he parameter LD50 is defined as the amount of poison that kills half the population studied. Because the mass of the subject varies within a species and between species, the LD50 is expressed in terms of mg per kg of body mass. So, for example, when comparing mice and men, at least the mass ratio is taken into account, even though lots of other characteristics may differ. In human studies, it is generally young and healthy males that are studied. The old and the young are more sensitive, and females have different sensitivities than males.