongly suspect that ozone does not deserve the reputation which commercial interests are endeavoring to foist upon it and that as a panacea it is destined to follow into oblivion phylacteries and amulets, blue glass and the rabbit's foot.
Carbon dioxide within man's body performs certain useful purposes. Generated in all of his tissues and passed from his cells into the lymph and blood that bathe them, when in not too large quantity it reacts upon the tissues as a hormone, or excitant, to stimulate them to greater activity. It thus for a time, during the earlier stages of a task and before fatigue sets in, augments our working power. One of its most striking services is that of acting as the stimulus to that part of our nervous systems which presides over our respiratory movements. After each expiratory act the accumulating carbon dioxide within our blood excites our respiratory center to a subsequent inspiration, and except for this substance within us respiration would be impossible. But when in larger quantity than is required for these needs carbon dioxide is poisonous to our tissues, causing fatigue and depression of working power, and for the good of the organism must be expelled. It is, therefore, carried to the lungs and passed into the air. Thus much of the carbon dioxide of the atmosphere represents a waste and poisonous product of protoplasmic activities, of no use to man, however valuable it is to the green plants. When breathed in excessive quantity it may cause a feeling of suffocation, headache, nausea and other unpleasant sensations, and may ultimately even be fatal. But the poisonous properties of carbon dioxide have been exaggerated. Thus while normally it is present in free air in only about three hundredths of one per cent., the breathing for hours of more than thirty times this amount does not appear to be detrimental to the individual. In fact it has recently been shown by Crowder that the air immediately before our faces is contaminated with expired carbon dioxide in varying quantities which in extreme cases may reach one per cent. Except where ventilation is very vigorous, as in facing the breeze of an electric fan, we are thus habitually rebreathing a portion of the air which has previously entered our lungs. In the face of such facts the minute variations of carbon dioxide in unconfined air are altogether negligible from the hygienic standpoint. Thus the proportion of this gas is greater in the air of cities than of the country, in night air than in that of day, in fogs than in clear weather, with a low than with a high barometer, at the foot than on the tops of mountains, and inland than at the seaside. But all such differences amount to merely a few thousandths of one per cent, and are probably of no importance whatever in the life of the individual. There is a larger proportion of the gas in the more or less confined air of crowded assemblies, school rooms and industrial work ​rooms, but even here it very rarely reaches four tenths of one per cent., or ten times its usual amount, and this is still well below the harmful limit. It thus appears that carbon dioxide, like oxygen, may be eliminated from the problem of fresh air, except under the rarest and most extreme circumstances.
The amount of carbon dioxide present is often regarded as a convenient and proper index of the degree of vitiation of air by human beings, and a limit to this is sometimes established by law for factories where many employees work together. Our country unfortunately has not reached that stage of governmental control of its industries in which legal standards of ventilation are established and maintained. We ought to do this in the interests of the health of the workman, but when we are prepared for it we should select some other index of the air's impurity than the amount of carbon dioxide present in it.
There has long existed a belief—and it has been strengthened by the advocacy of competent men of science—that air that has once been breathed by human beings is poisonous apart from its content in carbon dioxide, and this belief has fixed upon a hypothetical unknown organic constituent, a toxic protein, which is supposed to be produced within the body, volatilized and then cast out with the outgoing breath. Various attempts have been made during the past twenty-five years to support this belief experimentally. The most of such experiments have consisted in condensing expired air, injecting it into animals and obtaining symptoms of intoxication. Notwithstanding their seeming conclusiveness one by one these apparently positive results have been explained on other grounds than as due to the presence of an expired organic poison coming from the lungs and, moreover, they have been offset by more conclusive experiments terminating negatively. The latest of these researches finds no evidence whatever to support the theory of an organic poison, a "crowd poison," as it is sometimes called; and we must believe that the theory represents one of those erroneous conclusions which science frequently draws from incomplete evidence and then proceeds to utilize in discovering the truth.
Another feature of vitiated air is odor. Odor is always due to the existence of material, in the form of either gas or very finely divided solid particles, which has the power of stimulating the delicate terminals of the olfactory nerves in the walls of the nasal passages. Pure air contains nothing that can stimulate these nerve terminals and therefore is wholly free from odor. Odor may be introduced into air through decaying organic matter, through illuminating gas or the products of its combustion, through various foreign substances used in industrial procedures, and through emanations from the human body. These last are many and varied, both in quality and origin, and together they give to the air of a crowded assembly which lacks adequate ventilation the ​peculiar unpleasant crowd odor with which all are familiar. Our sense of smell is subjected continuously to slight stimulation, but it is peculiarly and vividly responsive to unpleasant changes in our odorous environment. Thus on entering a crowded, close and stuffy room the odor often seems to us intolerable, and we at once assume that the air is very bad for any one who breathes it. We rush to the window and throw it open, or complain to the janitor, or retreat in disgust. Well, the air may indeed be very bad, but this is not because of its odor, except as to the odor's possible psychic effect. There is a peculiar relation between one's sense of smell and one's esthetic sense, and an unpleasant odor by rudely shocking the esthetic part of our nature may interfere with our efficiency; but there is no evidence in support of the idea that the odoriferous elements in crowd air are physically or chemically harmful to us. Our sense of smell, however it may disturb us, is probably the least valuable of all our senses in contributing to our physical welfare and it can the most readily be dispensed with—a too sensitive nose is really an affliction. This sense is in fact extremely subject to fatigue, and hence on confinement in crowd air our olfactory aversion to it soon ceases—a provision of nature which is not altogether an evil.
Strangely enough it is only within a period of scarcely more than thirty years that the occurrence and the significance of atmospheric dust have become accurately known. Dust has now been shown to exist in air everywhere: in uninhabited as well as inhabited regions, though the more where man and his works are; at the tops of lofty mountains; and over the largest of oceans. The numbers of dust particles found by different observers in a single cubic centimeter of air have varied from 157 at the summit of the Swiss Bieshorn to more than 200,000 in a Parisian garden. On dusty streets and within doors, especially in dusty trades, still more dust may exist, and it is estimated that a single puff of tobacco smoke discharges into the atmosphere 4,000,000,000 particles. Dust may consist of inorganic and lifeless organic matter, as well as bacteria and other living organisms. It may be carried long distances. The most striking known example of this is the fine pumice which was sent into the air to tremendous heights and in enormous quantities at the time of the extraordinary eruption of the East Indian volcano Krakatoa in 1883. This fine dust was carried completely around the earth and from the extreme north to the extreme south of the largest continents. Moreover, it continued to exist for several years as a component of the earth's atmosphere. To dust particles we owe the existence of clouds, fog and haze, the beautiful colors of the sunset, and in large part the blueness of the sky. Dust is thus our constant companion and with every breath we inhale much of it. Our bodies are prepared for this and possess defensive agencies for our protection. With these agencies in proper order the greater part of the ordinary inhaled ​dust is harmless. It is not so, however, with the dusts produced abundantly in various trades, such as in the manufacture of cutlery, pottery, porcelain, glass, copper, iron, steel, brass and lead wares, in stone-cutting and cotton manufacture. Some of these industrial dusts are poisonous; some are mechanically irritating to the walls of our air passages. In dusty occupations such diseases as bronchitis, tuberculosis and pneumonia are unduly prevalent, and there is no doubt that their beginnings lie in local injuries to the lungs produced by the inhaled particles and that these injuries are followed by the lodgment of the specific bacteria.
Of the bacteria of the air and their relation to disease I must speak at greater length. From the earliest times the belief has existed that bad air is a prolific source of disease. The word "malaria" literally means "bad air," and the disease malaria was the type of those diseases that were supposed to be spread through the atmosphere. In the early days of the germ theory air was regarded as the chief medium of the transmission of disease germs. As one writer graphically put it, disease is "literally borne on the wings of the wind." The great surgeon Lister accepted this notion and conceived the idea of improving surgical technique by maintaining a continual and very fine spray of carbolic acid in the air in the immediate vicinity of the operation. Thus antiseptic surgery arose. Although surgery has now gone far beyond this stage and no longer regards the air as a source of operative infection, the general notion of aerial infection still prevails. But a multitude of facts, gradually accumulated, show that this notion must be revised. It is true that bacteria may be moved through the air, and this may occur under three conditions: when they are freely floating, when they are attached to particles of dust, and when they are contained within the bodies of flying insects. The dissemination of disease germs by insects is a serious fact—the mysterious miasma of malaria lies only within the body of the mosquito, and malaria is still the type, but in a new sense, of certain diseases that are spread through the atmosphere. But there are many reasons for believing that the danger of infection through germs freely floating in air or attached to particles of dust has been much exaggerated. Living organisms, it is true, may be found in the atmosphere of inhabited localities under almost any circumstances. To capture them it is only necessary to expose to the air for a few moments a sterilized plate covered by a layer of nutrient agar on which the floating particles may fall. If the plate be then covered and transferred to a warm place, the organisms will proceed to multiply and develop colonies. It is then found that they comprise bacteria and some other microscopic forms. By far the greater number are quite harmless, but pathogenic or disease-producing species do occur. These may include germs of tuberculosis, diphtheria, typhoid fever, dysentery, ​anthrax and suppuration. The mere fact that such germs have at times been found, however, is of little significance in the matter of possible aerial infection. They never occur in any considerable numbers, and considerable numbers of germs are usually necessary to produce a disease. It is known that many bacteria on being cast out into the air from an infected source lose their virulence in the process of drying and soon die. Evidence that disease germs pass through the air from room to room of a house or from a hospital to its immediate surroundings always breaks down when examined critically. It is indeed not rare now to treat cases of different infectious diseases within the same hospital ward. The one place of possible danger is in the immediate vicinity of a person suffering from a disease affecting the air passages, the mouth, throat or lungs, such as a "cold," or tuberculosis. Such a person may give out the characteristic microbes for a distance of a few feet from his body, not in quiet expiration, for simple expired air is sterile, but attached to droplets that may be expelled in coughing, sneezing or forcible speaking. In this manner infection may, and at times probably does, occur, the evidence being perhaps strongest in the case of tuberculosis. But apart from this source there appears to be little danger of contracting an infectious disease from germs that float to us through the medium of the air—aerial infection in the most of those diseases with which we are familiar is, in the authoritative words of Chapin, "under ordinary conditions of home and hospital a negligible factor." Avoid all forms of physical contact with disease germs or germ-laden articles; keep hands and dishes clean; beware of infected food and water; if you can detect him shun the bacteria-carrier, he who unwittingly carries within his body the germs without the disease and may deposit them where subsequent physical contact is possible; but do not be tormented any longer by the unnecessary specter of germladen air.
I might add a few words concerning sewer gas. Sewer gas consists simply of air containing volatile substances which are given off by the decomposing organic matters that occur in sewage. There is nothing mysterious about the components of sewer gas except in the minds of those who stand in dire dread of it. It may contain carbon dioxide, the ill-smelling hydrogen sulphide and ammonium sulphide, marsh gas, ammonia and certain other gaseous substances—all of these in variable, and, with the possible exception of carbon dioxide, usually small amounts. There is no excessively poisonous gas among them. Bacteria exist abundantly in sewage, but these appear to be given off to the air only when the liquid sewage is mechanically splashed and then only in very small numbers and usually not to great distances. Winslow has made a most careful experimental study of this subject, and has come to ​the conclusion that "the chance of direct bacterial infection through the air of drains and sewers is so slight as to be practically negligible." He says:
If one were to breathe for twenty-four hours the undiluted air of a house drainage system, at any point not immediately infected by mechanical splashing, it appears that less than fifty intestinal bacteria would be taken in. . . . There would be less danger of contracting disease from continuously breathing the air of a vent pipe, except where liquid is actually splashing, than from drinking New York water.
Workmen in sewers are notoriously strong, vigorous, healthy men, with a low death rate among them. With such facts before us the specter of an invisible monster entering our homes surreptitiously from our plumbing pipes and sapping our lives and the lives of our children must be laid aside; we need no longer leave saucers of so-called "chlorides" standing about our floors to neutralize in an impossible manner mysterious effluvia that do not exist; and when we return to our town houses in the autumn we may enter them with no fears that we are risking our lives by coming into a toxic, germ-infected, sewer-gas-laden, deadly atmosphere.
Our consideration so far of the qualities of air and their relation to human beings has been mainly destructive. I have tried to make it clear that we must give up some of our long-cherished notions. We have seen that while air may be rendered unsuitable for respiration by Tery unusual means, such as the addition of poisonous gases or excessive quantities of irritating dust or bacteria, the vitiation of air by the presence within it of human beings is not due, except under the most rare and exceptional circumstances, to chemical changes produced therein by respiration, such that when the air is rebreathed it reacts harmfully through the blood on the bodily tissues. The claims that are sometimes made by the venders of commercial chemical air-purifiers on behalf of their machines are based upon entire ignorance of the facts.
Nevertheless, that the air of a confined, ill-ventilated room when crowded with human beings soon becomes bad can admit of no question, and we are forced to search further for its bad qualities. Science has in recent years apparently found them in the physical, rather than the chemical action of such air on the body. This conclusion has been reached, not so much through inability to find the evil in other features, as through the very positive results of many experiments made by different investigators.
The human body is constantly burning fuel within itself and producing heat in the process. The amount of heat thus produced during twenty-four hours by an average adult man, when at rest, is about 2,400 calories, which is equal to the heat evolved by four or five ordinary Tungsten electric lamps during the same time. Such a man doing hard ​physical work generates more than twice this amount. The heat generated within man's body is not wasted energy; it keeps his bodily tissues at the temperature (37° C.; 98.6° F.) at which nature has decreed that they shall do their best work. But more heat is produced within than is needed for this purpose, and if this excess were allowed to accumulate unchecked, man's tissues would very soon become unduly heated, the protoplasm of his living cells would become coagulated, and death would be the end of him. He possesses, however, a very efficient regulating mechanism by which his body is enabled to give off heat constantly and in quantity just sufficient to maintain an equilibrium, notwithstanding the varying amounts which he produces from minute to minute. This constant output of heat takes place partly through expired air, but chiefly by direct radiation into the air from the skin, by conduction from the skin to the clothing, and by the evaporation of perspiration poured upon the surface of the skin by the sweat glands. The skin is thus the medium by which the excess of the bodily heat is carried away. But the action of the skin is dependent upon the action of the nervous system in regulating both the amount of hot blood sent to it and the activity of the sweat glands. Whenever, therefore, the body works harder than before and produces more heat, not only does the breathing intensify, but through the nervous system the cutaneous blood vessels are dilated, more blood is sent to them, more perspiration is made in the glands, poured out and evaporated, and thus the excess of heat is passed out to the clothing and the air. By these provisions our bodily temperature is kept fairly constant, whether we do much or little work, whether we live indoors or outdoors, in summer or in winter, whether we labor beside molten metal at a temperature of 250° F. or are exposed to the polar air with its 75° F. below zero.
Nevertheless, it is obvious that there are external essentials to this physiological power of regulation and that these are the possibilities of the radiation and the conduction of the heat and the evaporation of the perspiration. The body is ever ready to do its share, but the surrounding air must be in such a physical condition as to supplement the body's activities. If the air be cool and moderately dry the best conditions exist for the body's well-being; if the air be hot and dry, or cool and moist, within certain limits the body can protect itself; but if the air be hot and at the same time contain much moisture a condition exists against which the body is imperfectly equipped. If the external temperature be as high as or higher than the bodily temperature, bodily heat can not be given out by radiation and conduction, and if at the same time the air be saturated with moisture, bodily heat can not be given out by the evaporation of perspiration; and thus with the two principal avenues of heat loss obstructed and with the fires still burning within, the temperature rises and the unfortunate individual passes ​into a fever. This fever is accompanied by abnormal chemical changes within his tissues and the production of toxic substances, which in turn react upon his tissues diminishing their working power, inducing early fatigue, and upsetting the normal equilibrium of his organs. The result of such a disturbance of his bodily mechanism, if very pronounced, is the production of a pathological condition which is called heat stroke.
But the extreme condition of air at a temperature above the bodily temperature and completely saturated is not necessary for inducing the pathological symptoms. We may witness them under somewhat more moderate conditions in the frequent cases of sunstroke which occur in the streets of our American cities on hot and humid days. The observations of Kubner, one of the foremost German hygienists, indicate that even at 75° F., or more than 23 degrees below bodily temperature and with a humidity of only 80 per cent, of saturation an untrained man can continue comfortable only by refraining from physical work. The performance of work under these conditions would throw a tax upon his powers of adaptation. Even at still lower degrees of temperature and humidity the unfavorable symptoms may begin to appear, indeed the point at which our environing air ceases to be comfortable and begins to force us to make special efforts at accommodation to it is one that is not outside our range of frequent experience.
Many experiments, some of them striking, seem to make it clear that it is to these two features of heat and humidity, the same features which are responsible for sunstroke, and not to others, that all the evil effects of the air of crowded, ill-ventilated rooms are actually due. These experiments have usually consisted in confining and observing men, perhaps several together, in comparatively small experimental chambers. Sometimes these chambers have been little more than bare boxes; sometimes they have been rooms provided with elaborate devices for varying the quantity and qualities of the air. Sometimes the subjects of the experiments have been obliged to breathe over and over again the same air; sometimes the air has been kept under careful control and changed in various ways. The effects of the various conditions have then been observed and recorded. These observations upon human beings have been supplemented by a variety of experiments on animals, and these animal experiments have added greatly to our knowledge of the qualities of the air which human beings ought to breathe.
One of the notable and fruitful investigations was an American one, carried on between 1893 and 1895 by Billings, Mitchell and Bergey with the aid of the Smithsonian Institution in Washington. The Billings of this investigation was the efficient organizer and first librarian of the New York Public Library and the Mitchell was our famous physician-author, Dr. Weir Mitchell. Another helpful American contribution is that of Benedict, whose work with the respiration ​calorimeter at Wesleyan University was so prophetic of worthy contributions to science that he was chosen to organize and direct the work of the Nutrition Laboratory of the Carnegie Institution, which is situated in Boston. The Germans, as ever, have also been leaders in this experimental work, an important contribution having come from the laboratory of Professor Flügge of Breslau. The English have been and are still contributing some of the most significant facts, especially a group of men led by Dr. Haldane of the University of Oxford. The work of Dr. Leonard Hill of London has become widely known through the public prints. The eminence of these various men is indicative of the interest which the homely subject of fresh air can arouse in us.
Several of the investigators have placed men within small closed experimental chambers, arranged with tubes passing through the walls to the outside air, so that the subjects within can at will rebreathe the hot, close, confined air or take in the fresh air from outside. Under such conditions it is found that confinement within and breathing of the unventilated air soon brings on the usual symptoms. If the subjects then breathe through the tube the fresh cool air from outside they obtain no relief. If they step outside relief comes instantly. If, on the other hand, a person standing in the fresh air outside breathes through the tube the stale air of the chamber, which has been breathed over and over again by the subjects within, the unpleasant symptoms do not appear; if he steps inside, they begin to appear at once. If with subjects within feeling the ill symptoms electric fans be started and the stale air be vigorously stirred, thus driving the hottest air away from the skin, relief comes at once. These fundamental experiments have been performed in varied ways, and have been supplemented by many others. Their results have accorded well with one another and allow but one general conclusion, namely, that the evil effects exerted upon human beings by air that has become vitiated by human beings result not from a lack of oxygen, not from an increase of carbon dioxide, not from the presence of an organic poison, not from any chemical features of such air acting through the lungs on the tissues, not in any manner from the rebreathing of such air, but solely from the physical features of excessive heat and excessive humidity interfering with the proper action of the skin in regulating bodily temperature. The problem of bad air has thus ceased to be chemical and pulmonary, and has become physical and cutaneous.
With this knowledge before us it is clear that in the ventilation of the future attention should be focused less upon the chemical purity of air, although of course there are ultimate limits to chemical purity, and more upon the maintenance of a physiologically proper temperature and humidity. What here constitutes physiological propriety varies with individuals, with age, with clothing, with occupations and with ​habit. Undoubtedly our American houses during the winter months are usually kept too hot to maintain the highest efficiency of the individual. We are in far better physical condition when surrounded by a house temperature of 65° to 68° F. than of 70° F. Some of the British authorities advise a house temperature as low as even 60° F. Young persons can live efficiently in a lower temperature than those of middle life, while aged persons require warmer air. A lower temperature is better where physical work is being done. The following temperatures of heated rooms are recommended by American ventilating engineers:
| Occupants at Rest | Degrees F. | Occupants Physically Active | Degrees F. |
| Living rooms, offices, schools | 68 | Gymnasiums | 60 |
| Lecture halls | 61-64 | Work shops, moderate exertion. | 61-64 |
| Sleeping rooms | 54-59 | Work shops, vigorous exertion. | 50-59 |
| Bath rooms | 68-72 |
As to humidity, a percentage of 60 with air of 68° F. is rational. But the amount of moisture that air is capable of absorbing varies greatly with the temperature, hence it is impossible to establish a single standard of humidity that can apply to a range of temperatures. The surest single index of the physiological quality of the atmosphere at any moment is the reading of the wet-bulb thermometer. In this thermometer the bulb is covered by thin muslin or silk soaked with pure water. The evaporation of the water cools the bulb. The position of the mercury in such an instrument depends on two factors: first, the temperature of the air; and secondly, the amount of evaporation of the water immediately surrounding the bulb, which in turn varies inversely with the amount of moisture in the air generally—the more moisture in the air the less evaporation from the bulb. The wet-bulb thermometer is thus an index, at once, of both temperature and humidity. The most efficient simple instrument for the determination of humidity is the combination of dry bulb and wet bulb thermometers known as the sling psychrometer, but a fairly satisfactory indicator for household use is the instrument sold commercially under the name of hygrodeik. For our living rooms a wet-bulb reading of 60° F. is favorable to the maintenance of a comfortable and efficient physiological state. We can usually keep the temperatures of our rooms within reasonable limits by the aid of our heating systems and air admitted through windows; but the humidity can not be so perfectly controlled without more elaborate means than most private houses are provided with. With the increase in size of our American buildings, whether apartment houses, office buildings, school houses or factories, the provision of ventilation by means of more or less elaborate apparatus has become a necessity, and the profession of heating and ventilating engineer has become one of dignity and importance.
​If I were to add to this lecture a paragraph of practical hints, I would say, first of all, keep your houses and offices cool, never above and usually well below 70° F. Unfortunately here a difference between men and women sometimes causes trouble. Woman possesses a perpetual blanket of adipose tissue between her skin and her muscles, which is usually less developed in man, and hence women can dress more thinly than men, and are usually comfortable at a lower temperature. I have seen more than one happy home in danger of wrecking from this unfortunate difference. As a married man I am tempted to plead for greater charity on the part of wives; as a physiologist I realize that a lower temperature is more healthful. Keep room air in motion. An electric fan or a current of air from a window is a great aid in keeping down one's bodily temperature, and preventing sleepiness and bodily discomfort from stagnant air; with electric fans in use there would be fewer naps in churches and lecture halls. Air in motion promotes efficiency. Accustom yourselves to draughts, and especially big draughts. A small blast of cold air directed against a small area of warm skin may do harm, but the larger the current the more the harm gives way to benefit. Air of constantly uniform temperature is monotonous and debilitating. An occasional and considerable cooling, a flushing of the room by a sudden large inrush of outside air is, like a cold bath, stimulating. Do not be afraid of opening the windows of sleeping rooms at night. The prejudice against night air, which arose naturally enough from the belief in the existence of nocturnal disease-bearing miasms, in the light of present knowledge is a foolish prejudice and must give way to the rationalism of scientific fact. The increasing employment of cool outdoor air both night and day as a therapeutic agent in the treatment of disease is based on scientific principles and is justified by its results. And, finally, the whole moral of the modern physiological doctrine of fresh air may be expressed tersely in the two short words, keep cool.
I have thus endeavored to present to you a fair picture of the present attitude of science toward the problem of fresh air and its relation to health. Such a consideration 'affords an unusually fruitful opportunity to witness the ways in which science progresses, forming hypotheses, testing them and then retaining, rejecting or refining them, as the evidence derived from observation and experiment warrants. Of the subject before us there are still many gaps in our knowledge, and these gaps must be filled. Present knowledge is never final, and our present ideas of what constitutes fresh air may yet require revision. There has recently been brought together in the City of New York under the influence of the Association for Improving the Condition of the Poor and with governmental recognition a group of representative men of science ​constituting the New York State Commission on Ventilation. These men are keenly alive to the many interests involved in the general problem of air in its human relations and are now beginning in this city an extended experimental investigation of them in the hope of obtaining results of value to both science and humanity. The man of science who thus successfully investigates feels the keen and satisfying joy of pushing back a little farther the barriers between the known and the unknown; and the multitude who look on reap a benefit from his labor in seeing pointed out a way to more healthful living.