LD50 is typically reported in units of

• A. mg/kg
• B. ppm
• C. L/kg
• D. mg/L

Answer: (A)

LD50 is a dose-based toxicity measure: it represents the amount of substance required to kill 50% of a test population. To compare toxicity across animals of different sizes, the dose is normalized to body weight, giving units of milligrams of substance per kilogram of body weight (mg/kg). This per-body-weight basis reflects how much substance an organism can absorb relative to its size, which is what drives toxicity.

Other units shown are concentrations, not doses relative to body weight. Ppm and mg/L describe how much substance is present per unit of air or liquid, not how much substance per kilogram of an organism. L/kg would be a volume per kilogram and is not the standard way to express a lethal dose. Therefore, mg/kg is the correct and standard unit for LD50.