At its least hazardous level, microbiological activity may result in unpleasant odors. Controlling odors from any source is not an easy job. Odor thresholds vary over six orders of magnitude, from parts per billion to parts per thousand. Individual susceptibilities and responses also vary widely.  Sources of indoor microbiological odors include contaminants in the outdoor air and the growth or decay of microbes indoors. Within the ventilation system, microbial-generated odors can spread from several locations, including HVAC filters contaminated with moisture and organic dust, window-unit ventilators contaminated with mold, and contaminated cooling coils or drain pans. On the positive side, a properly operating HVAC system can be used to provide fresh clean air to dilute odors to below their odor thresholds. Five conditions must exist in order to attribute complaints to viable microorganisms like mold, mold spores, or bacteria. When these are absent, the primary cause of IAQ complaints is not likely to be viable microorganisms:

1. There must be a source of organic nourishment, such as pollen, insect parts, or shed-skin particles, collected on the surface of an improperly filtered cooling coil or in the drain pan.
   
2. There must be a reservoir or suitable environment, such as water condensed on a cooling coil in the HVAC system or stagnant water in the drain pan under a cooling coil.
   
3. There must be amplification, or the growth of the microorganism—for example, mold growing on a wet, dirty cooling coil or in a clogged drain pan.
   
4. There must be dissemination, such as aerosolization of water containing the microorganisms when contaminated water is stripped off the dirty cooling coil by the air passing through the coil.
   
5. There must be a pathway for microorganisms to reach people—for example, the duct-work delivering contaminated air to occupied spaces.

Because microorganisms like mold spores are always present in the air, it is the excess quantity of microbes that should be of concern. Even indoor concentrations as much as five to 100 times that of the outdoor concentration do not automatically produce occupant complaints. Check the literature for trigger concentrations for specific types of bioaerosols and typical human reactions. Common ventilation system sources of microbiological contamination include contaminated cooling coils, humidifiers, and air washers; high velocity air passing through contaminated wet cooling coils; the stripping of water droplets off the coil and into supply air; the absence of adequate preventive maintenance; improperly maintained or inaccessible air-handling equipment; wet porous fiber insulation inside air-handling and fan-coil ventilation equipment; wet contaminated acoustic insulation in supply ductwork; stagnant water in drain pans; excessive humidity (typically greater than 70 percent in the air); recirculation and buildup of human-shed skin particles; air transport of viruses and bacteria of human-origin (from coughing and sneezing, for example); use of cool-water room humidifiers; water runoff from windows into unit ventilators; locating outdoor-air intakes near external bioaerosol sources (like cooling towers, for example); and the building under negative pressure, which allows warm humid air to seep into and condense on cooler surfaces inside the building walls. The following suggested measures can reduce microbial problems in ventilation systems:

Occupants themselves can have a positive impact on the control of indoor moisture. It is recommended that occupants take such actions as:

• Be sure that air-supply and return registers are open and that air is flowing at all times.
• Use exhaust fans when using bathrooms, showers, and tubs.
• Avoid the use of personal room humidifiers unless management is involved.
• Avoid spilling water at sinks and coffee areas and clean up any spills.
• Keep work spaces clean, dry, and well-vacuumed. 
• Bring a portable air purifying HEPA filter unit from home for use if you suffer from allergies or asthma.
• Report any moisture—for example, water condensed on perimeter windows or basement walls and water spots on ceiling tiles, which could suggest a leaking pipe in the air return plenum above the ceiling.
• Keep an eye on potted plants; mold sometimes grows in wet potting soil.
• Use the “scratch and sniff” test for any location suspected of being contaminated. (For example, occupants can scratch a wall or carpet surface and then quickly smell the results; mold smells or other odors should be reported to management.)

As you can see, moisture is a common thread running through the fabric of bioaerosol contamination of indoor air. In many cases, a cooperative effort among occupants, building owners, managers, and vent system operators can help keep moisture problems and IAQ complaints minimized.  You can read more about these issues and how to deal with them in my , published by and available from AIHA.