The LD50 value is typically expressed in which units?

• A. mg/kg body weight
• B. g/kg body weight
• C. mg/L
• D. ppm

Answer: (A)

The LD50 is a dose measure, referring to the amount of substance that would kill 50% of a test population. Because toxicity scales with body size, the dose is normalized to body weight, so the standard expression is milligrams of substance per kilogram of body weight (mg/kg). This unit lets you compare toxicity across animals of different sizes and species on a consistent basis. While you might see grams per kilogram (g/kg), the conventional and more precise unit in toxicology data is mg/kg. Units like mg/L or ppm describe concentration in a medium (air or liquid) rather than dose per body weight, so they aren’t appropriate for LD50 values.