Haber's rule

Haber's rule is a mathematical statement of the relationship between the concentration of a poisonous gas and how long the gas must be breathed to produce death, or other toxic effect. The rule was formulated by German chemist Fritz Haber in the early 1900s.

Haber's rule states that, for a given poisonous gas, $$C\times t = k$$, where $$C$$ is the concentration of the gas (mass per unit volume), $$t$$ is the amount of time necessary to breathe the gas, in order to produce a given toxic effect, and $$k$$ is a constant, depending on both the gas and the effect. Thus, the rule states that doubling the concentration will halve the time, for example.

Haber's rule is an approximation, useful with certain inhaled poisons under certain conditions, and Haber himself acknowledged that it was not always applicable. It is very convenient, however, because its relationship between $$C$$ and $$t$$ appears as a straight line in a log-log plot. In 1940, statistician C. I. Bliss published a study (Bliss, 1940) of toxicity in insecticides in which he proposed more complex models, for example, expressing the relationship between $$C$$ and $$t$$ as two straight line segments in a log-log plot. However, because of its simplicity, Haber's rule continued to be widely used. Recently, some researchers have argued (Miller et al., 2000) that it is time to move beyond the simple relationship expressed by Haber's rule and to make regular use of more sophisticated models.