A well-maintained HEPA filter should be capable of intercepting the vast majority of fungal spores. Filtration provides the primary defense against fungal spores entering a building ventilation system. Pre-filters can be effective against most fungi, even when in the spore form. If a higher degree of protection is required, HEPA filters can be very effective, provided they are tightly installed, and well maintained. If the fungus is already growing inside the building or ventilation system, the problem becomes somewhat more difficult.
Fungi require moisture for growth.
The source of the moisture must be identified and then controlled. Cooling coils, drains pans, and water pans for humidifiers are likely locations for fungal growth, especially when there is standing water. These components should be disassembled and cleaned with a strong disinfectant, such as chlorine, when fungal or bacterial growth is found. Clogged drains are often a cause for standing water. Condensation on ductwork or other components is another likely source of moisture.