Besides lowering the temperature of the passenger cabin, the air-con also serves the purpose of de-humidifying the air in the passenger compartment. The system then removes the moisture through an evaporator beneath the dashboard.

Due to the fact that moisture is collected internally, it leads to the growth of fungus and bacteria in the system over time. The presence of such micro-organisms in the ducts is what produces that nasty smell every time the air-con system is switched on. Apart from its offensive smell,in severe cases, these bacteria can even lead to infections of the ear, nose and throat of passengers or what some refer to as "sick car syndrome".