CHAPTER XVIII.

THE EXAMINATION OF AIR.

There are various methods of ascertaining the quality of the air in enclosed spaces, of which not the least useful is the information furnished by the _sense of smell_, on entering a room from the external air. Besides the evidence given by the senses, _chemical_ and _microscopical_ examination of the air gives important information, while the _thermometer_ and _hydrometer_ ascertain the temperature and degree of moisture.

=Examination by the Senses.=—The dull grey haze hanging over a town, when it is viewed from a distance, indicates comparative impurity of its atmosphere, and the presence of a considerable amount of suspended matter, including smoke.

The _smell_ of a stagnant atmosphere is a good preliminary guide to its condition. The fact that a room has been occupied for some time without efficient ventilation can be at once detected on entering a room from the external air. The sense of smell is extremely delicate; it has been estimated that the 3∕100,000,000 part of a grain of musk can be apprehended by it. But nothing is so soon dulled as the sense of smell. An atmosphere which did not appear to be unpleasant while remaining in a room, is intolerable when one returns to it after a few minutes in the open air. It is important not to confound the “closeness” perceived by the sense of smell, with the oppression due to the high temperature of a room. The two are easily distinguished (unless the two co-exist) by a reference to the thermometer, which ought always to be placed in rooms inhabited during the evening. The remedy for a close room is to allow free entry of fresh air, and _not_ allow the fire to go down, as is so commonly done, under the impression that the closeness is due to heat.

De Chaumont has made many experiments, shewing how accurate is the information given by an acute sense of smell. Carbonic acid is destitute of odour, but as its amount is usually proportionate to that of the organic matter producing closeness, it may be taken as an index of the amount of impurity present in living rooms. De Chaumont found that the limit of smell is reached when carbonic acid amounts to 6 parts in 10,000 of air, or half as much again as in the external air. In the following extracts from his experiments, there was a close accordance between the evidence of his sense of smell and the amount of carbonic acid:—

┌────────────────────────────────────────────┐ │_At_ 14·8_ per_ 10,000 { _Extremely close │ │ { and unpleasant._ │ │ „ 10·90 „ _Extremely close._ │ │ „ 9·62 „ _Very close._ │ │ „ 9·21 „ _Close._ │ │ 8·43 „ _Not very foul._ │ │ „ 8·04 „ _Close._ │ │ „ 6·58 „ _Not very close._ │ │ „ 5·68 „ _Not close._ │ └────────────────────────────────────────────┘

He also found that humidity of the air had marked influence in rendering the smell of organic matter perceptible, even more than a rise of temperature. The sense of smell is doubtless aided in detecting impurities in the air, by the _besoin de respirer_, a feeling of oppression caused by the deficiency of interchange between the blood and air. The state of cleanliness of the room as well as of the persons in it influences smell; hence there may not be in particular instances exact correspondence between excess of carbonic acid and of organic matter.

=Chemical Examination.=—The estimation of nitrogen and oxygen in air is usually unnecessary, as these vary but little. The oxygen is, however, reduced in frequently re-breathed air. The ill effects of an often-breathed atmosphere are due not only to deficiency of oxygen, but also to the addition of carbonic acid and organic matters, rendering difficult the interchange between oxygen and the blood.

=The Estimation of Carbonic Acid= is of great importance, as under ordinary circumstances, its amount is a fairly exact indication of the amount of contamination in the air.

=Pettenkofer’s Method.=—A carefully dried glass vessel containing a gallon of water is filled with the air to be examined, by emptying the water in the room, the air of which is to be examined. Fifty cubic centimetres of clear freshly prepared baryta water are then added, and the stopper of the bottle then replaced. It is then well shaken, and afterwards allowed to stand for an hour. The carbonic acid combines with part of the baryta to form barium carbonate; and the baryta water remaining is consequently diminished in alkalinity. Given the alkalinity of the baryta water before and after the experiment, and the difference will give the amount of baryta which has combined with carbonic acid.

The alkalinity of the baryta is estimated by a standard solution of oxalic acid, of such a strength that 1 c.c. is the equivalent of 0·5 c.c. of CO₂. The indicator used in making this test is phenolphthalein, which colours baryta water red, but its colour disappears when neutralization is reached.

The following example is taken from “Pakes’ Laboratory Text Book of Hygiene,” p. 292:—

The jar is found to contain 3,950 c.c.

As 50 c.c. baryta water were run into the jar, the air experimented on = 3,950-50 = 3,900 c.c.

On titrating 25 c.c. of the original baryta water, 22·50 c.c. standard acid solution were required to neutralise it.

The baryta water in the jar required 19·35 c.c.

22·50-19·35 = 3·15 c.c. = difference of acid used.

But 1 c.c. acid = 0·5 c.c. CO₂ at 0° C. and 760 mm. of mercury.

Therefore CO₂ taken up by 25 c.c. of baryta = 3·15∕2 = 1·575 c.c.

As 50 c.c. were used the CO₂ absorbed by the baryta = 3·15 c.c. This was present in 3,900 c.c. of air. Therefore the CO₂ = 0·80 per cent.

Correction may be required for variations from the normal pressure of 760 mm. and normal temperature of 0° C., in accordance with ordinary rules.

=In Lunge and Zeckendorf’s Method=, the air to be examined is pumped through a glass bottle in which is 10 c.c. of a N/500 solution of Na₂CO₃ containing phenolphthalein as an indicator. The air is pumped by a hand pump through this solution until the phenolphthalein is decolourized. The number of times the ball of the pump has been squeezed indicates the amount of CO₂ present in accordance with a table prepared from separate experiments by Pettenkofer’s method.

=Dr. Angus Smith’s plan= for the estimation of carbonic acid in air is similar in principle to the last calculations. It is based on the fact that the amount of carbonic acid in a given volume of air will not render turbid a given amount of lime water, unless the carbonic acid is in excess.

TABLE.—_To be used when the point of observation is “No precipitate.”_ Half an ounce of lime water containing ·0195 gramme lime.

AIR AT 0° C. AND 760 M. M. BAROMETRIC PRESSURE.

┌───────────────┬────────────────┬─────────────────┬─────────────────┐ │ CARBONIC ACID │VOLUME OF AIR IN│SIZE OF BOTTLE IN│SIZE OF BOTTLE IN│ │ IN THE AIR │ CUBIC │ CUBIC │ OUNCES │ │ PER CENT. │ CENTIMETRES. │ CENTIMETRES. │ AVOIRDUPOIS. │ ├───────────────┼────────────────┼─────────────────┼─────────────────┤ │ ·03 │ 571 │ 584 │ 20·63 │ │ ·04 │ 428 │ 443 │ 15·60 │ │ ·05 │ 342 │ 356 │ 12·58 │ │ ·06 │ 285 │ 299 │ 10·57 │ │ ·07 │ 245 │ 259 │ 9·13 │ │ ·08 │ 214 │ 228 │ 8·05 │ │ ·09 │ 190 │ 204 │ 7·21 │ │ ·10 │ 171 │ 185 │ 6·54 │ │ ·11 │ 156 │ 170 │ 6·00 │ │ ·12 │ 153 │ 157 │ 5·53 │ │ ·13 │ 132 │ 146 │ 5·15 │ │ ·14 │ 123 │ 137 │ 4·82 │ │ ·15 │ 114 │ 128 │ 4·53 │ │ ·20 │ 86 │ 100 │ 3·52 │ │ ·25 │ 69 │ 83 │ 2·92 │ │ ·30 │ 57 │ 71 │ 2·51 │ └───────────────┴────────────────┴─────────────────┴─────────────────┘

The foregoing table shows how to apply this method. The first and second columns state the ratio of carbonic acid in a quantity of air which will give no turbidity or precipitate in half an ounce of lime water; the third column gives the corresponding size of the bottle in cubic centimetres; and the fourth column gives the same in ounces. Thus different sized bottles, each containing half an ounce of lime water, will indicate with a fair degree of accuracy the ratio of carbonic acid in the air containing them, by giving no precipitate when the bottle is well shaken. For instance, if a pint bottle is used and there is no precipitate with half an ounce of lime water, it indicates that the ratio of carbonic acid does not amount to ·03 per cent.; if an eight-ounce bottle be used, and there is no precipitate, it indicates that the ratio does not amount to ·08 per cent., and so on. The air of a room ought never to contain more than six parts of carbonic acid in 10,000 of air, or ·06 per cent., _i.e._ a 10½ ounce bottle full of the air shaken up with half an ounce of clear lime water ought to give no precipitate.

_Dr. Haldane_ has recently described (_Journal of Hygiene_, No. 1, 1901) a method of estimating CO_2, which, although it appears complicated, is really both simple and convenient. For particulars, see the above _Journal_.

=The Estimation of Organic Impurities= may be accomplished approximately by drawing a definite amount of air by means of an aspirator, through a dilute solution of permanganate of potassium of known strength. The result is stated by giving the number of cubic feet of air required to decolourise .001 gramme of the permanganate in solution. Sulphuretted hydrogen, sulphurous acid, and other substances in air likewise decolourise the permanganate; these ought to be separately tested for, and allowance made.

=The Estimation of Ammonia=, whether free or derived from albuminoid impurities, is a matter requiring very delicate processes. It is accomplished in the same way as the estimation of ammonia in water, the air being drawn through perfectly pure distilled water, and then the analysis proceeded with as a water analysis. The mere presence of free ammonia may be determined by exposing to the air strips of filtering paper dipped in Nessler’s solution, which become brown if there is any ammonia in the air.

=Microscopical Examination= is required for the detection of suspended matters. These are the most potent for harm, containing sometimes the germs of infectious diseases. The suspended matters scattered throughout the air may be collected by Pouchet’s aeroscope. This consists of a small funnel drawn out to a fine point, under which a slip of glass is placed moistened with glycerine. Both funnel and glass are enclosed in an air-tight chamber, connected by tubing with an aspirator, by means of which when water is allowed to escape from it, air is drawn through the funnel and its particles impinging on the glycerine are there arrested. Glycerine may be objectionable from the foreign particles previously contained in it. Various other plans have been devised, one of which is to draw the air through a small quantity of pure distilled water and then examine a drop of it. By microscopic examination large particles can be detected. For the detection of bacteria and their spores more delicate methods are required.

The =Bacteriological Examination= of air is usually conducted as follows. Air is drawn through a wide glass tube (Hesse’s tube), which has been previously sterilised, and on the inner side of which liquid gelatine has been allowed to solidify. The air as it passes over the gelatine deposits any germs present in it. The entrance of any further germs is prevented by closing the tube, and it is then left to stand for two or three days. Moulds and colonies of bacteria will develop in the gelatine, and these can be counted and differentiated by their appearance and by further tests. In closed rooms the number of microbes (_i.e._, bacteria and moulds) ought not to be more than 20 per litre of air in excess of those in the outside air; and the ratio of bacteria to moulds ought not to exceed 30 to 1.

=Examination of Temperature and Moisture.=—The temperature should be observed at the point most remote from an open fire-place, and compared with the external temperature. For methods of estimating moisture, see page 240.

It may be useful to recapitulate at this point the desiderata in an inhabited room. The temperature should be 60-62° Fahr., the amount of carbonic acid should not exceed ·06 per cent. and the humidity should range between 73 and 75 per cent. of the amount required to produce saturation. The dry bulb thermometer should read 63-65° Fahr., the wet bulb 58°-61° Fahr., and the difference between the two should not be less than 4° or more than 8°.