Which LC50 value indicates the product is most toxic to trout?

• A. 100 ppm
• B. 50 ppm
• C. 10 ppm
• D. 1 ppm

Answer: (C)

LC50 is the concentration of a substance in water that kills 50% of the trout under a defined exposure. The smaller this value, the more toxic the chemical is, because only a small amount is needed to reach the lethal threshold for half the population.

Looking at the numbers presented, the smallest LC50 is 1 ppm, which means that at this concentration, half the trout would be expected to die under the tested conditions. The higher the LC50, the less toxic the substance is under the same conditions; for example, 10 ppm would be less toxic than 1 ppm but more toxic than 50 or 100 ppm.

If the answer key lists 10 ppm as the most toxic, that would not align with the standard interpretation, since 1 ppm is more toxic than 10 ppm. The correct approach is to identify the smallest LC50 value as indicating the greatest acute toxicity to trout.