Fumigation of buildings, grain stores, and ships kills rodents and insects with gases that may cause variable welfare outcomes depending on the agent used.
Fumigation welfare is largely invisible — the deaths occur in sealed spaces and are rarely observed. For rodents, phosphine fumigation causes prolonged death from multi-organ failure rather than rapid unconsciousness. CO2 at sufficient concentration is more humane. For insects, the welfare assessment depends on one's position on invertebrate sentience. Practically, the most significant welfare improvement available is selecting fumigants and methods that cause faster loss of consciousness and death for mammals accidentally or deliberately targeted. Non-lethal alternatives (inert gas storage for grain, physical exclusion for buildings) eliminate the welfare issue entirely where feasible.