• Mold and mildew grows inside air conditioners.aire acondicionado image by Antoni Traver from Fotolia.com
  
  Almost nothing is as bad as having a mildew smell coming out of the air conditioning vents. Besides the bad smell, mold and mildew are known health hazards. Reasons exist for this adverse condition. By understanding these reasons, you can eliminate the growth and thus eliminate the smell.
  
  

Environment for Growth

• Mildew needs a dark, damp environment with stale air for growth. The insides of an air conditioner and ductwork are exactly that. It is a perfect incubator for mold, slime mold and mildew. Air conditioners are not used continuously but rather are shut down in the fall and started back up in the spring. Bear in mind that most molds and mildew do not die when the temperature falls below freezing but instead enter into a dormant state. When the temperature goes above freezing, growth resumes.
  
  

Contaminated Intake

• The air intake to your air conditioning unit is possibly contaminated. If the unit is in a crawlspace, the environment is damp, dark and has stale air. The intake of the air conditioning unit is in a mildew-laden environment, and it is sucking up mildew. The cure for this is to relocate the intake to a well-lit area that has good natural airflow, or relocate the intake to an outside source of air.
  
  

Water Leaks

• According to Energy Star, water leaks lead to mildew growth. Since an air conditioner cools the air, condensation occurs inside the unit. If the condensation pan is overflowing, it can overflow into the ducts. This usually happens when the drain tube becomes plugged up with slime mold. If the unit is located in an attic, the water may even drip onto the ceiling. A pool of stale water, combined with a dark environment, again is an ideal incubator for mildew growth. To avoid this, ensure the drainage lines from the drip pan to the drain are clear.