Sometimes, humidity or dampness (water vapor) in the air can supply enough moisture for mold growth. Indoor relative humidity (RH) should be kept below 60 percent -- ideally between 30 percent and 50 percent, if possible.
The photo shows mold growth on walls and ceilings as a result of high humidity because the home was closed up with little or no air conditioning.
To the untrained eye, air conditioners cool your home by blowing cool air into it; the truth is that air conditioners remove the heat and humidity from your living spaces while blowing cool air into it. ACs are able to remove heat and humidity due to the refrigerant in the system.