Fungi and Bacteria in Ventilation Systems
Fungi growing in ventilation systems may contaminate indoor environments and cause a variety of problems. Some fungi can cause lung infections. 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 raely a threat to healthy people. They can, however, be a nosocomial problem. Low levels of airborne fungi can be a primary or contributing cause of Sick Building Syndrome (SBS) and poor Indoor Air Quality (IAQ). The photomicrograph at the right shows lung tissue infected with a growing mycelium of aspergillus. Fungi differ significantly, in certain respects, from most other airborne pathogens, such as bacteria, viruses, and protozoa. Fungi do not cause secondary contagious infections; only the person inhaling the fungi is at risk. Fungi can exist outdoors and enter the building through the air intakes. No other respiratory pathogens can exist outdoors - viruses and bacteria are carried and transmitted indoors by human or animal hosts, with anthrax being the one exception. 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 are actually plants that contain no chlorophyll -- see this chart.

The true fungi are the Eumycetes. Some of the fungi, the mushrooms, yeasts and some of the molds are extremely beneficial to us. They assist the production of cheese, antibiotics, yogurt, wine and beer. Some fungi, like the blights, can cause extensive crop damage. Dutch Elm disease is, in fact, a fungus. In buildings, the ones that cause problems when they get into the wrong place are usually certain of the ascomycetes.
Airborne Pathogenic Fungi
Only certain fungi can produce infections, and only a few of these have been noted to travel via the airborne route, or become entrained in the airflow of intake ducts. The following chart list those fungi which are of primary concern:

Fungi from Outdoors
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 airstream. 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. At the right is an image of a colony of Candida albicans that has produced a number of large and small spores.
Fungi are ubiquitous in the outdoors, but occur in high concentrations only in hot Southern climates, especially during dry spells. Florida, Louisiana, Texas, New Mexico and southern California often experience high seasonal mold spore levels. Generally, when the ground dries after a period of moisture, the winds can overturn the top layers of soil and disperse large quantities of mold spores. These can be carried aloft into urban areas, where they are drawn into air intakes and building ventilation systems. The photo at left shows the long growing branches of mycelium that are characteristic of Nocardia asteroides, from a sputum sample of an infected patient. Nocardia are bacteria called actinomycetes, which greatly resemble fungi in characteristics, and they also produce spores.
Even though the problem is more common in southern states, it only takes the right conditions for microscopic quantities of fungi to gain a foothold in a ventilation system. This situation has occurred across the US, regardless of climate. Many, if not most, cases of poor IAQ and SBS can be tied directly to the occurrence of mold spores either in the ventilation ducts, or in the walls of buildings. Sometimes, fungi are merely a contributing factor when the ventilation is inadequate -- normal levels of airborne fungi are not removed from the building air.
Dealing with Fungi in Ventilation Systems
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. Fungus can grow on HEPA filters as well as other ventilation components and, if unchecked, can actually contribute to the problem. The image at right shows a layer of actinomyces mycelium growing on a surface.
Fungus or fungal spores from the outdoors can be dealt with easily, as described above. If, however, 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 must be treated as necessary with proper disinfectants. Some systems provide built-in UVGI lights for continuous disinfection. 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. The ductwork must be inspected for fungal growth and cleaned with a disinfectant. The cause of the condensation must be identified. Often, it results from inadequate insulation, or leakage into, or out of, the ductwork. Sometimes return air can leak into the supply air duct and result in localized condensation. Sometimes the insulation itself can absorb and hold moisture, resulting in fungus growth that may then directly or indirectly produce contamination of the building air. Smoke tests, or airflow measurements, and/or pressure tests can determine duct leakage.

In the absence of water they may reduce to spore form, which makes them even more subject to air entrainment. Therefore, a cycle of condensation and dehydration may exacerbate a fungal dispersion problem. In this case, the problem might be perplexing to isolate -- sometimes the duct and components will appear dry, while cases of respiratory irritation or infection may occur in irregular cycles that could ultimately depend on humidity variations. Every situation can be unique and must be studied carefully.
Air Sampling and Testing
Sampling of airborne microorganisms can be inconclusive. There are no absolute standards, and decisions on whether a building has a fungus problem or not are often made arbitrarily. Methods of collection can give divergent results and are therefore heavily subject to interpretation. Swabbing a sample from a duct or an exhaust grille will yield some concentration of fungal or bacterial cells, but doesn't exactly correlate with airborne concentrations.
Measuring airborne concentrations can likewise produce results that depend on interpretation. Often, the testing agencies will not identify the specific microorganisms, but will merely state that colonies were formed, or that there is a potential contamination problem. Most fungi are unique and have distinctive characteristics. The photomicrograph at right shows a colony of actinomyces, in which the radiating rays of the mycelium are clearly visible around the central granule.
Any studies contracted to be performed, such as on schools or office buildings, should be required both to state the types of microorganisms discovered, their probable airborne concentrations, and how these compare with standards or typical concentrations in normal, or "healthy," buildings.

For further information please follow the following Link:
http://www.cancerfungus.com/pdf/fungi-nexus.pdf
http://www.naima.org/pages/resources/library/pdf/RP032.PDF
http://www.library.ca.gov/crb/01/notes/v8n1.pdf