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      Respired Air
      Air Mixture
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Respired Air






It has Jong been known that respired air is unsuitable to support human or animal life, and the question is – why?

Assuming respired air to have the composition -

Carbon dioxide4-5 per cent
Oxygen15 per cent
Nitrogen76 per cent
Water-vapour5 per cent


it is not at first sight easy to understand why it should be unwholesome. The oxygen content is sufficiently high, for we have already seen that even 14 per cent, of this gas is ample for normal respirative purposes.7 We cannot therefore complain of oxygen shortage. Neither can we argue that the carbon dioxide is excessive, for 8 to 10 per cent, of the pure gas may be breathed for protracted periods without any injury.

Brown-Sequard and d'Arsonval in 1889 concluded, from a series of experiments, that respired air contains small quantities of a powerful organic poison, and that the unsuitableness of it for further respiration lies, not so much in its carbon dioxide content, as in the presence of this organic toxin. This view was supported by Merkel in 1892, but since then a large number of physiologists have brought forward a formidable array of arguments, indicating that the unwholesome properties of respired air may be more readily explained in other ways. Let us briefly consider these.

There are at least three important factors to consider.


The Influence of Moisture on Respiration

The presence of moisture in air has a two-fold action upon respiration.

First, there is the very obvious fact that if warm air saturated with moisture enters the lungs, the latter will have great difficulty in discharging their superfluous moisture, and a sense of oppression must result. This explains the heavy feeling produced upon entering hot-houses, in which the air, apart from its moisture content, is perfectly good, and very free from carbon dioxide. Cold air, even if saturated with moisture, will not have anything like the same effect, for upon entering the lungs the temperature rises proportionately higher, and the air is thus able to take up much more moisture before becoming saturated, thereby allowing the lungs full opportunity to relieve themselves. Secondly, air containing moisture cannot take up carbon dioxide from the blood as easily as dry air, consequently the ventilation of the lungs is retarded by inhaling moist air.

These two factors working together are sufficient to show that respired air, being saturated with moisture at a warm temperature, cannot be wholesome, and L. E. Hill and his co-workers regard the moisture content as the main cause of the discomfort of ill-ventilated buildings.

The Influence of Carbon Dioxide on Respiration

Mention has been made of the fact that relatively dry air containing 8 to 10 per cent, of pure carbon dioxide can be breathed for a long time with impunity. For the reason given in the preceding paragraph, however, it follows that breathing warm air both saturated with moisture and containing some carbon dioxide must be attended by a greater difficulty in ventilating the lungs, and that a proportionately smaller quantity of carbon dioxide will be capable of being breathed with impunity. Since respired air may contain anything from 4 to 5.4 per cent, of this gas, it is easy to understand how this may be the potent cause of the hyperpncea and headache, resulting from inhabiting ill-ventilated rooms. Haldane and Smith are evidently of this opinion.

The Influence of Organic Material on Respiration

There can be no reasonable doubt that expired air contains, in addition to moisture and carbon dioxide, some organic bodies of more complex composition. Thus Weichardt finds that by breathing into distilled water, or better still, into glycerine, a ponderable residue is obtained upon evaporation, which contains organic products of high molecular weight; and numerous other investigators have found evidence of organic matter in respired air. It is no doubt this, together with exhalations from the mouth (consequent upon bad teeth), and secretions from the skin, such as perspiration, etc., which give an unpleasant odour to the air of ill- ventilated rooms, tending to produce sickness and faintness.

The above three factors are quite sufficient to account for the unwholesomeness of respired air without assuming the presence of any particularly toxic organic poisons; and when it is remembered that, in addition to these, micro-organisms of a harmful nature are hurled into the air by people sneezing, coughing, spitting, and even talking, the need for very thorough and systematic ventilation becomes evident.

How much Fresh Air must each Individual have?

In very warm weather the windows may be thrown wide open and the air of rooms made almost as pure as that outside. But on cooler days when artificial heating is essential and draughts must be avoided, a certain amount of vitiation of the air is unavoidable. The question which now arises is - To what extent may that vitiation be permitted without harmful effects? It is reasonable to assume that if the air of a room is so pure as not to affect the sense of smell, when a person enters it direct from breathing the fresh air outside, there cannot be much wrong with it - provided, of course, the only impurity likely to be present is respired air, and not inodorous gases such as pure carbon dioxide obtained from chemical sources, or carbon monoxide from heated stoves, etc., which gases have no action upon the olfactory nerve. De Chaumont investigated this limit very carefully in 1875, and concluded that, on the average, air containing 0.02 per cent, or more of respired carbon dioxide possesses a detectable odour. This, of course, depends upon a variety of factors, such as the humidity, temperature, sensitiveness of the nasal organ, and last, but by no means least, the personal cleanliness of the persons who have been breathing the air. This must be evident when we remember that it is not the pure carbon dioxide that we smell, but the organic impurities respired with it. Let us, however, take 0.02 per cent, of carbon dioxide as the maximum amount of respired carbon dioxide that ought to be permitted in the atmosphere of inhabited rooms.

In order to determine the amount of fresh air to be supplied to each individual on this basis we require to know the amount of carbon dioxide exhaled per hour. This may be stated roughly as in table.

As an average for a mixed community we may take 0.6 cubic feet. If now we divide this by the permissible respired impurity, namely, 0.02 per cent., we have the amount of air required per hour per person, namely -

0.6/0.0002 = 3000 cubic feet per hour;
and this is the standard generally adopted.
For general purposes the equation may be expressed as
C/R = F
where C = Amount of carbon dioxide exhaled by each person per hour;
R = Respired impurity that may be allowed;
F = Amount of fresh air required, in cubic feet per hour.
For children we have
0.4/0.0002 = 2000 cubic feet per hour.

The U.S. Book on School Architecture allows only half this amount, namely, 1000 cubic feet per hour for children.

IndividualCubic Feet of Carbon Dioxide exhaled per Hour.
Adult males in heavy work1.84
Adult males in light work0.95
Adult males at rest0.72
Adult females at rest0.6
Children0.4

The Influence of Artificial Light

The chemical changes induced by the combustion of coal-gas, as a source of artificial illumination, may be enumerated as follows:
  1. Oxygen is consumed, each cubic foot of coal-gas using up almost exactly its own volume of oxygen during combustion.
  2. Carbon dioxide is produced, 1 cubic foot of coal-gas yielding 0.4 to 0.5 cubic feet (The mean of many experiments carried out by the writer on Worcester City coal-gas. Rideal (next reference) gives 0.6 c. ft. carbon dioxide and 295 grains, i.e. 19.1 grams, of water for London coal-gas.).
  3. Water-vapour is evolved, 1 cubic foot of gas yielding at normal pressure and room temperature approximately 26 grams of water.
  4. Small quantities of sulphur dioxide result by the oxidation of traces of sulphuretted hydrogen, etc., normally present in coal-gas.
  5. Organic impurities are incinerated by the flame.
From (4) and (5) above, we gather that if sufficient air is forthcoming to supply the necessary oxygen and to dilute down the carbon dioxide, a coal-gas flame should be more healthy than electric incandescent lamps. This has, in practice, proved to be the case. The sulphur dioxide is present in too minute a quantity to injure human beings, but, according to Rideal, it can and does tend to jnirify the air, and in conjunction with the incinerating action of the flame it reduces very sensibly the micro-organism content. Haldane drawrs attention to the small vitiation of air produced when incandescent gas mantles are used, as compared with any other form of gas burner. This is attributed to the very complete combustion of the gas on the surface of the mantle and to the incinerating activity.

In addition to these chemical effects, we have two important physical factors to consider, namely -
  1. The rise in temperature of the air; and
  2. The consequently increased rapidity in the circulation of the air.
By having artificial gas lights above the heads of the inhabitants of a room the warm air is impelled upwards, and if suitable ceiling ventilation is provided there is considerably less danger of the bad air cooling and sinking, and thus vitiating the purer air below. In the absence of good ceiling ventilation, the air cools near the walls and slowly creeps down them.

It is customary to regard, from the ventilation point of view, the presence of a gas flame as equivalent to that of a definite number of persons. As we have already learned, however, it is the organic impurities as well as the moisture and carbon dioxide content which renders respired air injurious. Consequently we ought not strictly speaking to express gas flames in terms of people. For the sake of convenience, however, and for want of a better method, we must continue to do so. It is usually agreed that 0.1 per cent, of carbon dioxide produced by combustion of coal-gas, and hence accompanied by water-vapour, is as much as ought to be permitted in the air of a room. Such being the case, an average burner, consuming 5 or 6 cubic feet of gas per hour yields from 2.5 to 3 cubic feet of carbon dioxide, and thus requires from 2500 to 3000 cubic feet of fresh air per hour. In other words, in calculating the air required, a gas flame may be taken as numerically equivalent to a human being.
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