1. n. [Drilling Fluids]

The lethal concentration of a substance, reported in ppm, that kills 50% of a population of test organisms, such as mysid shrimp, in a standard, controlled laboratory bioassay test. In offshore drilling operations, the LC₅₀ number is used to determine whether waste mud or cuttings can be discharged into the water. The larger the LC₅₀ ppm number from the test, the less toxic the sample is to the organism. For example, if LC₅₀ number is 1,000,000 ppm, the sample is presumably nontoxic according to the test protocol.

See: bactericide,  bioassay,  cuttings,  Minerals Management Service,  mysid shrimp,  ppm