Microbiological Contamination

Exposure to microorganisms, such as fungi, bacteria, viruses, and their biological by-products, may cause disease and allergic responses in building occupants. Human exposure to pathogenic microorganisms and their by-products in an indoor environment usually occurs by inhalation and contact with the mucous mem­branes. In order to achieve acceptable indoor air quality, airborne exposure to fungi and other pathogens should be minimized.

For airborne exposures to microorganisms to occur, several events must happen. First, there must be a reservoir (i.e., a loca­tion where an unusually high concentration of microorganisms is present). Second, the microorganisms must be allowed to repro­duce. Favorable conditions are needed for reproduction to occur. For example, fungal growth is usually optimized when moisture levels are high. Last, the microorganisms must be released into the air. For example, Legionella is released into the environment when cooling tower fans blow contaminated water mist into the air. Since all three steps are needed for exposure to occur, prevention of one or more of the steps from occurring will minimize airborne expo­sures to microorganisms.

Certain conditions contribute to microbial contamination in an indoor environment:

  • Location of fresh air intakes adjacent to outdoor microbial reservoirs
  • Excessive indoor relative humidity levels (greater than 60 percent)
  • Stagnant water in air-handling units or other HVAC components
  • Wet building materials such as carpet, gypsum board, insulation, or ceiling tiles
  • Wet furniture
  • Recent flooding within a building
  • Inadequate building vapor barriers that allow entry of moisture into the building
  • Voids in exterior insulation or cracks in buildings that allow cold outdoor air to enter the building and cool interior surfaces, which create condensation and promote microbial growth

Several controls are needed to minimize microbial contamination within a building. The HVAC system should be properly sized and installed. The system should be capable of handling the cool­ing and heating loads within the building. The building should be positively pressurized with respect to the outdoors to prevent the uncontrolled infiltration of moisture and airborne contaminants into the building. A preventive maintenance program should be implemented that provides regular inspections of HVAC compo­nents and prompt response to faulty equipment. All moisture leaks should be repaired immediately. Moisture should not be allowed to accumulate within wall cavities. The EPA recommends that water-damaged items should be discarded or dried within 24 to 48 hours to prevent mold growth.


Fungi include yeasts, molds, and mushrooms. Molds are ubiquitous organisms; their most common source is the outdoor environment. Building occupants are exposed to airborne molds outdoors and indoors. Indoor air quality complaints increase when mold growth proliferates in an indoor environment. Mold can cause discolor­ation and degradation of building materials, odor problems, and allergic reactions in building occupants. Some building occupants may be hypersensitive to certain species of fungi. The key to mini­mizing indoor exposure to airborne molds is to prevent the ampli­fication and dissemination of the organisms.

Some patients in hospitals are highly susceptible to developing infections from airborne microorganisms. One such infection is aspergillosis, which is caused by several species of the fungus Aspergillus. Aspergillus is a ubiquitous organism in the indoor and outdoor environments. Aspergillosis is a fungal infection of the tissue in the lungs and respiratory tract. Patients with suppressed immune systems (e.g., bone marrow transplant and organ trans­plant recipients) are at increased risk of developing aspergillosis. Aspergillosis can be fatal in immunocompromised patients. It may be associated with dust exposure from renovation and con­struction activities within hospitals; consequently, engineering and administrative controls should be in place to prevent patient dust exposure.


Another potentially pathogenic microbial agent is Legionella. Legionella pneumophila is capable of causing legionellosis. Legionellosis has two distinct forms: Legionnaires’ disease and Pontiac fever. Symptoms for Legionnaire’s disease range from a mild cough and low fever to pneumonia and death. Pontiac fever produces flu-like symptoms, including fever and muscle aches. Legionellosis is usually contracted by inhalation of aerosolized water contaminated with Legionella. The likelihood of contracting the disease depends on the concentration of the bacteria in the water source, the susceptibility of the exposed occupants, and the duration of exposure.

Legionella bacteria are commonly found in natural water sources. Under the right conditions, Legionella can proliferate in domestic water systems used for heating, cooling, and drinking. According to the OSHA Technical Manual, several conditions may be attributed to promoting the growth of Legionella in water systems:

  • Stagnation
  • Water temperatures between 68°F and 122°F
  • pH between 5.0 and 8.5
  • Presence of other microorganisms that supply nutrients or harbor Legionella
  • Presence of sediment that promotes the growth of other micro flora

Water sources that may provide optimal growth conditions for Legionella include cooling towers, evaporative condensers, humidi­fiers, decorative fountains, spas, whirlpools, and domestic hot water systems. All of these sources are capable of releasing aerosolized water into an occupied environment. To minimize the chance of disease, HVAC equipment should be cleaned periodically and maintained. Domestic water temperatures should be within accept­able temperature ranges. Water stagnation should be avoided. ASHRAE Guideline 12-2000: Minimizing the Risk of Legionellosis Associated with Building Water Systems provides detailed informa­tion regarding the treatment of contaminated systems, periodic cleaning procedures for equipment, and water sampling guidelines.

Proposed ASHRAE Standard 188P (4th Edition): Legionellosis: Risk Management for Building Water Systems, outlines the minimum legionellosis risk management requirements for the construction, commissioning, operation, maintenance, repair, replacement, and expansion of new and existing buildings and their associated water systems and components. The standard outlines the principles of a water management program utilizing various risk management principles. Hospital managers and directors must actively partici­pate in the creation and operation of a legionellosis water risk management plan. Elements of a water management plan include the following:

  • Program team: Identify persons responsible for the program development and implementation
  • Description of water system and flow diagrams
  • Analysis of building water systems
  • Control measures: Determine locations where control measures must be applied and maintained to stay within established limits
  • Monitoring: Establish procedures for monitoring whether control measures are operating within established limits and if not, take corrective actions
  • Corrective actions/documents
  • Documentation

Excerpt from: Mechanical Systems Handbook for Health Care Facilities
J. Robbin Barrick, PE, and Ronald G. Holdaway, PE
ASHE copyright 2014. Available at the ASHE Store.

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