Ventilation and Managing Indoor-Air Bioaerosol Hazards
Allergies and Asthma Are Disease Syndromes Often Associated with Indoor Bioaerosol Exposures

Studies by NIOSH, EPA, and others have confirmed that indoor air quality problems in employee occupancies (like offices, commercial buildings, and schools) regularly involve the presence in air of bioaerosols—also known as microbes, microbiological agents, microorganisms, biogenic particles, airborne organisms, organic dusts, and viable pathogenic aerosols, all of which are particles of biological origins.

Typical bioaerosols found in indoor air include fungal and mold spores; bacteria and viruses suspended in tiny water particles; cat, dog, and rodent dander; insect parts; tree, grass, and weed pollens; human shed skin particles; and other organic dusts. Airborne bioaerosols usually range in size from 0.1 to 100 micrometers in diameter.

Symptoms of microbiological activity indoors often include allergic reactions like sinus trouble, runny nose, eye irritation, and other irritations of the upper respiratory tract. The primary cause of allergies is immunological sensitization to aerosolized mold spores, insect parts, animal dander, and pollen. Asthma, a more serious respiratory disease characterized by shortness of breath and wheezing, can also be triggered by indoor bioaerosols. According to the Asthma and Allergy Foundation of America, bioaerosols cause allergies for up to ten percent of the population and asthma for about 3-4 percent. Ventilation systems are always involved, both as a source of problems and their solutions, so the occupational health and safety professional must be aware of how the ventilation system operates in order to provide bioaerosol control.
is an IH engineer with broad experience in ventilation used for emission and exposure control. He is the author of many books and training courses and is current chair of the ANSI Z9.2 and Z9.10 subcommittees. His full biography can be found
. He can be reached via

Indoor Environmental Quality/Indoor Air Quality
American Biological Safety Association
. Asthma and Allergies Foundation of America:
Control Indoor Allergens to Improve Air Quality
(September 2015). Asthma and Allergies Foundation of America:
Mold Allergy
(October 2015). Asthma and Allergies Foundation of America:
Pet Allergy: Are You Allergic to Dogs or Cats?
(October 2015). Asthma and Allergies Foundation of America:
Pollen Allergy
(October 2015). EPA:
An Office Building Occupants Guide to Indoor Air Quality
Indoor Environmental Quality
Indoor Air Quality
Symptoms of microbiological activity indoors often include allergic reactions like sinus trouble, runny nose, eye irritation, and other irritations of the upper respiratory tract.

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:
  • Prevent moisture collection in HVAC components.
  • Remove stagnant water and slime from mechanical equipment.
  • Use clean steam for central humidifying systems.
  • Avoid use of water sprays in HVAC systems.
  • Maintain a relative humidity of less than 70 percent in the building and HVAC equipment.
  • Do not allow water to condense in any part of the air handling system—for example, in ductwork.
  • Use filters with a collection efficiency rating of MERV 12 or higher.
  • Keep filters dry and clean.
  • Provide appropriate flowrates of fresh, clean outside air to dilute indoor air contaminants and odors.
  • Remove personal room humidifiers or keep them under surveillance.
  • Provide frequent inspections and preventive maintenance for all components of the HVAC system.
  • Provide pigeon and bug screens on air intakes and exhausts.
  • Prevent buildup of moisture in occupied spaces.
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
IAQ and HVAC Workbook
, published by and available from AIHA.