Any organic material that is high %RH for +24 hours has a high potiential for mold growth. Mold spores and organic dust are everywhere. A couple hours of drying out the ducts every 12 hours could stop mold spore from growing. A/c ducts in attics with moisture saturated cooling coils will keep the ducts near saturated for days on end. Completely drying out the ducts by operating the fan a couple hours twice a day stops mold. Fans in the "on" mode 24/7 are most likely to be clear of mold. There are other problems.
Also ducts in attics during cold weather also have mold potiential from condensation on the ducts when the duct surface is near the dew point of interior air dew point. Again operating the fan several hours each day will dry these surfaces and prevent mold spore germination.
Good indoor air quality requires fresh air change when occupied. An air change in 4-5 hours is usually adequate when occupied.
Air filtering to keep the equipment clean. Merv +11 is suggested for inside and fresh make-up ventilation.
Maintaining <50%RH throughout the inside of a home avoids mold and dust mites. Air ducts spend many hours with 80%-near 100%RH during cooling hours. Several hours everyday of <50%RH everyday will reduce mold/bacteria to minimable level.
In green grass climates, a whole house ventilating dehumidifier like the Ultra-Aire supplys fresh filtered air, blends the filtered dry air with filtered house air, circulated the conditioned air throughout the home via the a/c ducts. This procces maintains <50%RH throughout the all of the equipment.
In most cases the 100 watt fan of the dehumidifier can operate 24/7 which keeps everything dry during the "off" cycle. The real long term effects of UV lites on sensitive occupants is unknown.
Fresh filtered air, keep every dry when not in use, and low circulation is effective for indoor air quality.
Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"