DEVELOPMENT OF THE GAS MASK
While in ordinary warfare the best defense against any implement of
war is a vigorous offense with the same weapon, Chemical Warfare
presents a new point of view. Here it is very important to make use of
all defensive measures against attack. Because of the nature of the
materials used, it has been found possible to furnish, not only general
protection, but also continuous protection during the time the gas is
present.
The first consideration in the protection of troops against a gas
attack is the provision of an efficient individual protective appliance
for each soldier. The gas attack of April 22, 1915 found the Allies
entirely unprepared and unprotected against poisonous gas. While a few
of the men had the presence of mind to protect themselves by covering
their faces with wet cloths, the majority of them became casualties.
Immediately steps were taken to improvise protective devices among
which were gags, made with rags soaked in water or washing soda
solution, handkerchiefs filled with moist earth, etc. One suggestion
was to use bottles with the bottom knocked off and filled with moist
earth. The breath was to be taken in through the bottle and let out
through the nose; but as bottles were scarce and few of them survived
the attempt to get the bottom broken off, the idea was of no value.
The first masks were made by the women of England in response to the
appeal by Lord Kitchener; they consisted of cotton wool wrapped in
muslin or veiling and were to be kept moist with water, soda solution
or hypo.
ENGLISH MASKS
=The Black Veiling Respirator.= The first form of the English mask is
known as the Black Veiling respirator and consisted of cotton waste
enclosed in a length of black veiling. The waste was soaked in a
solution of:
Sodium thiosulphate 10 lbs.
Washing soda 2.5 lbs.
Glycerine 2 lbs.
Water 2 gals.
The glycerine was put in to keep the respirator moist, thus obviating
the need for dipping before use.
[Illustration: FIG. 42.—Early Gas Protection.]
The respirator was adjusted over the mouth and nose, the cotton waste
being molded to the shape of the face and the upper edge of the veiling
pulled up so as to protect the eyes. These respirators were used in
the attacks of May 10th and 12th, 1915 and were reasonably efficient
against the low concentration of chlorine then used; they were
difficult to fit exactly to the face, which resulted in leakage. The
cotton waste often became lumpy and had to be shredded out or discarded.
=The Hypo Helmet.= The next development of the British protection was
the so-called Hypo helmet. This is said to have resulted from the
suggestion of a Canadian sergeant that he had seen a German pulling a
bag over his head during a gas attack. It consisted of a flannel bag
soaked in the same solution as was used for the veiling respirator and
was fitted with a pane of mica as a window. The helmet was tucked down
inside the jacket which was then buttoned up tightly around the neck.
As may be seen from Figure 43, this would not prove very satisfactory
with the American type of uniform.
This helmet had many advantages over the veiling respirator but the
window often became cracked or broken from the rough treatment in the
trenches. Later the mica was replaced by celluloid and still later by
glass eyepieces set in metal rings. These were very effective against
chlorine in the field.
=The P and PH Helmets.= During the summer of 1915 it became evident
that phosgene-chlorine mixtures would be used in gas attacks and it
was therefore necessary to provide protection against this. The hypo
helmet, which offered no protection against phosgene, was soaked in
an alkaline solution of sodium phenolate (carbolic acid) containing
glycerine, and with this new form of impregnation was called the P
helmet. It protected against 300 parts of phosgene in a million of air.
Since this solution attacks flannel, two layers of flannelette were
used. The helmet was further improved by the addition of an expiratory
valve, partly to prevent the man from breathing any of his own breath
over again and partly to prevent the deterioration of the alkali of the
mask by the carbon dioxide of the expired air.
The protection was later further increased by the addition of
hexamethylenetetramine, and this mask is known as the PH helmet. This
increased the protection to 1,000 p.p.m.
The early types of helmet offered no protection against lachrymators.
For this purpose goggles were used, the later types of which had glass
eyepieces and were fitted around the eyes by means of rubber sponge.
While intended for use only after a lachrymatory bombardment, the
troops frequently used them during and after an ordinary gas attack
when the mask should have been worn. Consequently they were withdrawn.
The PH helmet was unsatisfactory because of the following reasons:
(1) It was warm and stuffy in summer;
(2) It deteriorated upon exposure to air;
(3) It was incapable of further development;
(4) It had a peculiar odor and, when wet, frequently
burned the foreheads of the men;
(5) It offered practically no protection against
lachrymators.
[Illustration: FIG. 43.—Method of Wearing the P. H. Helmet.]
[Illustration: FIG. 44.—Early Type of Standard (British) Box Respirator
(S. B. R.)]
=Box Respirator.= The increasing concentration of gas from cylinder
attacks and the introduction of shell, with such gases as chloropicrin
and superpalite, led, early in 1916, to very definite and constructive
efforts on the part of the British to increase the protection offered
by the mask. The result was a “polyvalent” respirator of the canister
type (the Standard Box Respirator). This mask was probably the result
of experience with oxygen apparatus in mine rescue work. The lines on
which this canister was modeled involved the use of a canister filled
with highly sensitive absorbent charcoal mixed with or alternating
in layers with oxidizing granules of alkaline permanganate. It was
the result of innumerable experiments, partly conducted in France
but mostly in England under the direction of the late Lieut. Col.
Harrison, who was almost entirely responsible for the wonderful
production of this respirator.
The respirator (Figure 44) consisted of the canister mentioned above,
which is attached by a flexible tube to a facepiece or mask. The
facepiece is made of rubberized fabric and fits the face so that there
is little or no leakage. This is secured by means of tape and elastic
bands which fit over the head. The nose is closed by means of clips,
which are wire springs with rubbered jaws covered with gauze (Fig.
45). Breathing is done through a mouthpiece of rubber; the teeth close
on the rubber tabs and the rubber flange lies between the teeth and
the lips. The expired air finds exit through a rubber flutter valve
in an angle tube just outside the mask. This arrangement furnishes a
double line of protection; if the face piece is punctured or torn,
gas-containing air cannot be breathed as long as the noseclip and
mouthpiece are in position.
The early English canister was packed with 675 cc. of 8-14 mesh war gas
mixture, 40 per cent of which was wood charcoal and 60 per cent reddish
brown soda-lime granules. The metal dome at the bottom of the can was
covered with a thin film of cotton. At two-thirds of the distance to
the top was placed a paper filter and a heavy wire screen which differs
from our heavy screen in that it is more loosely woven. The mixture
was covered with a cotton filter pad and a wire screen, over which was
placed the wire spring.
The use of this mask ensures that all the air breathed must enter the
lungs through the canister. This air passage is entirely independent
of leaks in the facepiece, due either to a poor fit about the face
or to actual leakage (from a cut or tear) of the fabric itself. The
facepiece is readily cleared of poison gases which may leak in. This is
accomplished by taking a full inspiration, releasing the noseclip, and
exhaling through the nose, which forces the air out around the edges of
the facepiece.
On the other hand, this type of mask possesses a number of very obvious
disadvantages, particularly from a military point of view:
The extreme discomfort of the facepiece. This discomfort arises from a
number of causes certain of which are inherent in this type of mask,
among them being: (_a_) the noseclip, (_b_) the mouthpiece, and (_c_)
the lack of ventilation within the facepiece chamber.
Aside from the actual physical discomfort of the noseclip and
mouthpiece, which becomes intense after long periods of wearing, this
combination forces upon the wearer an unnatural method of respiration
to which it is not only difficult to become accustomed, but which also
causes extreme dryness of the throat. The mouthpiece greatly increases
salivation and as swallowing is rather more difficult with the nose
closed, this adds another extremely objectionable feature.
[Illustration: FIG. 45.—Interior of S. B. R., Showing Cotton Wrapped
Noseclips.]
[Illustration: FIG. 46.—French M-2 Mask.]
The lack of ventilation in the facepiece chamber entraps the heat
radiating from the face and retains the moisture which is constantly
evaporating from the skin. This moisture condenses on the eyepieces,
and even if cleared away by the use of a so-called anti-dimming paste,
usually makes vision nearly impossible.
FRENCH MASKS
=M-2 Mask.= The early protection of the French Army was obtained from a
mask of the type M-2 (Fig. 46).
This mask consists of a number of layers of muslin impregnated with
various absorbent chemicals. A typical mask was made up of 20 layers of
cheesecloth impregnated with _Greasene_ and 20 layers impregnated with
_Complexene_. These solutions were made up as follows:
Complexene: 39.0 lbs. Hexamethylenetetramine
37.5 lbs. Glycerine
27.5 lbs. Nickel sulfate (NiSO₄.7 H₂O)
11.8 lbs. Sodium carbonate (Na₂CO₃)
Water
Greasene: 107.0 lbs. Castor oil
81.0 lbs. Alcohol (95%)
10.7 lbs. Glycerine (90%)
3.1 lbs. Sodium hydroxide (NaOH)
This mask fits the face tightly and as a consequence the inhaled air
can be obtained only by drawing it through the pores of the impregnated
fabric. There is no outlet valve. The exhaled air makes its escape
through the fabric. The eyepieces are made of a special non-dimming
celluloid. The mask is protected from rain by a flap of weather proof
fabric, which also protects the absorbent chemicals from deterioration.
At the beginning of the war the United States experimented considerably
with the French mask. Several modifications of the impregnating
solutions were suggested, as well as methods of application. One of
these was to separate the components of the complexene solution and
impregnate two separate layers of cloth; this would make a three-layer
mask. In view of the phosgene which was in use at that time, the
following arrangement was suggested:
20 layers of hexamethylenetetramine,
10 layers of nickel sulfate-sodium carbonate,
10 layers of greasene.
This arrangement was more effective than the original French mask and
offered the following protection when tested against the following
gases (concentration 1 to 1,000, rate 30 liters per minute):
Phosgene 65 minutes
Hydrocyanic acid 60 minutes
Chlorine 60 minutes
[Illustration: FIG. 47.—Interior View of M-2 Mask.]
[Illustration: FIG. 48.—French Artillery Mask, Tissot Type.]
=Tissot Mask.= The French deserve great credit for their development of
the Tissot type mask. This was first issued to artillerymen, stretcher
bearers, and certain other special classes of soldiers to furnish them
with protection and yet enable them to work with greater efficiency
because of the decrease in resistance to breathing. The mask (Fig.
48) resembles the British box respirator in that it consists of a
canister and rubber facepiece, but differs in that the mouthpiece and
noseclip are lacking. The inhaled air enters the mask from two tubes
which open directly under the eyepieces and allow the air to sweep
across them. This removes, by evaporation, the condensed moisture of
the breath from the eyepieces, which otherwise would obstruct the
vision. The circulation of the fresh air in the mask also removes and
dilutes lachrymatory gases which may filter through the mask. The
exhaled air escapes through a simple outlet valve. This type of mask is
advantageous because:
(1) The facepiece is tight and comfortable.
(2) The eyepieces do not become dimmed.
(3) There is no difficulty in speaking.
(4) Salivation is eliminated because of the absence of the
mouthpiece.
(5) It is generally more comfortable than the box respirator.
This mask, however, was made of thin rubber of great flexibility which,
while affording a perfect fit, did not possess sufficient durability to
recommend it as the sole defense of the wearer.
The canister is markedly different from all other canisters described
in this chapter in that a highly hygroscopic chemical absorbent is
used. An approximate determination showed about 70 per cent sodium
hydroxide. The use of caustic soda in the canister is made possible by
the intermixing of steel wool with the granules of caustic. A layer of
absorbent having the appearance of vegetable charcoal is placed at the
top of the canister.
The canister has the shape of a rectangular prism 8 × 6½ × 2½ inches;
and, owing to the use of steel wool, is large in proportion to the
weight of absorbent contained. Valves are supplied which prevent
exhalation through the canister. When not in use the opening in the
bottom of the canister is plugged with a rubber stopper to protect the
absorbents from moisture. The canister is carried against the body and
is connected to the facepiece with a flexible rubber-fabric tube.
=A. R. S. Mask (Appareil Respiratoire Special).= One of the latest
types of French mask is the so-called A. R. S. mask, which is based
upon, or at least resembles closely, the German mask. This is a frame
mask made from well rubberized balloon material, provided on the inside
with a lining of oiled or waxed linen and fitted with a drum which
is screwed on. The mask is provided with eyepieces of cellophane,
fastened by metal rings into rubber goggles, which are sewed in the
mask. A metal mouth-ring is tied in the mask with tape. This ring is
placed somewhat higher than in the German mask, in this way reducing
the harmful space under the mask. An inlet and outlet valve is placed
in the mouth-ring; the first is of mica while the other, which is in
direct communication with the interior of the mask, is of rubber. On
the inside of the mask, in front of the valves, a baffle is sewed in,
whereby the inhaled air is forced to pass in front of the eyepieces to
prevent dimming and, at the same time, condensed vapor is prevented
from entering the valves.
[Illustration: FIG. 49.—French A. R. S. Mask.]
The mask or head straps are arranged in the same way as on the latest
M-2 mask, i.e., one elastic band is placed across the top of the head
and the other across the back; the two are joined by an elastic. Below
these two straps is an adjustable elastic neck band. The drum is made
of metal similar in shape to the German drum and fits in the mouth-ring
by means of a thread. It is made tight by a rubber ring as in the
German mask. The thread differs from that on the German mask, making
an interchange of canisters impossible. The canister or drum includes
a bottom screen, springs and wire screens between the layers. It is
closed by a perforated bottom. There are three layers. On the top is
a thin layer of absorbent cotton. Beneath this is a central layer of
charcoal, which is a little finer than the German charcoal. The lower
layer consists of soda-lime, mixed with charcoal and zinc oxide and
moistened with glycerine.
GERMAN MASK
The early type of German mask probably served as the model for the
French A. R. S. mask. The facepiece was made of rubber, which was later
replaced by leather because of the shortage of rubber. The following is
a good description of a typical German facepiece:
“The facepiece of the German mask was made of one piece of leather,
with seams at the chin and at the temples, giving it roughly the shape
of the face. The leather was treated with oil to make it soft and
pliable, also to render it impervious to gases. The dressed surface
was toward the inside of the mask. A circular steel plate, 3 inches
in diameter, was set into the facepiece just opposite the wearer’s
nose and mouth, with a threaded socket into which the drum containing
the absorbents screwed. A rubber gasket (synthetic?) held in place by
a sort of pitch cement, secured a gas-tight joint between the drum
and the facepiece. There were no valves, both inhaled and exhaled air
passing through the canister. The eyepieces were inserted by means
of metal rims with leather washers, and were in two parts: (_a_) a
permanent exterior sheet of transparent material (‘cellon’) resembling
celluloid, and (_b_) an inner removable disc which functioned as an
anti-dimming device. This latter appeared to be of ‘cellon’ coated on
the side toward the eye with gelatin, and was held in position by a
‘wheel’ stamped from thin sheet metal, which screwed into the metal
rim of the eyepiece from the inside. The gelatin prevented dimming by
absorbing the moisture, but wrinkled and blistered and became opaque
after a few hours’ use, and could not be changed without removing the
mask. The edge of the facepiece all around was provided with a bearing
surface consisting of a welt of finely woven cloth about one inch wide
sewed to the leather. In some instances this welt was of leather of an
inferior grade. The edge of the facepiece was smoothed over by a coat
of flexible transparent gum, probably a synthetic compound.”
[Illustration: FIG. 50.—German Respirator.]
[Illustration: FIG. 51.—The German Respirator.]
1. Smoke Filter Extension.
2. Canister.
3. Ring for Protecting Eye Piece.
4. Anti-dimming Disc Envelope.
5. Carrying Case.
6. Cloth Wallet for Extra Canister (1918).
7. Can for Extra Canister (1916).
8. Assembled Respirator.
9. Face Piece.
10. Anti-dimming disc.
=German Canister.= The general appearance of the canister (Sept., 1916
Type) is that of a short thick cylinder slightly tapered and having at
the smaller end a threaded protrusion or neck by which it is screwed
onto the facepiece. The cylinder is about 10 cm. in diameter and about
5 cm. in length. In the canister are three layers of absorbents of
unequal thickness separated by disks of fine mesh metal screen. The
canister is shipped in a light sheet iron can 10 cm. in diameter and 8
cm. high. The can is shellacked and is lined with paper packing board.
The container is made air-tight by sealing with a strip of adhesive
tape.
[Illustration: FIG. 52.—Cross Section of 1917 and 1918 German
Canisters.]
ABSORBENTS.
Absorbent. Composition. Weight. Volume.
1917. No. 1. Chemical Absorbent. 66 gr. 105 cc.
No. 2. Impregnated Charcoal. 36 gr. 85 cc.
No. 3. Chemical Absorbent. 15 gr. 45 cc.
1918. No. 1. Impregnated Charcoal. 58 gr. 185 cc.
No. 2. Chemical Absorbent. 29 gr. 45 cc.
Total Volume of Absorbents, 1917, 235 cc. = 14.3 cu. in.
1918, 230 cc. = 14.0 cu. in.
Total Weight of Absorbents, 1917, 117 gr.
1918, 87 gr.
Volume of Air Space above Absorbents = 50 cc. = 3.1 cu. in.
=Body.= The body of the canister is made of sheet metal (probably
iron), which is protected on the outside with a coat of dark gray
paint and on the inside with a japan varnish. For ease in assembling
the sides of the canister have a gentle taper, and are formed so as to
supply a seat for each of the follower rings. The protrusion or neck
has about six threads to the inch, the pitch of the screw being 4 mm.
The lower part of the body is rolled so as to give a finished edge, and
the upper part of the cylinder is grooved to receive the top support.
The first screen is double, consisting of a coarse top screen five to
six mesh, per linear inch, and immediately below, a finer screen of
30-40 mesh, per linear inch. The top support is a rigid ring of metal
with two cross arms, which give added, strength to the ring and support
to the screens. It springs into a groove at the top of the body and
forms the support for the contents of the canister. Both screens are
made of iron wire and the top support is made of iron (probably lightly
tinned).
The second screen, which separates the second and third absorbents, is
double, consisting of two disks of 30-40 mesh iron screen. Both screens
are held in place by a follower ring.
The third screen is single, but otherwise it is exactly similar to the
second screen. It serves to keep separate the layers of absorbents No.
1 and No. 2.
The fourth screen (30-40 mesh) is made of iron wire and is held to the
bottom support by six cleats which are punched from the body of the
support. The bottom support is simply a flanged iron cover for the
bottom of the canister. It is punched with 79 circular holes each 4
mm. in diameter and is painted on the outside to match the body of the
canister. The screen and the inside of the bottom support or cover are
coated with a red paint.
AMERICAN MASK
At the entrance of the United States into the war, three types of masks
were available: the PH helmet, the British S. B. R. and the French
M-2 masks. Experiments were made on all three of these types, and it
was soon found that the S. B. R. offered the greatest possibilities,
both as regards immediate protection and future development. During
the eighteen months which were devoted to improvement of the American
mask, the facepiece underwent a gradual evolution and the canister
passed through types _A_ to _L_, with many special modifications
for experimental purposes. The latest development consisted in an
adaptation of the fighting mask to industrial purposes. For this
reason a rather detailed description of the construction of the
facepiece and of the canister of the respirator in use at the close of
the war (R. F. K. type) may not be out of place. The mask now adopted
as standard for the U. S. Army and Navy is known as the Model 1919
American mask, with 1920 model carrier, and will be described on page
225.
[Illustration: FIG. 53.—Diagrammatic Sketch of Box Respirator Type
Mask.]
=Facepiece.= The facepiece of the R. F. K. type Box Respirator is made
from a light weight cotton fabric coated with pure gum rubber, the
finished fabric having a total thickness of approximately ¹/₁₆ inch.
The fit of the facepiece is along two lines—first, across the forehead,
approximately from temple to temple; second, from the same temporal
points down the sides of the face just in front of the ears and under
the chin as far back as does not interfere with the Adam’s apple. In
securing this fit, the piece of stock for the facepiece is died out
of the felt and pleated up around the edges to conform to this line.
After this pleating operation, the edges of the fabric are stitched to
a binding frame similar to a hat-band made up of felt or velveteen
covered with rubberized fabric. All the stitching and joints in the
facepiece are rendered gas-tight by cementing with rubber cement. This
facepiece is made in five sizes ranging from No. 1 to No. 5, with a
large majority of faces fitted by the three intermediate sizes, 2, 3, 4.
=Harness.= The function of the harness is to hold the mask on the face
in such a way as to insure a gas-tight fit at all points. Because of
the great variations in the conformation of different heads, this
problem is not a simple one. Probably, the simplest type of harness, as
well as the one which is theoretically correct, consists of a harness
in which the line of fit across the forehead is extended into an
elastic band passing around the back of the head, while the line of fit
around the side of the face and chin is similarly extended into another
elastic tape passing over the top of the head; these should be held in
place by a third tape, preferably non-elastic, attached to the mask at
the middle of the forehead and to the middle points of the other tapes
at a suitable distance to hold them in their proper positions.
The discomfort of the earlier types of harness has been remedied, in
a large measure, by the development of a specially woven elastic web
which, for a given change in tension, allowed more than double the
stretch of the commercial weaves. There is still much room for valuable
work in developing a harness which will combine greater comfort and
safety. The following points should always be observed in harness
design:
(1) The straps should pull in such a direction that as large a
component as possible of the tension of the strap should be available
in actually holding the mask against the face.
(2) The number of straps should be kept to a minimum in order to
avoid tangling and improper positioning when put on in a hurry by an
inexperienced wearer.
=Eyepieces.= One of the most important parts of the gas mask, from the
military point of view, is the eyepiece. The primary requirement of a
good eyepiece is that it shall provide a minimum reduction in clarity
of vision with a maximum degree of safety to the wearer. The clarity
of vision may be affected in one of several ways: (1) by abrasion of
the eyepieces under service conditions; (2) irregularities in the
surface and thickness of the eyepiece, causing optical dispersion;
(3) absorption of light by the eyepiece itself; (4) dimming of the
eyepieces due to condensation of moisture radiating from the face or in
the exhaled air.
Three types of eyepieces were used but by the end of the war the first
two types had been abandoned.
(1) Ordinary celluloid.
(2) Various hygroscopic forms of celluloid, known as non-dimming
eyepieces.
(3) Various combinations of glass and celluloid, known as non-breakable
eyepieces.
Celluloid was used first, due to its freedom from breakage. It is not
satisfactory because it is rapidly abraded in use, turns yellow, thus
increasing its light absorption, has relatively uneven optical surfaces
and becomes brittle after service.
The various forms of non-dimming lenses function by absorbing the water
which condenses on their surfaces, either by combining individual drops
into a film which does not seriously impair vision, by transmitting
it through the surface and giving it off on the exterior or by a
combination of these mechanisms. With the exception that they are
non-dimming, they are open to all the objections of the celluloid
eyepiece and, as a matter of fact, were never tried out in the field.
The so-called non-breakable eyepieces are formed by cementing together
a layer of celluloid between two layers of glass.[25] This results in
an almost perfect eyepiece. Any ordinary blow falling upon such an
eyepiece does no more than crack the glass, which remains attached to
the celluloid coating. Except in extreme cases, the celluloid remains
unbroken and there is relatively slight danger of a cracked eyepiece of
this sort leaking gas.
[Footnote 25: So-called “Triplex” glass.]
In the matter of flying fragments, the type of eyepiece consisting of
a single layer of celluloid and glass with the celluloid placed next
to the eye, has probably a slight advantage over the type in which
there is glass on both sides. However, the superior optical surface of
the latter type, coupled with its greater freedom from abrasion of the
surface led to the adoption of this type known as “triplexin” in the
mask produced in the later part of the American manufacturing program.
It should be pointed out in connection with this type of eyepiece that
it is possible to make it as perfect optically as desired by using the
better grades of glass. While the optical properties of these eyepieces
undoubtedly suffer somewhat with age, due to the discoloration of
the celluloid, it can be safely said that this material, located as
it is between the layers of glass and relatively little exposed to
atmospheric conditions, will probably be far less affected in this way
than is the ordinary celluloid eyepiece.
[Illustration: FIG. 54.—American Box Respirator, Showing Improved
Rubber Noseclip.]
The position of the eyepiece is very important; the total and the
binocular fields of vision should be kept at a maximum.
=noseclip.= The noseclip is probably the most uncomfortable feature
of the types of mask used during the War. While a really comfortable
nose pad is probably impossible, the comfort of the clip was greatly
improved by using pads of soft rubber and springs giving the minimum
tension necessary to close the nostrils.
=Mouthpiece.= The design of the mouthpiece should consider the size
and shape of the flange which goes between the lips and teeth; this
should be such as to prevent leakage of gas into the mouth and should
reduce to a minimum any chafing of the gums. The opening through the
mouthpiece is held distended at its inner end by a metallic bushing
to prevent its collapse, if, under stress of excitement, the jaws
are forced over the flange and closed. Rubber has proved a very
satisfactory material for this part of the facepiece.
=Flexible Hose.= The flexible hose leads from the angle tube to the
canister. This should combine flexibility, freedom from collapse, and
extreme physical ruggedness. These specifications are met successfully
by the stockinette-covered corrugated rubber hose. The angular
corrugations not only give a high degree of flexibility but are
extremely effective in preventing collapse. The flexibility gained by
this construction is not only lateral but also longitudinal; a hose
having a nominal length of 10 inches functions successfully between
lengths of 8 and 12 inches. The covering of stockinette, which is
vulcanized to the rubber in the manufacturing process, adds materially
to the mechanical strength by preventing incipient tears and breaks.
=Exhalation Valve.= The exhalation valve allows the exhaled air to pass
directly to the outside atmosphere. (This valve is not found on the
German mask.) This valve has the following advantages:
(1) It tends to reduce very materially the dead air space in the mask.
(2) It prevents deterioration of the absorbent on account of moisture
and carbon dioxide of the expired air.
(3) It reduces the back pressure against expiration, since it is
unnecessary to breathe out against the resistance of the canister.
The disadvantage, which may under certain conditions be very serious,
is that, if for any reason the valve fails to function properly,
inspiration will take place through the valve. It can be readily seen
that any failure of this nature will allow the poisonous atmosphere to
be drawn directly into the lungs of the wearer.
The type of valve generally used is shown in Fig. 55, which shows one
of these valves mounted and unmounted. While it is rather difficult
to give a clear description of its construction, the valve may be
considered as a flattened triangular sack of rubber, whose altitude
is two or three times the base and from which all three corners have
been clipped, each giving openings into the interior of the sack. The
opening at the top is slipped over the exhalation passage of the angle
tube, and the air passes out through the other two corners. Closure is
obtained by the combination of two factors,—first, the difference in
atmospheric pressure, and second, the tension due to mounting a section
which has been cured in the flat over an elliptical opening.
[Illustration: FIG. 55.—American Type Exhale Valve, Mounted and
Unmounted.]
In order to protect the flutter valve from injury and from contact with
objects which might interfere with its proper functioning, the later
types of valve were provided with a guard of stamped sheet metal.
CANISTERS
During the development of the facepiece, as discussed above, the
American canister underwent changes in design which have been
designated as _A_ to _L_. These changes were noted by the different
colored paints applied to the exterior of the canister.
Type _A_ canister was exactly like the British model then in use,
except that it was made one inch longer because it was realized that
the early absorbents were of poor quality. The canister was made of
beaded tin plate and was 18 cm. high. The area of the flattened oval
section was 65 sq. cm. In the bottom was a fine wire dome 3.4 cm. high.
The valve in the bottom was integral with the bottom of the container,
there being no removable plug for the insertion of the check valve. The
absorbents were held in place by a heavy wire screen on top and by two
rectangular springs.
[Illustration: FIG. 56.—American Canister, Type _A_.]
Inhaled air entered through the circular valve at the bottom of the
canister, passed through the absorbents and through a small nipple at
the top.
The filling consisted of 60 per cent by volume of wood charcoal,
developed by the National Carbon Co., and 40 per cent of green soda
lime, developed and manufactured by the General Chemical Company,
Easton, Pa. The entire volume amounted to 660 cc. The early experiments
with this volume of absorbent showed that ⅖ soda-lime was the minimum
amount that could be used and still furnish adequate protection against
the then known war gases. It was, therefore, decided to use ⅖ soda-lime
and ⅗ charcoal by volume and this proportion has been adhered to in all
of the later types of canisters. It is interesting to note that these
figures have been fully substantiated by the later experimental work on
canister filling.
The charcoal and soda-lime were not mixed but arranged in five layers
of equal volume, each layer, therefore, containing 20 per cent of the
total volume. The layers were separated by screens of crinoline. At the
top was inserted a layer of terry cloth, a layer of gray flannel, and
two steel wire screens. The cloth kept the fine particles of chemicals
from being drawn into the throat of the person wearing the mask.
This canister furnished very good protection against chlorine and
hydrocyanic acid and was fairly efficient against phosgene, but it was
useless against chloropicrin. These canisters were never used at the
front, but served a very useful purpose as experimental canisters and
in training troops.
It was soon found that better protection was obtained if the absorbents
were mixed before packing in the canister. This procedure also
simplified the method of packing and was used in canister _B_ and
following types. Among other changes introduced in later types were:
The integral valve was replaced by a removable check valve plug which
enabled the men in the field to adjust the valve in case it did not
function properly. The mixture of charcoal and soda-lime was divided
into three separate layers and these separated by cotton pads. The pads
offered protection against stannic chloride smokes but not against
smokes of the type of sneezing gas. The green soda-lime was replaced
by the pink granules. In April, 1918, the mesh of the absorbent was
changed to 8 to 14 in place of 6 to 14.
About July 1, 1918, the authorities were convinced by the field forces
of the Chemical Warfare Service that the length of life of the chemical
protection of the standard _H_ canister (the type then in use) was
excessive and that the resistance was much too high. Type _J_ was
therefore adopted, July 27, 1918. In this the volume of the absorbent
was reduced from 450 cc. to 300 cc. It was packed in two layers, ⅔ in
the bottom and ⅓ in the top. One pad was placed between the layers and
one on top. This change gave a lowering of the resistance of 27 per
cent (to 2.5 inches) at a sacrifice of 50 per cent of the length of
life of the canister, but not of protection during the shortened life.
Type _L_ differed from this only in having 325 cc. of absorbent, a
change made to decrease leakage about the top cotton pad.
[Illustration: FIG. 57.—U. S. Army Canister, Type _J_.]
The following table shows the relative efficiency of various canisters:
-----------------+----------+----------+----------+----------+------
| | U. S., | British, | French, |German
| p. p. m. | Type _H_ | S. B. R. | A. R. S. |
-----------------+----------+----------+----------+----------+------
Chloropicrin | 1000 | 770 | 17 | 2 | 43
Phosgene | 2500 | 85 | 54 | 5 | 16
Hydrocyanic acid | 500 | 70 | 90 | | 10
Mustard gas | 100 | 1800 | | 35 | 195
-----------------+----------+----------+----------+----------+------
The figures represent time in minutes till the first traces of gas
begin to come through.
[Illustration: FIG. 58.—Type _J_ Canister and Contents]
MANUFACTURE
The following description of the manufacture of the gas mask at the
Long Island plant is taken from an article by Col. Bradley Dewey[26]:
[Footnote 26: _J. Ind. Eng. Chem._, =11=, 185 (1919).]
“INCOMING INSPECTION—A thorough 100 per cent inspection was made of
each part before sending it to the Assembly Department. The inspectors
were carefully chosen and were sent to a school for training before
they were assigned to this important work. Every feature found to
be essential to the manufacture of a perfect gas mask was carefully
checked.
“The incoming inspection of the flexible rubber hose leading from the
canister to the facepiece can be taken as an illustration. Each piece
of hose was given a visual inspection for buckles or blisters in the
ends or in the corrugations; for cuts, air pockets, or other defects
on the interior; for loose seams where fabric covering was cemented
to the rubber tube; for weaving defects in the fabric itself; and for
careless application of the cement. Special tests were conducted for
flexibility, as a stiff hose would produce a strain on the soldier’s
mouth; for permanent set to insure that the hose was properly cured;
for the adhesion of the fabric covering to the hose; and for kinking
when the hose was doubled on the fingers. Finally each piece was
subjected to a test for leaks under water with a pressure of 5 lbs. per
sq in.
“Each eyepiece and the three-way metal connection to the facepiece
were subjected to a vacuum test for leakage. The delicate exhalation
valve was carefully examined for defects which would be liable to cause
leakage. Fabric for the facepiece was given a high-tension electrical
test on a special machine developed at the plant to overcome the
difficulty met in the inspection of this most important material. It
was of course necessary that the facepiece fabric be free from defects
but just what constituted a defect was the source of much discussion.
The electrical test eliminated all personal views and gave an impartial
test of the fabric. The machine consisted of two steel rolls between
which a potential difference of 4,000 volts was maintained; the fabric
was led through the rolls and wherever there was a pinhole or flaw the
current arced through and burned a clearly visible hole.
“PRELIMINARY FACEPIECE OPERATIONS—Blanks were died out from the
facepiece fabric in hydraulic presses. Each face blank was swabbed to
remove bloom and the eye washers were cemented about the eyeholes. The
pockets for holding the noseclips were also cemented to the blanks. The
bands which formed a gas-tight seal of the mask about the face were
died out from rubberized fabric to which a felt backing was attached.
The harness consisting of elastic and cotton tapes was also sewed
together at this point.
“FACEPIECE OPERATIONS—The sewing machine operations were next
performed. First the died out blanks were pleated to form the
facepiece. The operator had to register the various notches in the
blank to an accuracy of ¹/₃₂ in. and to locate the stitches in some
cases as closely as ¹/₆₄ in. The band was next sewed to the periphery
of the facepiece after which the harness was attached. The stitches
on the outside of the facepiece were covered with liquid dope, which
filled the needle holes and made the seams gas-tight.
“In addition to the inspection of each operation, the completed
facepiece was submitted to a control inspection to discover any defects
that might escape the attention of the inspectors on the various
operations.
“ASSEMBLY OPERATIONS—The facepieces were now ready for assembly and
were sent for insertion of the eyepieces, which was done in specially
designed automatic presses. The eyepieces had to be carefully inserted
so that the facepiece fabric extended evenly around the entire
circumference.
“Before manufacture began on a large scale, the most satisfactory
method of conducting each assembly operation was worked out and
the details standardized, so that operators could be quickly and
efficiently trained. No detail was considered too small if it improved
the quality of the mask. The assembly operations proceeded as follows:
“The exhalation valve was first joined to the three-way metal tube
which formed the connection between the facepiece, flexible hose, and
mouthpiece. Each valve was then tested for leakage under a pressure
difference of a one inch head of water. No valve was accepted which
showed leakage in excess of 10 cc. per min. under these conditions.
“The metal guard to protect the exhalation valve was next assembled,
followed by the flexible hose. The three-way tube was then assembled
to the facepiece by means of a threaded connection and the rubber
mouthpiece attached. To illustrate the attention to details the
following operation may be cited:
“The contact surfaces between each rubber and metal
part were coated with rubber cement before the parts
were assembled. The connection was then tightly wired,
care being taken that none of the turns of wire should
cross and finally the wire was covered with adhesive
tape so that no sharp edges would be exposed.
“The masks, completely assembled except for the
canisters, were inspected and hung on racks on
specially designed trucks which prevented injury in
transit, and were delivered to the Finishing Department.
“CANISTER FILLING—Meanwhile the canisters were being filled, in another
building.
“The chemicals were first screened in such a way that the fine and
coarse materials were separated from the correctly sized materials.
They were then carried on a belt conveyor to the storage bins, whence
they were fed by gravity through pipes to various mixing machines. A
special mixing machine was developed to mix the carbon and granules in
the proper proportions for use in the canister. The mixed chemicals
were then led to the canister-filling machines. There was a separate
mixing machine for each filling machine, of which there were eighteen
in all.
“The can-filling department was laid out in six units. Each unit had a
capacity of 20,000 cans per day. A system of double belt conveyors was
installed to conduct empty canisters to the machines and carry away the
filled ones.
“Each filling operation was carefully inspected and special stops were
placed on the belt conveyors so that a canister could not go to the
next operation without having been inspected. Operators and inspectors
were stationed on opposite sides of the belt. The chemicals were placed
in the canister in three equal layers which were separated by pads of
cotton wadding. The first layer was introduced from the filling machine
(which delivered automatically the proper volume of chemicals), the
canister was shaken to pack the chemicals tightly, the cotton baffle
inserted, the second layer of chemicals introduced and so forth. On top
of the top layer of chemicals were placed a wire screen and a specially
designed spring which held the contents of the canister securely in
place. The metal top was then fitted and securely soldered.
“Each canister was tested under water for possible leaks in joints or
soldering, with an air pressure of 5 lbs. per sq. in. A test was also
made for the resistance which it offered to breathing, a rate of flow
of air through the canister of 85 liters per min. being maintained and
the resistance being measured in inches of water.
“The filled canisters were then painted a distinctive color to indicate
the type of filling.
“FINISHING DEPARTMENT—In the finishing department, the filled
canisters, were conducted down the middle of the finishing tables and
assembled to masks.
[Illustration: FIG. 59.]
“The finished masks were then inspected, placed in unit boxes, ten to a
box, and returned for the final inspection.
“FINAL INSPECTION—Final inspection of the completely assembled masks
was as rigid as could be devised, and was closely supervised by army
representatives. Only the most painstaking, and careful women were
selected for this work and the masks were examined in every detail
to discover any defect that might have escaped previous inspection.
Finally, each mask was inspected over a bright light in a dark booth
for small pinholes which the ordinary visual inspection might not have
detected.
“As a check on the quality on the final inspectors’ work a reinspection
of 5 per cent of the passed masks was conducted. Where it was found
that a particular inspector was making numerous mistakes, her eyes were
examined to see whether it was due to faulty eyesight or careless work.
Masks containing known defects were purposely sent to these inspectors
to determine whether they were capable of continuing the inspection
work. In this way the desired standard was maintained.
“A daily report of the final inspection was sent back to each of the
assembly departments involved so that defects might be eliminated
immediately and the percentage of rejects kept as low as possible.
“After the final inspection the masks were numbered, packed in
knapsacks, and the filled knapsacks placed in packing cases,
twenty-four to a case.”
TISSOT MASK
The French, as has already been pointed out, early recognized that
certain classes of fighting men, as the artillerymen, needed the
maximum of protection with the minimum decrease in efficiency. The
result of this was the Tissot Mask. Before the United States entered
the war, the British standard box respirator had reached a greater
degree of perfection, with far greater ruggedness and portability. It
was therefore adopted as the American standard. At the time of the
invention of the British box respirator and practically up to the time
the United States entered the war, masks were worn only during the
sporadic gas attacks then occurring and only for a brief period at a
time. As the war progressed, the men were compelled to wear their masks
for much longer periods (eight hours was not uncommon). It was then
seen that more comfort was needed, even at the expense of a little
safety.
The principle of the Tissot mask was correct so far as comfort was
concerned, since it did away with the irritating mouthpiece and
noseclip, but the chief danger in the French mask arose from the fact
that the facepiece was made of thin, pure gum rubber. The Research
Division, together with the Gas Defense Division, developed two
distinct types of Tissot masks. The first of these was the Akron
Tissot, the second the Kops Tissot. The best features of these have
been combined in the 1919 Model.
[Illustration: FIG. 60.—American Tissot Mask, Early Type.]
[Illustration: FIG. 61.—American Tissot Mask, Interior View.]
1919 MODEL AMERICAN MASK
=Facepiece.= This facepiece is made of rubberized stockinet about
one-tenth inch in thickness. The stockinet is on the outside only and
is for the purpose of strengthening and protecting the rubber which is
of very high grade. The facepiece is died out as a single flat piece
from the stockinet which is furnished in long rolls. The die is of such
shape that when the facepiece is sewed there is but one seam, and that
between the angle tube opening and the edge under the chin. This seam
is sewed with a zigzag stitch with the stockinet sides flat together.
The seam is then stretched over a jig, so as to form a flat butt joint.
This seam is then cemented with rubber cement and taped, inside and
out, to make it thoroughly gas-proof.
The eyepiece openings are of oval shape with the longer axes horizontal
and considerably smaller than the finished eyepieces. The eyepieces
being circular, the cloth is stretched to accommodate them, giving the
necessary bulge to keep the cloth and metal of the eyepieces away from
the face. The harness has three straps on each side. Instead of the
single strap over the top of the head, two straps lead from directly
over the eyes, both being made of elastic the same as the other straps.
All six straps are brought together around a pad of felt and cloth
about 2½ × 3½ inches at the back of the head. This pad makes the
harness much more comfortable.
The rubberized stockinet is reinforced on the inner or rubber side with
thin bits of cloth at all points where the straps are sewed on. The
strap across the temples just above the ears is sewed at two points,
one about one-half inch from the edge and the other about two inches
from the edge. This is for the purpose of helping press the cloth
against the temples, thereby adding to the gas-tightness for those
heads that have a tendency to be hollow at the temples. The lower strap
is just above the chin and is for the purpose of giving gas-tightness
in that vicinity. All of the straps except the two over the top of the
head are attached to the pad with buckles, and are thus capable of
exact adjustment.
The eyepieces are of triplex glass in metal rings with rubber gaskets.
In pressing the rings home, the rubberized stockinet is turned and held
securely so that there is no possibility of pulling them out. The angle
tube containing the outlet valve and the connection to the corrugated
tube connecting with the canister is the same as with the latest model
R. F. K. mask. The only difference as regards the corrugated tube is
that a greater length is needed with the new carrier under the left
shoulder. The total length of the tube for this model is about 24
inches. On the inside of the facepiece and connected to the angle tube
inlet is a butterfly baffle of rubber, so arranged that the incoming
air is thrown upward and over the eyepieces, thus keeping them clear no
matter how much the exertion or what the temperature, except in certain
rare cases when the temperature is down at zero F. or below.
[Illustration: FIG. 62.—1919 Model American Mask.]
CANISTER
The canister is radically different from the canisters used in the
R. F. K. and earlier types. In the first place, it is longer, the
total length finished being 8 inches. It has two inlet valves at the
top end protected by a tin cover instead of the single inlet valve
at the bottom of the earlier types. The two inlet valves are each ⅝
inch in diameter and are made up of square flat valves on the end of a
short rubber tube. The rubber tube is fitted over a short metal tube.
Gas-tightness is obtained both by the pressing of the valve against
the round edge of the metal tube and by the pressure of the edges
against each other. These valves, while delicate, are proving very
satisfactory, and being simply check valves to prevent the air going
back through the canister, they are not vital. In case of failure, the
eyepieces would fog somewhat and the dead air space be increased by
that held in the inlet tube.
The canister consists really of two parts—an outer casing that is
solid and an inner perforated tin casing. Around the perforated tin is
fitted a filter of wool felt ³/₁₆ of an inch in thickness. This wool
felt is very securely fastened by turning operations to solid pieces
of tin, top and bottom, so that no air can get into the chemicals
without passing through the filter. Thus the air coming through the
inlet valves at the top circulates around the loosely fitting outside
corrugated case to all parts of the filter and after passing through
the filter continues through the perforations of the tin into the
charcoal and soda-lime granules.
The chemicals are packed around a central wedge-shaped tube extending
about two-thirds the length of the can. The wedge is enlarged at the
top and made circular where it passes through the top of the can to
connect with the corrugated tube. The wedge-shaped inner piece is made
of perforated tin and is covered with thin cloth to prevent dust from
the chemicals passing into the tube and thus into the lungs. The cans
are filled from the bottom and are subjected to two mechanical jarring
operations in order to settle the chemicals thoroughly before the
spring which holds them in place is added. The outer tin cap protecting
the inlet valves has two openings on each side but none at the ends of
the canister.
[Illustration: FIG. 63.—1919 Model American Mask after Adjustment.]
The carrier is a simple canvas case nearly rectangular, about one foot
wide and 15 inches in length. The width is just sufficient at the back
to hold the canister and the front part to hold the extra length of
corrugated tube and the facepiece. There are two straps, one passing
over the right shoulder and the other around the body. The one passing
over the right shoulder has two “V” shaped seams at the top so as to
change the direction of the strap over the shoulder in order that it
will pull directly downward instead of against the neck. The flap
closing the case opens outward.
It has the usual automobile curtain fasteners. A secondary fastener at
the top of the opening is arranged so that when the tube is adjusted to
the proper length and the mask is adjusted to the face of the wearer,
the flap can be buttoned tightly over the corrugated tube and held
tightly. This prevents water from entering the case.
Figures 62 and 63 show the position of the carrier both with the
facepiece in the carrier and after adjustment. It will be noted that
the carrier does not interfere with the pack nor with anything on the
front of the body. The left arm hangs almost entirely natural over
the case. It has been thoroughly tried out by the Infantry, Cavalry,
Artillery and Special Gas Troops and adopted as eminently satisfactory.
SPECIAL CANISTERS
=Navy.= The early Navy canister is a drum much like the German
canister. The container is a slightly tapered metal cylinder, 9 cm.
in diameter at the bottom. The most satisfactory filling for this
drum consists of two layers, 98 cc. in each, of a standard mixture of
charcoal and soda-lime, separated by cotton wadding pad. The filling is
6-20 mesh, instead of 8-14 mesh. A later type is shown in Figure 41.
=Carbon Monoxide.= This canister is discussed in Chapter XI.
=Ammonia.= Ammonia respirators were needed by the Navy and also by
the workmen in refrigeration plants. Early protection was obtained by
the use of pumice stone impregnated with sulfuric acid. This had many
disadvantages, such as the amount of heat evolved, the caustic fumes
produced, high resistance and corrosion of the canister. To overcome
these, the “Kupramite” canister was developed. The filling consists of
pumice stone impregnated with copper sulfate. Pumice stone, 8 to 14
mesh, and technical copper sulfate are placed in an evaporating pan
in the ratio of one part by weight CuSO₄·5H₂O to 1.5 parts pumice,
and the whole is covered with sufficient water to dissolve the salt
at boiling temperature. The mixture is then boiled down with constant
stirring until crystallization takes place on the pumice and the
crystals are nearly dry. The pumice thus treated is then removed from
the dish, spread out and allowed to dry in the air. The fines are then
screened out on a 14-mesh sieve. Care must be taken in the evaporating
process that the absorbent is still slightly moist when taken from the
pan.
[Illustration: FIG. 64.—Early Type Navy Mask. Contains noseclip and
mouthpiece.]
In packing the standard Army canister with kupramite a layer of
toweling is placed on top of the absorbent to filter out any fine
particles which might be drawn up from the absorbent, and the whole is
held in place by the usual heavy wire screen and spring. This method of
packing is to be used with the present mouthpiece type of army mask.
If the new Tissot type mask is used, a modification of the packing is
desirable in order to eliminate the trouble due to moisture given off
by the absorbent during service condensing on the eyepieces of the mask
and thus impairing the vision of the wearer. To remedy this defect a
1-in. layer of kupramite at the top of the canister is replaced by
activated charcoal or silica gel, preferably silica gel. This decreases
the humidity of the effluent air sufficiently to prevent dimming of the
eyepieces. If charcoal is used, a 2-8 cotton pad (Eastern Star Furrier
Co., Pawtucket, R. I.) is substituted for the toweling in order to
remove charcoal dust. The canister complete weighs about 1.7 lbs.
[Illustration: FIG. 65.—Ammonia Canister—“Kupramite.”]
A canister containing 45 cu. in. of this material will protect a man
breathing at rest for at least 5 hours against 2 per cent ammonia and
for 2½ hours against 5 per cent ammonia. Its advantages are large
capacity and activity, negligible heat of absorption, and cheapness.
PHYSIOLOGICAL FEATURES OF THE MASK
For some time after the introduction of gas warfare, the gases used
were of the so-called non-persistent type. Under these conditions it
was necessary to wear the mask for only relatively short periods,
after which the cloud dissipated. With the increasing use of gas and
the introduction of the more persistent gases, particularly mustard
gas, it not only became necessary to wear the mask for long periods of
time but also to do relatively heavy physical work, such as serving
artillery, when wearing the mask.
[Illustration: FIG. 66.—Ammonia Mask, Showing Relative Size of
Canister.]
Under these conditions, it became evident that the wearing of the
mask caused a very great reduction in the military efficiency of
the soldier. The reasons for this reduction in efficiency have been
made the subject of extensive research by a group of the foremost
physiologists and psychologists of the country. As a result of their
work, the causes contributing to this reduction in efficiency may be
grouped about the following main factors:
(1) The physical discomfort of the mask arising from causes such as
pressure on the head and face, due to improperly fitting facepieces and
harness, the noseclip, and the mouthpiece.
(2) Abnormal conditions of vision, due to poor optical qualities in eye
pieces and restrictions of vision, both as to total field and binocular
field.
(3) Abnormal conditions of respiration, among them being (_a_) the
unnatural channels of respiration caused by wearing the box respirator,
(_b_) increase in dead air space in respiratory circuit, and (_c_) the
increase in resistance to both inhalation and exhalation, the last two
mentioned being present to a greater or less degree in all types of
mask.
Of these general subdivisions the various phases of the first two are
so evident that no further discussion will be given. The effects of
the changed conditions of respiration are, however, less obvious, and
it may be of interest to present in a general way the results of the
research along this line, particularly as regards the harmful effects
of increasing the resistance and dead air space in the respiratory
tract above the normal.
The function of respiration is to supply oxygen to and remove carbon
dioxide from the blood as it passes through the lungs. This interchange
of gases takes place in the alveoli, a myriad of thin-walled air sacs
at the end of the respiratory tract where the air is separated by a
very thin membrane through which the gases readily pass. The volume
and rate, or in other words, the minute-volume, of respiration is
automatically controlled by the nerve centers in such a way that a
sufficient amount of air is supplied to the lungs to maintain by means
of this interchange a uniform percentage of its various constituents
as it leaves the lungs. It will be readily seen therefore, that
anything which causes a change in the composition of the air presented
to the blood in the alveoli will bring about abnormal conditions of
respiration.
Inasmuch as the gaseous interchange between the lungs and the blood
takes place only in the terminal air sacs it follows that, at the end
of each respiration, the rest of the respiratory tract is filled with
air low in oxygen and high in carbon dioxide, which on inspiration
is drawn back into the lungs, diluting the fresh air. The volume of
these passages holding air which must be re-breathed is known as the
anatomical dead air space.
Similarly, when a mask is worn the facepiece chamber and any other
parts of the air passage common to inspiration and expiration become
additional dead air space contributing a further dilution of oxygen
content and contamination by carbon dioxide of the inspired air in
addition to that occasioned by the anatomical dead space, which of
course, is always present and is taken care of by the functions
normally controlling respiration.
Major R. G. Pearce who directed a large amount of the research along
this line, sums up the harmful effects of thus increasing the dead air
space as follows:
1. Interpretation from the physiological standpoint:
(_a_) A larger minute-volume of air is required when breathing through
dead air space. This, interpreted on physiological grounds, means that
the carbon dioxide content of the arterial blood is higher than normal.
The level to which the content of carbon dioxide in the arterial blood
may rise is limited. Anything which wastefully increases the carbon
dioxide level of the blood decreases the reserve so necessary to a
soldier when he is asked to respond to the demand for exercise which is
a part of his daily life.
(_b_) A larger minute-volume of air must be pulled through the
canister, which offers resistance proportional to the volume of air
passing through it. If resistance is a factor of harm, dead air space
increases that harm, since dead air space increases the volume of air
passing through the canister.
(_c_) As will be noted below, the effect of resistance is a tendency to
decrease the minute-volume of air breathed. Dead air space increases
the minute-volume. Accordingly, if breathing is accomplished against
resistance and through a large volume of dead air space, the volume
of air breathed is reduced more in proportion to the actual needs of
the body than when breathing against resistance without the additional
factor of dead space; this, again, causes the level of carbon dioxide
in the blood and tissues to be raised to a higher level than normal,
and thus again there is some reserve power wasted.
2. Interpretation from the standpoint of the canister.
The life of the canister depends on the volume of the gas-laden air
passed through it. The dead space increases the minute-volume of air
passed through the canister and, therefore, shortens its life.
Physiologically, the reason for the harmful effects of breathing
resistance is more involved:
“The importance of resistance to breathing lies in: (1)
the effect on the circulation of the blood, and (2) the
changes in the lung tissue, which seriously interfere
with the gas exchange between the outside air and the
blood. Data have been presented to draw attention to
the seriousness of resistance to inspiration. In these
reports, it was suggested that the deleterious effects
on the body consist in changes in the blood pressure,
increased work of the right side of the heart, and an
increase in the blood and lymph content of the lungs.
Resistance also decreases the minute-volume of air
breathed and thereby increases the percentage of carbon
dioxide in the expired air. The foregoing changes are
all deleterious.
“Although the chief problem of resistance in gas mask
design concerns inspiration, nevertheless _resistance
to expiration_ is an important factor. The expired
air of the lungs contains carbon dioxide for which
means of escape must be provided. The expiratory act is
more passive than the inspiratory act, and resistance
to expiration is, therefore, more keenly felt than
resistance to inspiration. It is then imperative that
the exhale valve be so arranged as to allow for the
escape of the entire amount of air during the time of
expiration with the least possible resistance. The data
of the laboratory indicate that seldom, if ever, do
expiratory rates rise above a velocity of 150 to 175
per minute. The effect of resistance to exhalation upon
the vital organs of the body is not dissimilar to that
of inspiration.”