Deodorizing is the removal of unpleasant odors wafting in the air. What kinds of odor do humans find unpleasant? Living organisms are composed of proteins, and proteins are composed of various amino acids.
Decomposed protein is hazardous, so in order for people not to ingest it as food, we perceive the smell of volatile compounds—such as ammonia produced by the breaking down of amino acids—as unpleasant. Therefore, the very perception of a certain smell as unpleasant is a necessary function for humans, and because humans are such sensitive creatures, deodorizing becomes a very delicate process. For example, even if 50% of an odor-causing material is removed from the air, humans will still perceive the smell to have been barely reduced in strength. Furthermore, even if 99% of the odor-causing material is removed, humans will still perceive the remaining smell to be about 30% as strong.
This is one of the reasons that deodorizing is such a difficult task.