Car air-condition has a foul smell

The nature of an air-conditioner is that it generates condensation “water” as it cools the air passing through it. The dust in the air sticks to the water and then when the car is shut off, the water and dust stagnates. Consequentially, when the car is started the next day and the blower is turned on, you get a musty air smell. The older the car, the more dirt build up and the stronger the smell.