R. Dixon Associates - Management Resources for Health, Safety and Environment

The microbial contamination of indoor air can become a serious problem if not monitored and properly controlled. The environmental factors of high humidity, reduced ventilation, tightly sealed buildings, construction and renovation, all allow for the growth and distribution of microbes from both environmental and human sources.

A wide variety of pathogenic (disease-causing) micro-organisms such as fungi (moulds and yeasts,) bacteria, viruses, and protozoa can be found in the indoor environment. Some exist as viable (living) organisms; others are non-viable, including dead spores and sub-micron organic particulates carrying toxins. Contagious micro-organisms are brought into the environment and spread by infected persons - for example tuberculosis, and influenza. In poorly ventilated areas, the influenza virus can spread rapidly. However, viruses do not survive long outside the host, and transmission usually requires close contact with an infected person.

Ventilation (HVAC) systems with water or condensation equipment (humidifiers, cooling coils, etc,) can present special problems. Building related illnesses can occur following exposure to microbial antigens aerosolized from contaminated HVAC systems, or from other building components that may have been damaged by moisture. Condensation and water accumulation allow the growth of many fungal species that can induce allergies and other health problems not readily detected by current diagnostic procedures. Endotoxin-producing bacteria can occur in some types of humidification systems used in modern buildings, the genus Legionella being of particular concern.

In larger buildings, the HVAC system can transport micro-organisms some distance from the source to the occupants. For example, airborne particulate contaminated with bird or bat droppings may be taken into a building via the ventilating system. Older buildings and vacant properties having bird or bat infestations should be treated with caution.

Various respiratory illnesses have been reported from building-related fungal exposure. Although active fungal growth requires water, the release of fungal spores into air can take place for months after the water has disappeared.

Exposure to some fungi can induce allergic or asthmatic reactions, while other species can cause primary infectious diseases. Some fungi produce mycotoxins that accumulate on airborne propagules (fungal fragments.) The inhalation of spores containing mycotoxins has been shown to cause many of the symptoms typically associated with the "sick building syndrome;" for example the spores of Aspergillus fumigatus and Histoplasma capsulatum can cause hypersensitivity pneumonitis. Affected individuals often experience relief when they leave the building for several days.

Other products of fungal metabolism include volatile organic compounds (VOCs.) These substances (characterized by mouldy smells) arise only when there is active fungal growth. Allergenic fungal spores, especially Cladosporium and Alternaria, are common in outdoor air during the growing season.

In health care institutions, it is important to remember that some individuals (AIDS patients and immuno-compromised individuals, such as those on chemotherapy) are particularly susceptible to microbial exposures.

A walk-through survey can identify microbial reservoirs and amplification sites. Properly planned air sampling should be carried out to identify and control airborne microbes. However, air sampling alone is not an infallible means of determining microbial contamination. The bulk sampling of suspected contaminated materials is equally important. The identification of species is necessary for proper hazard determination. Remedial action should be carried out in problem areas as soon as possible. The services of an experienced occupational hygienist or mycologist should be used, along with a qualified analytical laboratory.

In bioaerosol sampling, quantities are normally measured in terms of colony-forming units per cubic metre of air (CFU/m³ ), measured by collecting organisms and allowing them to grow on a suitable agar medium. There can be difficulties in collecting viable organisms with different shapes, sizes, and masses. Some fungi produce fewer propagules than others for a given amount of fungal biomass. The spores of some species rapidly become non-viable (i.e. they die,) but can still release toxins.

Suitable sampling instruments include single or multi-stage impactors (Andersen,) impingers and centrifugal samplers. Samples should be taken while the HVAC system is operating normally. Early Monday morning may be a good time if the HVAC system has been turned off for the weekend. Samples should be taken at several locations throughout the area, including near air outlets, at desk level and the surrounding area. Potential source locations should be sampled, including the air supply plenum down-stream of the humidifier, and at the outside air intake. Surface samples should be taken with sterile swabs on the surfaces of diffusers, fan blades, coils, pans, and humidifiers. The samples are then plated on a suitable agar medium containing antibiotics to suppress bacteria.

It should be noted that dust mites are important sources of indoor allergens worldwide. In North America, the dust mites most commonly found are Dermatophagoides farinae (DF), D. pteronyssinus (DP), and Euroglyphus maynei (EM). It is not always clear what factors contribute to the development of large mite populations indoors. Methods of assessing indoor airborne concentrations of dust mites include the detection of the substance guanine and specific allergens carried in the dust

In Summary - Dealing with the Problem

Findings of large quantities of certain pathogenic fungi (e.g., Aspergillus fumigatus, Histoplasma and Cryptococcus,) are cause for concern. Bird or bat droppings near air intakes or in ducts, should be assumed to contain these pathogens. Action should be taken accordingly.

The persistent presence of significant numbers of toxigenic fungi, e.g., Stachybotrys chartarum (atra,) Aspergillus, Penicillium and Fusarium species indicates that further investigation and action is required.

The presence of one or more fungal species occurring as a significant percentage of indoor air samples and not present in simultaneous outdoor samples, is evidence of a fungal amplifier. Appropriate action should be taken.

The "normal" air mycoflora is qualitatively similar and quantitatively lower than that of outdoor air. In Canadian federal government buildings, the 3-year average has been approximately 40 CFU/m³ for Cladosporium, Alternaria, and non-sporulating basidiomycetes.

More than 50 CFU/m³ of a single species (other than Cladosporium or Alternaria) may be reason for concern. Further investigation is necessary.

Up to 150 CFU/m³ is acceptable if there is a mixture of species reflective of the concentration in outdoor air. Higher counts suggest dirty or low efficiency air filters or other problems.

Up to 500 CFU/m³ is acceptable in summer if the species present are primarily Cladosporium or other tree and leaf fungi. Values higher than this may indicate failure of the filters or contamination within the building.

The visible presence of fungi in humidifiers and on ducts, mouldy ceiling tiles and other surfaces requires investigation and remedial action, regardless of the airborne spore load.