Phosphine is a poisonous gas that has a strong, distinctively fishy smell. Exterminators sometimes use phosphine to kill pests or keep them from infesting shipments of grains.
Phosphine is an effective way to kill bedbugs, rats, and other pests, but it can also be very dangerous for humans who are exposed to the toxic substance. The chemical is mainly used in industrial extermination, often to fumigate the goods on container ships, but it's also used in the semiconductor industry and is sometimes an ingredient in flame retardants. People who work with phosphine have to be careful to avoid inhaling it.