A similar definition could be made for 1B. Note that 1B = 1 — 1a for a binary mixture. The range of values for 1A is 0.0 (pure B) to 1.0 (pure A).

Now consider an experiment in which the toxicity of a mixture is determined. The fraction 1A is held constant but the total dosage dt is varied to determine the LD50 of the mixture (LD50M). In general, LD50M will depend on the fraction 1A, and will equal LD50B when 1A is zero and will equal LD50A when 1A is 1.0. The question is: What happens in between?

We can define the situation expected when the two toxins act independently: Two toxins are said to show additive toxicity when the toxicity of a mixture of them (as measured by inverse dosage) varies linearly with the mixture fraction. Figure 19.4a shows this relationship. In the case where the dosage is the LD50, the equation of the straight line is

LD50,m LD503 VLD50.A LD503

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