If you compare two chemicals where one has LD50 of 5 mg/kg and the other 50 mg/kg, which is more toxic?

• A. The one with 50 mg/kg.
• B. They are equally toxic.
• C. It cannot be determined from LD50 alone.
• D. The one with 5 mg/kg.

Answer: (D)

LD50 is the dose required to kill 50% of a test population under specified conditions. A smaller LD50 means the substance is more potent and toxic because a much smaller amount is enough to reach the lethal threshold. So, a chemical with an LD50 of 5 mg/kg is more toxic than one with 50 mg/kg—the five-milligram dose is enough to be lethal in half the individuals, whereas it takes fifty milligrams per kilogram for the other chemical to have the same effect. This comparison assumes the same species, exposure route, and testing conditions; differences in these can change LD50 values, but within the same context the lower value indicates greater acute toxicity.