Chemical warfare

The pharmacology of war gases plays such an important part in chemical warfare that a brief discussion may well be given of the methods used in the testing of gases for toxicity and other pharmacological properties.

War gases may be divided into two groups: persistent and non-persistent, each of which may include several classes:

Each class necessitates special tests in order to determine whether or not it is suitable for further development.

Toxicity

One of the first points which must be carefully determined in investigating a substance is its toxicity. It is important that this be determined for numerous reasons:

1. To determine what concentrations are dangerous in the field.

2. To ascertain how effective protective devices have to be to furnish sufficient protection against the gas.

3. To furnish a basis for accurate experimental work on the treatment of gassed cases.

4. To decide whether or not the material is worthy of further development in the laboratory or in the plant.

These considerations necessitate the determination of the toxicity in the form of a vapor and not by the ordinary method of administration by mouth, through the skin (subcutaneously) or through the blood (intravenously). The simplest method of determining the toxicity of a substance as a vapor would be to place animals in a gas-tight box and introduce a known amount of the substance in the form of vapor. But by this method the concentration is not accurately known unless chemical analyses of the air are made, and then it is found to be much less than that calculated from the amount of substance introduced, because of condensation on the walls of the chamber, or absorption of the substance by the skin and hair of the animal and in some cases, of decomposition of the substance by moisture in the air. Moreover, it is found that the concentration decreases markedly with time. Because of these factors, the figures used for the concentration are more or less guess work. To overcome these difficulties, a chamber is used through which a continuous current of air, containing a known and constant amount of the poisonous vapor, is passed. Such an apparatus is shown in Fig. 116.

The flask E is a 300 cc. Erlenmeyer flask, with a ground glass stopper. The liquid to be tested is placed in this flask together with a sufficient quantity of glass wool to prevent splashing and the carrying over mechanically of droplets of the liquid. Air is passed through A and C (calcium chloride drying tubes) and the rate measured by the flow meter D. The air and gas are mixed in F before passing into the chamber G. This chamber is made of plate glass, is of about 100 liters capacity, and is air-tight. The entire flow of air and gas through the box, kept constant at 250 liters per minute, is measured at H. The gas is removed through K, which is filled with charcoal and soda-lime, in order that little gas may pass into the pump.

By weighing the flask E, and its contents before and after passing air through it, and knowing the total volume of the mixture passing through the chamber during the same period, the concentration of the substance can readily be calculated. This concentration, as determined by the “loss in weight” method, can be checked by chemical analysis (samples taken at M—M). The method has been found to give accurate values.

The concentration in the chamber reaches its constant level within 30 to 40 seconds after the apparatus is started.

With the flow of 250 liters per minute, the difficulties mentioned above are reduced to a point where they are practically negligible.

All toxicity tests on mice were made with an exposure of ten minutes, while dogs were exposed for thirty minutes. In case death did not occur during exposure, the animals were kept under observation for several days. Toxicity and all other figures are expressed in milligrams per liter of air, though parts per million (p.p.m.) was frequently used during the early work.

Another point of difficulty is the great individual variation in the susceptibility of animals. This is probably greater than when the poison is administered subcutaneously or intravenously. It necessitates the use of a large number of animals in making a determination of the toxicity of a gas. Again, the toxicity for different species may vary, and as the ultimate aim is a knowledge of the toxicity for man, a great many different species must be used. If the toxicity is widely different for different animal species, it is hard to arrive at a definite conclusion as to the toxicity for man.

With longer exposures than thirty minutes the lethal concentration is usually less, there being a cumulative effect. This is not true for hydrocyanic acid. If the concentration is not enough to kill at once, an animal can stand it almost indefinitely. Whether the action is cumulative or not depends on the rate at which the system destroys or eliminates the poison. If the poison is being eliminated as fast as received the concentration in the tissues cannot increase. It is stated, for example, that the amount of nicotine in a cigar would kill a man if taken in one dose. If it is spread over twenty minutes, the destruction or elimination of the nicotine is so rapid that no obviously bad effects are noted.

Another interesting thing about the work on poison gases is that in most cases a preliminary exposure to less than the lethal concentration does not seem to make the animal either more or less sensitive on a later exposure. This is quite unexpected, because we know that with irritating gases, especially lachrymators, men adapt themselves to much higher concentrations than they could stand at first. In view of the experiences of arsenic eaters, it is quite possible that the experiments, which showed no accustoming to toxic gases, were not continued long enough to give positive results.

Not only does the susceptibility of different animals of the same species vary greatly for a particular gas, but the susceptibility of different species varies greatly with different gases. Thus while the effects of certain gases on mice are quite comparable to the effects on man, it is very far from being true with other gases.

Lachrymators

While one cannot determine the lethal concentrations of poison gases for men, it is possible to determine the concentration that will produce lachrymation. The threshold value is that at which two-thirds of the observers experience irritation. The lachrymatory value is considerably higher than the threshold value.

A very satisfactory method for determining lachrymatory values is shown in Fig. 117. Air is measured at A and bubbled through the lachrymatory substance in B. The air and gas are mixed in D and pass into E, a gas-tight, glass-walled chamber of about 150 liters capacity. The gas is removed through Ef, by suction and the volume of the air-gas mixture measured by the flow meter, F.

After the apparatus has run a few minutes, and the concentration of the gas has become constant, the subject is instructed to adjust the mask, attached at H, and to tell whatever he notices just as soon as he notices it. The operator stands in such a position that he can manipulate the stopcock H without being observed by the subject. After breathing air for a time (H is a two-way cock, connected with the air through J, and to the chamber through Eg) both to become accustomed to the mask and to eliminate, as far as possible, any “psychological symptoms,” the subject is allowed to breathe the gas mixture for a maximum of three minutes. If the expected symptoms are produced in less than this time, the test is discontinued as soon as they develop.

For accurate work, it is necessary to work with a pure sample which is at least fairly volatile. Mixtures cannot be run by this method. In this case it is necessary to volatilize each separately, passing the vapors simultaneously into the mixing chamber E.

A spray method may also be used with satisfactory results. Types of sprays are shown in Fig. 118.

Odors

Because of the great value in detecting low concentrations of gases in the field, it is important to know the smallest amount of a gas that can be detected by odor. In some cases, this test is more delicate than any chemical test yet devised.

Odors may be divided into two classes, true odors, and mild irritation. By true odor is meant a definite stimulation of the olfactory nerve, giving rise to a sensation which is more or less characteristic for each substance producing the stimulation. Mild irritation defines the sensation which is confused with the sense of smell by untrained observers, but which is really a gentle stimulation of the sensory nerve endings of the nose. This so-called odor of substances producing this effect is not characteristic. Higher concentrations of these compounds almost invariably cause a definite irritation of the nose.

Examples of true odors are the mercaptans, mustard gas, bromoacetone, acrolein, chlorine and ammonia. Substances which cause mild irritation are chloroacetone, methyl dichloroarsine, ethyl iodoacetate and chloropicrin.

In making the test for odor, the same apparatus is used as for lachrymators. The time of exposure is shortened to 30 seconds, as the subject always detects the odor at the first or second inhalation.

In this connection the recent work of Allison and Katz (J. Ind. Eng. Chem. 11, 336, [1919]) is of interest. They have designed an instrument, “the odorometer,” for measuring the intensity of odors in varying concentrations in air. It is based on the principle given above. A measured volume of air is passed through the liquid and then diluted to a given concentration. The mixture is then passed through a rubber tube with a glass funnel at the open end. Only one inhalation of the mixture is used to determine the intensity of the odor. The position of any strength of odor on the scale depends upon the sensitiveness and judgment of the operator, but with one person conducting the entire test, the results have been found quite satisfactory. (See tables on pages 360 and 361.)

Skin Irritants

Substances which seem useful for producing skin burns are studied both on animals and on man. Dichloroethyl sulfide (mustard gas) is used as a basis of comparison. Several methods are available.

Direct Application. This method consists of the direct application of the compound itself to the skin, using a definite quantity (0.005 cc. or 0.005 mg.) over a definite area (5 square centimeters) of the skin. With such a quantity of mustard gas a rather severe burn on animals is produced. No precautions are taken to prevent evaporation from the skin since it is believed that in this way the test will approximate fairly closely the field conditions.

Vapor Tests. Preliminary tests with vapors of volatile compounds are best made by placing a small amount of the material on a plug of cotton in the bottom of a test tube enclosed in a larger test tube which acts as an air jacket. After about an hour at room temperature the mouth of the test tube is applied to the skin. The concentration is not known, but one is dealing practically with saturated vapor. If an exposure of from 30 to 60 minutes produces no effect, one is safe to assume that the compound is not sufficiently active to be of value as a skin irritant.

If quantitative results are desired, the apparatus shown in Fig. 119 is used. Dry air is blown through the bubbler, which is connected with a series of glass skin applicators. The concentration is determined in the usual way. The skin applicator consists of a small cylinder about 1.5 to 2 cm. in diameter and about 4 cm. long with a small glass handle attached on top. The opening is 1 cm. in diameter. When the concentration of the gas is constant, the exposure to the skin is made directly for any desired length of time. The skin irritant efficiency is judged by comparing the per cent of positive responses to approximately equal concentrations of the vapors, using mustard gas as a standard.

TABLE I—Physical and Physiological Properties
of Chemicals Used as Stenches

TABLE II—Results of Measurement of the Intensity
of Various Stenches

Touch Method. This method consists of dipping a small glass rod drawn to a needle-like end to the depth of 1 mm. in the compound and then quickly touching the skin. The method is qualitative only.

Use of Solutions. Alcohol, kerosene, olive oil, carbon tetrachloride and other solvents may be used for the purpose of determining the lowest effective concentration of a substance, and for the determination of the relative skin irritant efficiencies of various compounds. Since the skin irritants were scarcely ever used in this form in the field, that is, in solution, the method is not as satisfactory as the vapor method.