Popular Science Monthly/Volume 19/August 1881/School-Room Ventilation
VENTILATION is the supply of fresh air to an apartment, and the removal of impure or vitiated air therefrom. An adequate supply of free oxygen is absolutely necessary to animal life; and, the higher we ascend in the scale of that life, the greater the quantity of oxygen consumed, and the more urgent the necessity for its consumption. In the atmosphere this oxygen exists in a free state—in mechanical solution—and in the form and proportion in which it is most easily assimilable. From the atmosphere, the animal absorbs it by means of its breathing apparatus which provides for its absorption by the blood, and the blood carries it to the tissues. Pure air consists of a mechanical mixture of about four fifths nitrogen and one fifth oxygen, with traces of ammonia, and about one part in two thousand of carbonic-acid gas (CO2). These latter (ammonia and CO2), from their small amount, may be neglected.
Air becomes vitiated for breathing purposes by holding in solution other gases or substances whose presence interferes with the appropriation of oxygen by the animal, or, being themselves absorbed, exert a toxic influence upon the vital fluid and tissues of the body. Hence to secure an adequate supply of fresh air, and the removal of impurities that accumulate therein, are the objects of ventilation. In this paper school-room ventilation only will be considered.
A full-grown person breathes on an average about twenty times per minute, and takes in over twenty cubic inches of air at each inspiration. Boys and girls inspire somewhat less than twenty cubic inches, but breathe more rapidly than an adult—say twenty-five times per minute. In five minutes each will breathe over a cubic foot of air, and in a two-hours session nearly twenty-five cubic feet: so that, in a school of forty pupils, one thousand cubic feet will be inhaled every two hours. This is under, rather than above, the average.
Oxygen to the amount of nearly five per cent, of the quantity inhaled disappears at every breath, being absorbed by the blood—or twenty cubic inches per minute, for each individual—representing a total of fifty cubic feet for a school of forty pupils during a two-hours session. But, in addition to the consumption of oxygen, the air is further deteriorated by the exhalation of nearly as much carbonic-acid gas (CO2) as there is oxygen consumed—say forty-five cubic feet in two hours, about one fortieth of the total amount produced being thrown off by the cutaneous surface of the body. Each cubic foot of carbonic-acid gas contains nearly half an ounce of pure carbon, or twenty-three ounces in all: so that, by breathing, forty mouths—like veritable little chimneys—puff out in two hours an amount equal to about a pound and a half of solid carbon. This is injurious in two ways, each of which will be examined in the proper place.
The air occasionally contains many impurities, but only those usually found in the school-room will here be enumerated. They are carbonic oxide (CO), carbonic-acid gas (CO2), ammonia (NH3), sulphur (S), sulphuretted hydrogen (H3S) all in the gaseous form; to which must be added aqueous vapor, organic matters, inorganic matters, epithelial cells, and animal exhalations.
The most toxic of all these is undoubtedly carbon monoxide (CO). It is a product of the incomplete combustion of carbon (C), but happily it is not usually found in the school-room in any large amount. A fire is the result of the chemical combination of the carbon of coal, or other combustible, with the oxygen (O) of the air; the atoms of the gas rush into combination with those of the carbon, and the arrested motion is transformed into heat—aqueous vapor (H2O), carbon monoxide (CO), and carbonic-acid gas (CO2) being produced. If a sufficient supply of air has free access to the lower portions of the fire, carbonic-acid gas is directly formed; but this in its passage upward through the central portion of the fire, where the temperature is higher, takes up another atom of carbon (CO2 C CO CO) and becomes carbon monoxide, or carbonic oxide, as it is commonly called. This carbonic oxide, on reaching the upper surface of the fire, takes up an additional atom of oxygen from the air, and, burning with a bluish flame, becomes carbonic-acid gas once more, and makes its escape by the chimney. But usually a portion of the carbonic oxide fails to take up the additional atom of oxygen; and, when the supply of air is limited, the amount is increased, so that more or less carbonic oxide passes up the chimney along with the other gases of combustion. As the products of combustion are much lighter than the surrounding atmosphere—volume for volume—on account of their much higher temperature, and as the expansibility of gases is very great, they exert a pressure upon the sides of the pipe or flue through which they ascend. This being the case, these gases will escape through chinks, holes, or defective joints, along their course, like steam through a leaky conduit. Downward air-currents in the flue, and lateral currents from open windows, etc., occasionally blow large quantities of the gases of combustion through the open door of the stove, or through seams or cracks therein; and in these two ways—through stove and flue—sulphur, carbonic oxide, and carbonic-acid gas, may find their way into the room. It is claimed by some physicists that carbonic oxide will make its way through heated iron, and thus escape through the sides of the stove, but the quantity given out in this way—if, indeed, any is so given out, of which there is a reasonable doubt—must be so small that it is practically of no account, while quantities large enough to be decidedly injurious may issue through the door and other openings. Of course, these remarks apply only to schools heated by stoves; but it must not be forgotten that in rural districts, and many cities, all the schools are still heated in this way.
Carbonic oxide is a deadly poison, fixing itself in the blood-corpuscles and paralyzing them so that they can not carry on the function of respiration. To the inhalation of this gas is chiefly due the pale color of those who spend much time in apartments heated by stoves and poorly ventilated. Its presence can not be recognized by the senses, as it is tasteless, colorless, and inodorous.
Carbonic acid is produced in two ways, as before explained—by combustion and by breathing. The quantity thrown off in breathing is very much increased—often nearly doubled—during active digestion. As the fullest meal is taken at dinner, and digestion is most active soon after, it follows that the exhalation of carbonic-acid gas is greatest during the early part of the afternoon, and therefore during this time ventilation needs more attention. Of all the impurities found in the school-room, this is vastly the largest in amount, and popularly considered the most important. It is once and a half as heavy as air. At first sight, it might be supposed that, being heavier than air, it would sink to the floor and settle there in a layer of uniform height and density, like so much water. But this is not the case, for it is even more expansible than air. (Coefficient of expansion of air ·00366; of CO2 ·00371.) Now, the law which governs the mixtures of gases is this:
The mixture of gases in free communication, whatever their density, takes place rapidly, and is homogeneous—that is, the mixture contains the gases in the same proportion; so that the percentage of carbonic-acid gas is about the same in all parts of the room.
If ample provision is not made for the removal of the vitiated air, the proportion of carbonic-acid gas continues to increase; and, as it is much heavier than air, the density becomes greater. Now, this increase of the air's density interferes with and retards the diffusion between the impure gases held in solution in the blood and the oxygen of the atmosphere—in other words, interferes with respiration. The consequence is, that the blood is not purified of the carbonic-acid gas which it holds in solution and combination. Not being removed as fast as it is formed in the body, it accumulates in the blood; the blood carries it throughout the system, circulating it through the delicate tissues of the brain. As the brain is the organ of the mind, it is by and through the brain that we think, reason, memorize, learn. For its healthy and vigorous action, a full supply of pure blood is an imperious necessity. The effects produced by this gas, when circulating through the brain in excess, are drowsiness, dizziness, dull headache, an inability to fix the attention, a dislike for application, a weakening of the memory, and a general torpor of the intellectual powers. An explanation of how and why these effects are produced would involve certain principles of mental physiology—a subject not within the scope of this paper.
Special attention is requested to this statement by Dr. Routh: "Experiment has shown that if an animal be kept confined in a narrow, closed apartment, so that the air supplied is always more or less vitiated by the carbonic acid which it expires, however well fed that animal may be, tubercle (consumption) will be developed in about three months." If this be the case, a large percentage of cases of consumption should be met with among the inmates of badly ventilated schools. But, fortunately, the disease is comparatively infrequent under the age of fifteen, and added to this is the protecting influence of the active exercise in the open air usually indulged in by school-children. It is upon the teachers that its blighting effects are most apparent, as they are predisposed by age, they neglect exercise in the open air, and their mental labor is severe, and worry of mind exhausting. Of eleven teachers who died during the last eight years within the limits of one county in Pennsylvania, two died of acute disease, one of an overdose of an habitual narcotic, and of nine attacked by consumption, eight died—six ladies and one gentleman; the other, a gentleman, will recover, at least for a time.
The organic matters suspended in the air are derived (a) from the body; (b) from other sources. Epithelial cells or scales, very minute, arise by desquamation from the external cutaneous surface, and also from the mouth, pharynx, and bronchi. Being exceedingly light, they float in the air, and are inhaled, lodging in the throat, trachea, and even deep in the lungs. It is not pleasant to contemplate the fact that we inhale minute portions of each other's bodies, but it is true nevertheless. In diphtheria, scarlatina, small-pox, measles, etc., these epithelial scales come off in vastly greater quantities than in health, carrying with them, in greater or less virulence, the peculiar infection in the body whence they have arisen. The greater their number and the more favorable the nidus in which they become deposited, the more likely they are to become transplanted as primary centers of infection. Hence it is important to prevent their accumulation, as the greater their numbers the greater the probability of their successful transplantation; and as they float in the air they follow its currents, and are thus removed by ventilation. Other sources of organic matters are various and numerous, but, with the following exception, of little importance in the present connection.
The cutaneous surface and the lungs give out certain odors, sui generis, which are designated "animal exhalations." It is to these that the heavy, sickening smell noticed on first entering a crowded room is due. Odors being volatile and exceedingly light, these exhalations rise to the highest portions of the room; and, if not allowed to escape, accumulate there, saturating the air from above downward, and finally reaching the floor. Of all the noxious matters in the fouled air of a poorly ventilated school or public building, these are at once the most perceptible, the most offensive, and the most rapidly prostrating. They produce a sensation of stifling by their irritation of the branches of the pneumogastric nerve distributed to the lungs and larynx, and nauseate, probably by reflex action, through branches of the same nerve distributed to the stomach. A distinguished physician, writing of an infant nursery under his charge where the children did not thrive, and many died of diseases of the digestive organs, says: "One remarkable circumstance observed was that there was a faint odor always present in the room. Yet it was a large room, about fifty feet in length. One side of the room was made up of windows which went up about ten feet where the roof or ceiling beveled up in an inverted ∧ shape, which raised the room in the center seven or eight feet more. Do what I would, I could not get rid of this smell. One day, being much annoyed thereat, I procured some long steps which extended about three feet above the upper ledge of the windows. On walking up, no sooner had I got my head one foot above their level, than I found a terrible odor that made me feel giddy and sick; and I was glad enough to come down. I instantly sent for a workman, and desired him to remove three or four tiles at each end of the room, on a level with the highest point of the roof. He did so. In ten minutes all odor had disappeared; but his work was no sooner ended than he was taken very giddy and practically sick, so completely had he been overcome by the pestilential atmosphere." This incident will again be referred to in speaking of ventilators.
In regard to the moisture of the air, the following may be said: The drier the air, the more rapidly are the liquids of the body evaporated, and digestion and assimilation carried on, the more nervous is the temperament, and the more rapid the development. Generally speaking, the air is much drier in the United States than in Europe. This is the chief reason why our children are less repressible, livelier, and more nervous and precocious than those of Europe. Another reason is, that we use here more animal food, which is far more stimulating both to body and mind than vegetable. On the other hand, too dry an atmosphere is unhealthy. As children drink much water, they exhale much aqueous vapor—the sweat-glands and capillary circulation being more active than in the adult say to the amount of half a pint each, more or less, during school-hours. As such a large amount of invisible vapor arises, it serves a useful purpose by adding to the moisture of our dry air, rather than being injurious. In dwellings it is sometimes customary to place a vessel of water upon the stove to produce vapor, so as to diminish the dryness of the air; but, for the reason given above, it is perhaps unnecessary in a school-room. However, as water absorbs equal volumes of carbonic-acid gas, and four hundred and thirty volumes of ammonia, a shallow vessel of water may in this way be of some service.
The inorganic matters consist of chalk-dust, earth-dust, ashes, etc. Of late years, owing to the large amount of blackboard work done in schools, particularly in the primary departments, chalk-dust floats in large quantities in the air whenever the erasers are used. The particles of chalk-dust are comparatively large in size. When inhaled, it lodges in the posterior portion of the nasal passages and upper portion of the larynx; and when settled in large amount in these locations it gives rise to a good deal of irritation. The effect of this irritation is the secretion of a tenacious mucus that provokes distressing cough and unpleasant hawking. It is easy to understand how this exciting cause, long continued, may produce a chronic catarrh of these regions, especially in the posterior nasal passages, as they are prone to congestion and a low grade of chronic inflammation. The same remarks apply, but in a far less degree, to ash and earth-dust. The frequent cough and occasional sneeze heard among the audience in theatre, hall, or church, are provoked by the inhalation of fine dust suspended in the air, and might be prevented by careful sweeping and dusting after occupancy. The school-room should be swept every evening, and dusted at least an hour before opening. The blackboards should be erased as little as possible, and preferably by the so-called "dustless" erasers—though, strictly speaking, no eraser is really "dustless," being simply "less dusty"—and then gently in an up-and-down direction, so that the dust may not be dispersed through the room. The floor should not be disturbed by sweeping at any time during the day. Having examined briefly the different substances that vitiate or foul the air contained in a school-room, and the sources from which they are derived, the means of effecting their removal therefrom will next be discussed.
The chief factors in carrying on ventilation are (a) the difference in temperature between the outside air and that within the room, and (b) the diffusibility of gases.
It is the difference in temperature that produces a draught up a flue or chimney when a fire is lighted below; for the products of combustion have a very much higher temperature (several hundred degrees Fahr.) than the surrounding atmosphere. Being so much warmer, they are lighter in consequence (as will be explained presently), and therefore have a constant tendency to ascend—being compelled by the force of gravity—till, after cooling little by little, they reach a layer of their own temperature. Upon the same principle an inflated balloon ascends and a cork immersed in water constantly tends to rise to the surface. As the coefficient of expansion for gases equals about 273—i. e., they increase about 273 of their bulk for every degree centigrade increase in temperature, thus becoming lighter in proportion to their volume, and, becoming lighter (some being originally lighter) than the atmosphere, are compelled by gravitation to ascend. It is important that the pipe or flue, in rooms heated by stoves or grates, should be vertical or nearly so; also that it be not too wide, otherwise downward currents will be produced, and these interfere with the draught, and cause the gases of combustion to escape into the room. In a stove-pipe the elbows should be as few in number as possible, and rounded rather than acute; for a sharp or abrupt bend materially diminishes the velocity of the draught. Two or more pipes opening into the same chimney should have separate flues; when they open into the same flue, the pipe that draws best will interfere with the draught in the others, and set up downward currents.
The air consumed by combustion escapes by the chimney, and tends to create a vacuum in the room; but it is steadily replaced by the atmosphere which rushes in at every available opening. This rush is strongest at the lowest openings (those nearest the earth), and here the whole amount enters if the space is sufficient. On the other hand, and for the reasons before given, the warmer (lighter) and fouled air within has a constant tendency to escape at the highest points; and it is here, therefore, that ventilators should be placed to allow its exit. Thus it is that, when a door is opened, the warmer (foul) air escapes in a current at the top, and the colder (fresh) from the outside rushes in at the bottom. This may be shown by a lighted taper held in these situations—the flame in each case taking the direction of the current. When the outside air is the warmer, and per consequence the lighter, as on a very warm summer day, the direction of the currents, other things being the same, will be reversed—the fresh air coming in above, and the cool air within escaping below. But, owing to the large amount of heat radiated from the pupils—the normal temperature of the human body averaging 37·5° Cent., or 99° Fahr.—the lighter air is nearly always within. Therefore, if on the sheltered side a window is lowered at the top, or on any side if the air be calm, the foul air will escape above it; if raised from below, fresh air will enter beneath. But ordinarily it is sufficient to fully provide for the escape of the fouled air—the fresh, as a rule, wall not need so much attention; yet it is better to make ample provision also for this. The best method is by ventilators in the walls—say of a foot square in section, or thereabout—raised but a few inches above the floor below, and lowered but a few inches below the ceiling above; or otherwise at the highest points of the ceiling itself. In this way the currents that are likely to blow on the children's shoulders when the windows are raised are avoided, a matter of importance; for a draught of cold air, blowing upon the shoulders from behind, arrests the action of the skin—probably through the spinal sensory nerves—and causes what is commonly known as a "cold." Even when windows are lowered at the top, draughts will occasionally blow upon the pupils; and, the lower the windows are set in the wall, the stronger and more uncomfortable and injurious is the draught. In order to prevent these draughts, the windows should be set high in the wall and lowered on the sheltered side whenever possible. An ingenious contrivance for the prevention of draughts through open windows has been suggested by Dr. Swinburne, in a paper read before the last annual meeting of the New York State Medical Society. It consists in the attachment of one end of a strip of unstarched muslin to a spring roller fastened to the casement above, and the other end to the upper edge of the window itself. On lowering the window, the muslin is unrolled, and thus stretches across the vacant space. Being held tense by the spring of the roller, it effectually shuts off all draught, while it allows the escape of the foul gases within, and the slow but steady entrance of fresh air.
Even should there be no currents through ventilators or open windows, yet the foul gases will make their escape by diffusion; for, according to the law of diffusion, there is a rapid interchange between gases in free communication. Of course, the outflow of the inside air very materially hastens the rapidity of the interchange; but the outflow will not, can not, be very rapid if there is not sufficient provision for the entrance of fresh air other than through the same apertures through which the outflow itself takes place. Again, the warmer the day, the less the difference between the temperature of the inside and the outside air; hence the buoyancy of the inside air is less, and consequently the ventilation not so effective; so that more attention and greater facilities must be afforded it. Ventilators should never be placed in the hall; here they do but little good. The doors leading from the hall to the rooms are usually closed, and, even if open, the buoyancy of the air as a factor in ventilation is nearly eliminated; for there is a partition between the hall and the room, so that the light air and the lighter animal exhalations would be compelled to descend to the level of the top of the communicating door in order to escape. This they can not do, for it is in opposition to gravity. If no other outlet is provided, the only ventilation will be by diffusion through the doorway with the purer air in the hall. The animal exhalations will fill the room from the ceiling to the level of the top of the communicating door, and there remain. It would cost but a trifle to have one or two ventilators put in the ceiling of a school-room where there are none in the walls; and school directors could not make a better investment of the money. Children will not study, and can not be persuaded or compelled to study diligently, in the foul and stifling air of a crowded and wretchedly ventilated room. It may be safely asserted that in a majority of our schools the ventilation is insufficient, or not properly attended to, either on account of lack of knowledge or attention on the part of the teacher, or the defective construction of the building. A sanitary inspection should be made of every school in the State by a competent medical inspector; and all the schools found defective in this (or any other way injurious to health) should have all such defects remedied, or otherwise be condemned as unfit for school purposes, with the imposition of penalties for using them as such.
A school-room should have a high ceiling; contain from two hundred to three hundred cubic feet of air to each pupil; have one or more ventilators in the ceiling, or the walls near the ceiling; have long, high windows arranged to slide upward from beneath, and downward from above. All the children should be sent out at recess, if only for a short time, in order to have their clothing—saturated as it usually is by animal exhalations—exposed to the purifying influence of the open air, and doors and windows thrown open in order to completely change the air within. Stoves, chimneys, pipes, etc., should be carefully looked after, and any accident or defect promptly attended to, or immediately reported. Children convalescing from contagious diseases should be excluded from school for weeks, or months, according to the recognized limit of contagiousness of the disease. It should not be forgotten that the school and the church are the two great centers for the communication of contagious diseases; and that both are active in this way in direct proportion to the insufficiency of the ventilation.
- "Infant Feeding," Part IV, chapter iv.