Many fungi can cause allergic reactions in susceptible people and respiratory irritation in non-allergic people. Inhalation of fungal spores by highly susceptible people can have fatal consequences. Some environmental bacteria can grow in ventilation systems, but these are rarely a threat to healthy people. Fungi differ significantly, in certain respects, from most other airborne pathogens, such as bacteria, viruses, and protozoa. Fungi can exist outdoors and enter the building through the air intakes.
Fungi are normally harmless and non-parasitic.
Fungal infections inevitably result from fungi being in the wrong place, often as the result of poor cleanliness or improper design of ventilation system components. Fungi produce spores, in much the same way as bacteria do, and this enables them to survive harsh conditions while they travel or lie dormant. Spores are usually what enter the building air intakes and what can travel through the ventilation air stream. Fungal spores are smaller than fungal cells and can vary in size from 1 micron to 100 microns. A well-maintained HEPA filter should be capable of intercepting the vast majority of fungal spores.