The drain pan has a small drain if it is blocked, the condensate water will collect and remain in the pan below the coils. This is also a perfect breeding ground for mold.
The solution may be to clean your air conditioning systems evaporator coils. The cost to clean the coils will depend on the coil location, coil size and whether the coils can be cleaned in place or if they have to be removed to clean.
If your coils are older and rusted or corroded you may want to consider replacing them or the entire air handler because cleaning them and/or removing them could cause more damage then good. It's possible that the coils may begin to leak refrigerant or even fall apart.
Ultraviolet lights and certain air purifiers may also be a solution.
If you would like more information about these services contact Prestige Air Conditioning Systems, Inc.