The word 毒气 is formed by combining two meaningful characters: 毒, which represents the concept of "poison" or "toxin", and 气, which represents the concept of "gas" or "air". These two independent ideas are merged conceptually to create a single, unified notion, specifically, the phenomenon of "poisonous gas" or "toxic fumes". It is a direct, logical fusion where the first character describes the harmful nature and the second character specifies the gaseous form, resulting in a compound that efficiently conveys the idea of a hazardous airborne substance.