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.
Fig. 117.—Aeration Apparatus for Testing Lachrymators.
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.
