It can be quite disconcerting when the air quality in your home or business becomes foul because of the air conditioner. And this is particularly bad in hot climates like we have here in Arizona. Mildew and fungus growth are the most common causes of bad-smelling air being emitted from your system. It’s not surprising. As the AC removes excess moisture from the air during the cooling process, the water can collect in the unit if it does not properly drain. This can lead to odiferous fungi and mildew problems. Additional causes of bad AC smell include internal gas leaks, dead animals, and electrical shorting in the motor. In any case, it’s best to call in a professional.