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°.