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«»pondingly lighter than 450 mg. per cubic meter. The absolute quantity of hydrocyanic acid, according to the foregoing toxicity index, averages 20 mg.; but as dilution increases, the toxicity of gaseous hydrocyanic acid falls off rapidly, until at 0.03 gram per cubic meter the human organism is able finally to counteract wholly its toxicity; thus concentrations below this strength become actually innocuous. Considering the extreme volatility of carbon monoxide and hydrocyanic acid, and the resulting impracticability of creating effective concentrations of their vapors in the field, it is evident that these chemicals, while commonly regarded as highly poisonous, are unsuited to military usage.

The foregoing toxicity figures must be accepted with reservations, as they apply to one set of conditions only, i.e., to one animal (the cat) and to one rate of breathing. While experimental determinations of toxicity on animals yield valuable relative data as to certain classes of toxic agents, these data cannot always be applied to man, because

on experimental observations

The product cl = W is called the product of mortality (Todlich-keitsprodukl) or the lethal index of the particular toxic substance for the given animal. This product W varies inversely as the toxicity of the toxic substance, ue., the smaller the value of }V, the more toxic is the substance.

By taking a large number of observations on various animals exposed to constant concentrations of toxic gases for definite periods of time under carefully controlled conditions and tabulating the mortality results, it luus been found that the relation between the minimum lethal dose and time of cx|x>sure follows a definite curve for each toxic gas. Chart II shows this curve for phosgene on dogs and expresses the concentration required for each length of exposure in order to produce death. Such curvcs are generally known as toxicity curves and the form of curve shown in Chart II is typical of all such curvcs, and illustrates the tremendous increase in concentration required when the time of exposure is reduced below 10 minutes. For this reason, it is customary to base relative toxicity figures on 10 minutes for short exposures and 30 minutes for long exposures.

U|H)n examining internal physiological reactions to toxic vaj>ors, it api>ears that some differentiation must be made between those substances characterized by local effects as distinguished from those that induce general systemic poisoning. Compounds of the latter category, such as carbon monoxide and hydrocyanic acid, are in part neutralized by certain 'physiological counterreactions, and their reactions with the body tissues are reversible up to a certain ¡>oint. To allow for this

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