What is the reason why warm air smells bad?

Cars smell when they use warm air. The most common reasons are that the evaporator is moldy, the air conditioning duct smells, and the air conditioning filter element smells. The evaporator is moldy: the warm air of the car mainly comes from the heat of the water tank, which is in the evaporator. It is the cooling system of warm air engine. In the process of cooling the engine, heat is blown into the cab by the blower. After a long time, a small amount of moisture and dust may be attached to the evaporator, and the evaporator will become moldy and produce odor. Air conditioning duct has odor, air conditioning filter element has odor: air conditioning duct has odor. The smell of damp and moldy air conditioning pipes. The empty filter element smells. Empty filter elements have a service life and are usually replaced every two years. Otherwise, it will be moldy and produce odor after a long time. And bacteria will gradually accumulate, which is harmful to human health.