Indoor Air Quality in Office Buildings: A Technical Guide

5.2.8 Microbials

In indoor air, microbial contamination can be a serious problem. High humidity, reduced ventilation, tighter buildings, and HVAC systems that have water or condensation (humidifiers, cooling coils, etc.) allow for the growth and distribution of various microbials. This is a matter of concern because of the various associated human health and comfort implications.

A wide variety of microbials (micro-organisms) such as fungi (moulds, yeasts), bacteria, viruses, and amoebae can be found in the indoor environment. Contamination of indoor air with micro-organisms can occur under many circumstances. Such contamination most often occurs when a fault in the building, HVAC, or other system allows the proliferation of micro-organisms.

Viruses and bacteria cause diseases, but indoor air is not usually the cause of the viral infection (e.g., common cold). Viruses do not survive long outside the host, and transmission depends on contact with an infected individual. Bacteria such as Legionella and related species can, however, be a significant IAQ concern. Legionnaire's disease is an infection that can result in pneumonia if it is disseminated from an amplification site to the breathing zone of a susceptible host. Cooling towers, evaporative condensers, and hot water systems can be amplification sites for Legionella and can disseminate aerosols containing the bacteria into the indoor air. Endotoxin-producing bacteria can also occur in some kinds of humidification systems.

Inhalation of very large concentrations of fungal spores can cause hypersensitivity pneumonitis, but this rarely results from building exposure. Chronic exposure to most fungi can induce allergic or asthmatic reactions in humans, and a very few species can cause diseases directly. Some moulds are "toxigenic," producing mycotoxins that often accumulate in the spores. The inhalation of spores containing certain mycotoxins has been shown to induce many of the symptoms normally associated with SBS.

Other products of fungi include certain VOCs. Such compounds (characterized by mouldy smells) occur only when there is active and considerable fungal growth. There is some evidence to suggest that these can contribute to SBS.

It is important to remember that some individuals (AIDS patients and immuno-compromised individuals, such as those on chemotherapy) are very susceptible to certain microbial exposures.

5.2.8.1 Background

Microbial evaluation of indoor air started in the late 1950s, when secondary (nosocomial) infections of patients became a major concern in many hospitals. One cause was airborne micro-organisms spread via ventilation systems.

In Europe and North America, there have been reports of a number of building-associated outbreaks of influenza-like illness ("humidifier fever"), in which affected individuals manifest symptoms such as malaise, fever, shortness of breath, cough, and muscle aches. These illnesses

usually occur as an acute response to microbial antigens aerosolized from contaminated HVAC system components or from other building components that may have been damaged by recurrent floods or moisture problems. Various respiratory illnesses have been reported from building-related fungal exposure. Affected individuals often experience relief when they leave the building for several days.

Fungal spores, especially Cladosporium and Alternaria, are common in outdoor air during the growing season, and the principal fungi that grow on leaves constitute 60-70% of the spores in air. These fungi can induce allergies, but most people are not particularly affected.

Species of fungi that have the physiological ability to grow and accumulate indoors or in air handling equipment are quite different from the common plant and leaf fungi. Condensation and water accumulation allow the growth of many fungi that can induce allergies and other health problems not readily detected with current medical procedures.

The presence of micro-organisms in the indoor environment in sufficient numbers or kinds to cause health or comfort problems is dependent on a number of factors. Fungal spores are ubiquitous because they reside in the soil. HVAC systems are complex and offer a number of environments where microbial populations may flourish. Water spray humidifiers containing stagnant water, filters packed with organic dusts, cooling system condensate pans, and interiors characterized by excessive humidity may all offer suitable environments for microbial proliferation under appropriate circumstances. In large buildings, the HVAC system will serve to transport micro-organisms from the locus of contamination to the vicinity of the occupants.

Aspergillus fumigatus, Histoplasma capsulatum, and some other fungi can cause certain diseases. Although this is rare in urban environments, buildings exposed to quantities of bird or bat droppings are at risk. Roosts in or near air intakes need to be eliminated. Older buildings and vacant properties having bird or bat infestations should be retrofitted or demolished with caution.

5.2.8.2 Checklist

In a walkthrough, possible microbial reservoirs and amplification sites should be determined:

• air intakes, filter units, cooling/heating fans and coils, spray humidifiers, reservoirs, ducts, insulation, induction and fan coil units, drain and condensate pans and sumps that are either dirty or wet
• mouldy, damp odours, evidence of a previous flood or water leak
• portable humidifiers and water coolers containing slime or algae
• mouldy, dirty or wet ceiling tiles, plaster/gyprock, carpet, window sills/frames.

Remedial action should be carried out in problem areas as soon as possible.

5.2.8.3 Measurement Methods and Equipment

The variety of microbial organisms in the air is enormous; some exist as viable particulates, others are non-viable and include dead spores, toxins, and submi-cron particulates. Although active fungal growth requires water, release of fungal spores into air can take place for months after the water has disappeared. Air sampling is undertaken in order to recognize and control microbial air contamination and as a means of quantitative and qualitative monitoring. Air sampling is not an infallible means of reliably determining microbial contamination, and caution must be used in interpreting the results.

Identification of the fungal species is critical for a complete determination of whether an abnormal or hazardous condition exists. This process requires a mycologist with expertise in IAQ. Excessive numbers of fungal propagules or modest numbers of certain disease-causing or toxigenic fungi can result in health or comfort problems. When fungi are growing in or on building surfaces or systems, removal is necessary.

Quantities of fungi have been assessed traditionally by the measure "colony-forming units" per cubic metre (CFU/m³) of air, measured by collecting spores and allowing them to grow on some type of agar medium. This method can be described as semi-quantitative, as there are difficulties in collecting spores that have different shapes, sizes, and masses. In addition, all media are selective to some extent. Further, some fungi produce fewer propagules than others for a given amount of fungal biomass/activity. The spores of some species rapidly become non-viable but can still pose a problem. The number of propagules in indoor air is highly variable and is a function of many factors, such as the activity in the room, the operation of the HVAC system, weather conditions, wind speed, and the microbial life-cycle. Research is in progress on these issues.

Sampling instruments include single or multi-stage impactor and centrifugal samplers from various manufacturers. Spore concentrations in buildings have been shown to vary over an order of magnitude in less than 1 minute. Similarly, recoveries of fungal species have been shown to be directly correlated with sampling time.

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 in the adjacent area. In addition, potential source locations in the mechanical equipment room should be sampled, including the air supply plenum after the humidifier and at the outside air intake.

Surface samples should be taken with sterile swabs (e.g., autoclaved, moistened cotton wool swabs individually packed in tubes) on the surfaces of diffusers, fan blades, coils, pans, and humidifiers. The samples are plated on a suitable agar medium containing antibiotics. This method is used to determine possible sources of contamination.

5.2.8.4 Interpretation of Results

Since 1989, the ACGIH Bioaerosols Committee has recommended rank order assessment as a means of interpreting air sampling data. This interpretation has been part of the practice in Government of Canada investigations since 1986. The presence of one or more species of fungi indoors, but not outdoors, suggests the presence of an amplifier in the building. Species identification is critical to the analysis. Because of the problems noted above, numerical guidelines cannot be used as the primary determinant of whether there is a problem. However, numerical data are useful under defined circumstances.

Information from a large data set obtained by experienced individuals using the same instrument has practical value. Investigations of more than 110 federal government buildings over several years has resulted in the creation of such a data set. Fungal data from about 3000 samples taken between 1986 and 1995 with a Reuter centrifugal sampler with a 4-minute sampling time have been used to prepare the following interpretation notes. Data acquired with other samplers require similar analysis. However, if a 4-minute sampling time is used, the numerical data from any proprietary sampler will probably be comparable.

• Significant numbers of certain pathogenic fungi should not be present in indoor air (e.g., Aspergillus fumigatus, Histoplasma and Cryptococcus). Bird or bat droppings near air intakes, in ducts or buildings should be assumed to contain these pathogens. Action should be taken accordingly. Some of these species cannot be measured by air sampling techniques.
• The persistent presence of significant numbers of toxigenic fungi (e.g., Stachybotrys atra, toxigenic Aspergillus, Penicillium and Fusarium species) indicates that further investigation and action should be taken accordingly.
• The confirmed presence of one or more fungal species occurring as a significant percentage of a sample in indoor air samples and not similarly present in concurrent 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 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 present. Further investigation is necessary.
• Up to 150 CFU/m³ is acceptable if there is a mixture of species reflective of the outdoor air spores. 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 in 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.
• There are certain kinds of fungal contamination not readily detectable by the methods discussed in this report. If unexplained SBS symptoms persist, consideration should be given to collecting dust samples with a vacuum cleaner and having them analysed for fungal species.

5.2.8.5 Strategy for Remediation

The principal guideline for microbial control is to keep fungal growth in buildings to a minimum. This can be accomplished in a number of ways:

• Remove water sources that encourage fungal growth. Prevent the accumulation of stagnant water in and around HVAC system mechanical components, such as under the cooling coils of AHUs. Maintain the relative humidity of indoor spaces at less than 60%. Repair all external and internal leaks promptly and permanently.
• Remove fungus-contaminated substrates. Remove and discard porous organic materials that are obviously contaminated (e.g., mouldy ceiling tiles, mildewed carpets). Wash all smooth surfaces that have been contaminated by fungi with diluted 5% bleach (250 mL/4 L water).
• In HVAC systems, use steam for humidification rather than recirculated water. Spray humidifiers where feasible. If spray systems are used, a rigorous preventive maintenance program must be employed, as these systems can easily become contaminated with bacteria and fungi. This includes maintenance of slime-free surfaces and the addition of potable water to the reservoir. Humidifiers should be drained and cleaned with chlorine bleach at intervals of 2-4 months. Rust and scale deposits should be removed from HVAC system components once or twice a year. HVAC systems should be turned off during cleaning operations, which should be scheduled during weekends and unoccupied periods.
• Porous synthetic insulation is often used to line ducts and air handling and induction units. The vapour barrier on fibreglass should be intact. There should be no standing water or condensation on these surfaces. Dirty, contaminated insulation should be removed, as the effectiveness of cleaning or encapsulation has not yet been verified.
• Personal portable humidifiers should not be allowed in offices, as they are seldom maintained properly and can easily become contaminated.
• The use of efficient filters to control the load of spores entering the air handling system is important. Use prefilters and extended surface-type secondary filters with dust-spot efficiency ratings higher than 85% when possible. Replace the filters at regular intervals. The prefilters are normally changed 4-6 times a year and secondary bag filters once a year, depending on outside conditions and retrofit activity.