Normal air contains 79 per cent of nitrogen, 20.96 per cent of oxygen, and .04 per cent (4 parts in 10,000) of carbonic acid.

Oxygen supports animal life; carbonic acid, vegetable life; and the use of the nitrogen, otherwise than as a diluent, is not known.

Very pure air contains 78.98 per cent of nitrogen, 20.99 per cent of oxygen, and .03 per cent of carbonic acid.

Air begins to be very bad when the oxygen is reduced to 20.60 parts in 100. In mines, where candles go out, oxygen is reduced to 18.50 parts in 100, and, in the worst specimen yet examined by Angus Smith, to 18.27. Air in which the percentage of oxygen has been reduced to 17.20 is very difficult to remain in for many minutes.

Aside from impurities due to local causes, the purest air is found from six to forty feet above the ground, and the most impure from seventy to ninety feet, where the air from chimneys is poured forth.

Air is contaminated by the products of respiration and the bodily emanations of healthy persons, and by the products of combustion.

An adult man, in ordinary work, gives off in twenty-four hours from twelve to eighteen cubic feet of carbonic acid, according to his size; women, children, and old persons less.

Edward Smith found that an adult asleep exhaled about nineteen grains of carbonic acid per hour, and, when he walked three miles an hour, the amount was increased to 100.6 grains.

W. R. Nichols, of Boston, found in passenger-cars 23.2 parts of carbonic acid to 10,000 parts of air, and in the Berkeley Street sewer 10.4 parts per 10,000. Wilson found in Portsmouth Prison, in cells containing six hundred and fourteen cubic feet of air, always occupied, 7.20 parts per 10,000, and in cells containing two hundred and ten cubic feet, occupied only at night, 10.44 per 10,000.

Besides the carbonic acid, there is exhaled from the lungs a small amount of organic matter, of unknown composition. It forms a glutinous coating on the furniture, walls, and windows of closed rooms, decomposes rapidly, imparts a peculiarly offensive odor to the air of a badly-ventilated room, and poisons those who inhale it. Its quantity is so small that it has so far defied analysis. In a room contaminated by respiration alone, the odor of this substance begins to be perceived when the carbonic acid has increased to about 7 parts in 10,000, and 10 parts in 10,000 may be considered the maximum amount of carbonic acid allowable in dwellings.

The following table shows how much carbonic acid artificial lights produce per hour:

A five-foot gas-burner produces as much carbonic acid per hour as five men.

As the most poisonous element of the breath can not readily be detected by analysis, the amount of carbonic acid is taken as a measure of the impurity of air contaminated by respiration.

Test for carbonic acid in air (Pettenkofer’s method):

Shake up a definite volume of the air in a closed vessel with a definite amount of lime-water. The carbonic acid unites with the lime, forming carbonate of lime. This compound, being insoluble in water, renders it turbid. The degree of turbidity may be judged of by looking through the water at a cross marked in lead-pencil on the inside of a piece of paper pasted on the opposite side of the bottle, and a standard may be fixed by shaking up ordinary external air in a sixteen-ounce bottle, as described below, which will show the degree of turbidity produced by 4 parts of carbonic acid in 10,000. Lime-water can be bought of a druggist, or made by shaking distilled water with slaked lime, allowing it to settle, and pouring off the clear liquid. With a common hand-ball syringe, the end of the rubber tube resting on the bottom of the bottle, pump in air, until the bottle is filled with the air to be tested. Put in half an ounce of lime-water, cork the bottle, and shake it up well. Let it stand for five minutes, and if the water becomes turbid, as if a little milk had been dropped into it, the presence of carbonic acid in the air will be indicated in the following proportions.

Dangers of such Contamination.

Air contaminated by the products of respiration and by bodily emanations (perspiration, etc.) contains substances which have been ejected from human bodies as useless or injurious. What all systems reject can not be healthy for any, and it is found that long-continued exposure in an atmosphere laden with these impurities produces anæmia, general debility, and poor nutrition, conditions likely to result in the development of scrofula and consumption. It is believed, too, that typhus fever may originate in this manner, while when such poisons are inhaled in a more concentrated form, as in the famous Black Hole of Calcutta, nausea, vertigo, convulsions, and even death are produced.

The air is at certain times and places contaminated by the products of respiration and the bodily emanations of diseased persons.

In certain diseases, commonly known as contagious, organic matters are thrown off by the lungs and skin of the sick, which tend to reproduce these diseases in the bodies of other persons. The exact nature of these poisons is in most cases unknown, but they are generally believed to be living microscopic organisms (bacteria, bacilli, micrococci, etc.), which multiply their kind in the blood of the person who has inhaled them.

Of such diseases, the dangerous ones are small-pox, measles, scarlet fever, typhus fever, and diphtheria, and their contagious quality is marked very nearly in the order in which they are here mentioned.

The less harmful of these diseases are whooping-cough, chicken-pox, mumps, and German measles.

There is strong evidence that consumption is contagious, though not as markedly so as the diseases above enumerated.

The air may be contaminated by the products of the decomposition of the excreta of healthy persons.

The contents of cesspools, privy-vaults, and sewers, are generally composed of discharges from the bowels and kidneys, various matters washed off from the bodies of animals and from culinary and household utensils, and dissolved soap, constituting a mixture which rapidly decomposes and affords a fine soil for the nourishment and propagation of microscopic organisms.

Air contaminated in this way, popularly known as sewer-gas, contains sulphide of ammonium and sulphureted hydrogen (which cause the characteristic odor of rotten eggs), carbureted hydrogen, nitrogen, and carbonic acid (odorless), and certain undetermined organic matters.

Professor Nichols analyzed the air of the Berkeley Street sewer in Boston, a type of a badly-constructed and badly-ventilated sewer. The sulphureted hydrogen, etc., were in too small quantity to be measured. The highest percentages found were, of oxygen, 20.90; of nitrogen, 79.26; of carbonic acid, .4 (40 parts in 10,000). The lowest were, oxygen, 20.48; nitrogen, 78.89; and carbonic acid, .05 (5 parts in 10,000).

Letheby found that sewer-water (containing 128.8 grains of organic matter to the gallon) excluded from air yielded, for nine weeks, 1.2 cubic inches of gas per hour. In one hundred volumes of this mixture there were 78.83 parts of marsh-gas (carbureted hydrogen), 15.90 parts of carbonic acid, 10.19 parts of nitrogen, and .08 of sulphureted hydrogen. Examination of sewage-mud in the Seine by Durand-Claye gave 72.88 parts of marsh-gas, 13.30 of carbonic acid, 6.70 of sulphureted hydrogen, 2.54 of carbonic oxide, 4.58 of nitrogen, and some other gases. Such mixtures are sometimes found in long-closed cesspools and privy-vaults, but not in sewers proper.

Of these gases, sulphureted hydrogen and carbonic acid are very poisonous, and when they are inhaled in concentrated form produce almost immediate unconsciousness, and often death. When less concentrated, sewer-air may cause nausea and vomiting, followed by a low fever which sometimes kills, and, if not, results in a tedious convalescence. As a rule, it is so largely diluted that it produces no immediate effects, excepting the discomfort due to offensive odor, and the mental anxiety resulting therefrom.

The effects usually attributed to the continued breathing of diluted sewer-air are general malaise, loss of appetite, anæmia, impaired nutrition, and therefore diminished power of resistance to attacks of disease which are not directly attributable to sewer-air poisoning. It is doubtful whether these effects are due to the constant introduction of sewer-air in minute quantities into the blood, or to the inhalation of particles of organic matter floating in such a contaminated atmosphere.

The greatest danger, however, in the breathing of sewer-air is that of inhaling with it the living particles (bacilli, etc.) contained or developed in the excreta of diseased persons.

The diseases believed to be propagated in this way are cholera, typhoid fever, and dysentery. The discharges both from the mouth (stomach) and bowels are known to be poisonous.

It is believed by many that the poisons of typhoid fever and diphtheria may be developed de novo by the decomposition of the mixtures found in cesspools and sewers.

There also seems to be a connection, imperfectly understood, between bad drainage and malarial fevers, and perhaps cerebro-spinal meningitis.

The origin of yellow fever is not yet ascertained.

Surgical erysipelas, puerperal fever, and hospital gangrene, are only developed on and about wounded surfaces, and seem to be due to the organisms developed in the secretions of such surfaces, where ventilation and drainage are bad.

Air may be contaminated by the products of organic decomposition rising from the ground and drawn into the house through furnace-flues, etc.

Ground-air contains from 1.49 to 80 parts per 1,000 of carbonic acid, and frequently contains products of organic decomposition. A damp soil is also very unhealthy, as shown by Bowditch and others. “A persistently low ground-water, say fifteen feet down or more, is healthy; a persistently high ground-water, less than five feet from the surface, is unhealthy; and a fluctuating level, especially if the changes are sudden and violent, is very unhealthy” (De Chaumont). Such soils are especially productive of consumption.

VENTILATION.

The contamination of the atmosphere by the respiration and bodily emanations of human beings and other animals is unavoidable, but the noxious matters thus added to the air are being constantly changed in the following ways:

1. Oxidation. The organic matters, which have been mentioned as especially injurious, are gradually decomposed by the oxygen of the air, and changed into harmless substances, which either remain as constituents of the atmosphere, or are washed into the earth by rains.

2. Vegetable growth. Plants absorb carbonic acid (which is composed of carbon and oxygen) through their leaves, and give back oxygen to the air, retaining the carbon for their own nourishment. There is thus a constant interchange between animals and vegetables, the former exhaling carbonic acid and appropriating oxygen, and the latter appropriating carbonic acid and exhaling oxygen. The small percentage of carbonic acid always found in the air is, therefore, essential to vegetable life, while harmless to animals.

It is necessary, for the proper purification of a contaminated atmosphere, that it should be largely diluted with fresh air. Hence arises the need of the constant change of air in dwellings.

Air expands when heated and so becomes lighter. Local differences of temperature, created by natural and artificial means, therefore bring about currents in the atmosphere, the cooler and heavier column of air always descending, and the warmer and lighter always rising. This fact is taken advantage of in ventilation.

It has been estimated that, to keep the air pure, three thousand cubic feet of fresh air per hour are required for a male adult, and that a sleeping-room should contain at least twelve hundred cubic feet of air-space for each occupant.

When the temperature of the external air is such that the doors and windows can be constantly open, they afford the best means of ventilation for dwellings. An exposure to draughts, however, is dangerous to many persons, and it is desirable, therefore, in cooler weather, to devise means of admitting fresh air without creating a draught. At a temperature of 60°, a draught is perceived when the air moves at a higher rate of speed than three feet a second. Now it is obvious that a draught may be rendered harmless if the entering current of air is guided in such a direction as not to strike the occupants of a room. This is accomplished simply and cheaply by either of two devices: If the lower sash of a window is raised a few inches (say four), and the space between the bottom of the sash and the window-sill is filled by an accurately fitted board, there will be a space between the panes of the two sashes, through which air will enter, spouting upward toward the ceiling and not falling until its momentum is so much diminished that it will not be felt as a draught. The other plan is to make the upper portion of the upper sash movable, so that it can be tilted inward at such an angle as to direct the entering current upward (essentially the Sherringham valve, though this is made of iron, with side-cheeks to prevent a lateral outflow of air).

There are various patent apparatuses for the admission of fresh air through windows without draught, but they are mostly modifications of the methods above mentioned.

In weather when artificial heat is necessary for comfort, thorough ventilation is not difficult, provided expense is not considered. As the removal of the foul air, however, involves a considerable waste of heat and consumption of fuel, the means of procuring the best ventilation at the least cost becomes a problem of great intricacy, which has not yet been satisfactorily solved.

Fireplaces, or open grates, are excellent ventilators. An ordinary fireplace renews the air of the room four or five times hourly, removing in that time from fifteen to twenty thousand cubic feet of air. But only about 12 or 14 per cent. of the heat given off by the fuel is utilized, the rest passing off by the chimney. The objections to the fireplace as a sole means of heating are, its wastefulness, and the fact that it warms only by radiation, so that the room is unequally warmed, and may be too cold in one place and insupportably hot in another.

Stoves and furnaces can not be relied on for ventilation, the ventilating power of a close stove being only one tenth of that required for a single adult.

Modern fireplaces are sometimes built with a metallic flue extending upward into the chimney. Between this flue and the masonry is an air-chamber opening to the external air and communicating with the room near the ceiling, so that fresh air from outside the house is continuously warmed, and discharged into the room at a temperature of 80° or 90°. The Galton fireplace (Fig. 1) is of this kind, and utilizes 35 per cent. of the fuel.

The best combined heating and ventilating arrangement at present seems to be that which warms the fresh air by means of a soapstone furnace or steam-coils, and removes the foul air through a fireplace. In milder weather, gas may be burned in the chimney at a slight expense. According to Morin, seven cubic feet of gas burned in a flue eleven inches square and sixty-six feet high, will draw thirteen thousand three hundred cubic feet of air per hour from a room.

The dampers of stoves should never be in the pipes, for they dam back the gases which ought to enter the chimney, and force them into the room. The fire should be regulated by dampers which prevent the access of air, and not its escape after contamination.

Ventilating flues in walls do very little good, unless special means are provided to heat them (e. g., gas lights or lamps).

The common whirling ventilators in window-panes are of very little use.

As a rule, fresh air should enter a room near the ceiling, and foul air be removed near the floor.

In very cold climates, dangerous draughts are often produced by the cooling of the air in contact with the window-panes, so that it falls and sweeps along the floor. This danger may be prevented by double windows, which also save fuel. Double windows may be utilized in ventilation, by raising the lower outer sash a few inches, and lowering the upper inner one.

Smoky Chimneys.

When a chimney smokes, the draught is downward. This may be, caused—1. By an obstruction in the flue or stove-pipe. 2. By a higher chimney in the same house, the air coming down the shorter chimney, and going up the other. The remedy is to equalize the heights, or close the doors between the two. 3. If, when the fire is started, the air outside is warmer than that in the chimney, the heavier column will of course fall. This effect will vanish in a few minutes, when the flue becomes heated. 4. The doors and windows of the room may be so tight as to prevent a sufficient supply of fresh air to burn the fuel. If so, they must be opened. 5. The chimney may be lower than the adjoining wall, and the wind from certain directions, striking the wall, may be directed down the flues. This may be remedied by extending the chimney above the wall, or by capping the flues with one of the various cowls that prevent a downward draught.