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Understanding Types, Sources of Biological Contaminants Essential In Building, HVAC Investigations

Paul Warden

Indoor Air Quality (IAQ) is one of the most significant environmental issues facing today's building owners and operators. The ubiquitous nature of IAQ pollutants, the glare of media attention and societal changes, including the increased amount of time we spend indoors at home and at work, increases in the elderly and infirm populations, and the emphasis placed on weather-tight energy efficient construction all contribute to the complexity of indoor air quality issues.

Typical sources of chemical contaminants include adhesives, carpeting, treated wood products, office machines, cleaning agents, and vehicle exhaust. Biological contaminants include organisms such as fungi, bacteria, viruses, protozoa, and their by-products such as pollen, dust mite and cockroach feces, and most important, volatile organic compounds (VOCs). However, of equal importance is the bioaerosol, or microbial, aspects of indoor air quality.

Types of Bioaerosols

Bioaerosol can be classified into the following groups; animals, bacteria, fungi, plants, protozoans and viruses.

• Animals of all types can contribute to human allergic reactions and indoor air quality problems. Animal by-products, such as dust mite fecal material have been shown to promote allergic responses in some people.

• Bacteria particularly, Gram negative bacteria, have been associated to a variety of indoor air quality bioaerosol problems. Typical diseases include pneumonia, allergic reactions, chronic bronchitis, Legionnaire's disease, hypersensitivity pneumonitis, and humidifier fever.

• Fungi are the most common type of organism related to indoor air quality problems because of their ability to colonize, multiply and disseminate (via spores) throughout the indoor air environment. Health issues related to fungi include pathogenic and opportunistic pathogenic fungi, sensitivity to chemical by-products of fungi, and allergic reactions to the spores themselves.

• Pollen used by plants to reproduce causes considerable discomfort to many people. Typically contaminants of this sort are not major problems in facilities with adequate HVAC systems.

• Protozoans have also been implicated in indoor air quality problems. Although most of these organisms are too large to be aerosolized, they may contribute to the amplification of other microorganisms. For example, free living amoebae may harbor legionella bacteria, shielding the bacterial target from a dose of biocide.

• Viruses are not considered to be living organisms, as they lack the systems required for independent reproduction and consist of only genetic material (DNA or RNA) in a protein shell. However, most groups of viruses are capable of respiratory transmission and cause a variety of human diseases and therefore are a component of indoor air quality.

Sources Of Problems

Moisture can enter a building through a breach in the building envelope, such as, flooding, roof leaks, and unsealed basements. Moisture can also escape from internal transport systems, as in leaky drinking water fountains, and dripping pipes.

Many of these problems are relatively easy to detect, and the potential for fungal growth is minimal if appropriate cleanup and corrective action is promptly taken. Moisture problems that develop in the non-trafficked areas of a building, which may go undetected until a complaint situation develop are more insidious. Condensation is one means of moisture accumulation and can occur in exterior walls, particularly if vapor barriers are improperly installed.

Improperly designed, operated or maintained HVAC systems are probably the most common sources of interior moisture problems. For example, an undersized chiller coil, which cools the incoming air to reduce its moisture content, may result in moisture bypass. Or maintenance personnel may turn off chillers in misguided attempts to save energy and reduce operating costs. Another common problem is a condensate drip pan which fails to drain properly creating an environment for microbial growth. Regardless of the source of the moisture, if the resulting relative humidity (RH) exceeds 65 percent, a fungal problem may result. EPA recommends a 45-50 percent RH and ASHRAE recommends 40-60 percent RH.

Poor ventilation, improper HVAC system design or operation can contribute significantly to indoor air quality problems. Elevated carbon dioxide (CO₂) levels, odors, complaints of "stale air," may be early warning signs of signs of inadequate air exchange, insufficient outdoor air. The occupants of a building also contribute to microbial indoor air quality problems. For example, human associated bacteria, fungal spores, and viruses, are all brought into a building by the staff on each work shift. With inadequate air exchange or filtration, and favorable growing conditions, these microbes may grow inside a building and result in an occupant complaint.

To counteract these problems, building owners and operators should educate themselves about the causes and warning signs of bioaerosol indoor air quality problems and establish preventative maintenance programs aimed at minimizing these problems. However, if a complaint situation arises, the following steps should be considered: site investigation, sample collection and analysis, data interpretation, remediation and monitoring.

Site Investigation

The goal of a site investigation is to acquire an overview of the condition of the building and the function of the HVAC system relative to the occupant's activities and complaints. To achieve this goal, information about the symptoms, the timing of their development, and recovery time, should be elicited from the occupants by means of interviews or questionnaires.

To relate the occupant information to bioaerosols, the investigator should proceed to examine the building beginning with the outdoor air intakes to see if they are properly located - e.g., their proximity to standing water, vehicles, building exhaust vents - and fully operational. The HVAC system should be investigated next. The heaters, chillers, humidifiers, dehumidifiers, filters, fans and mixing chambers should all be operational, dry and clean. What is the maintenance schedule? How often are the air filters replaced? How well do the replacement filters fit?

An examination of the ductwork should reveal whether the circulated air is dry and free of debris. Supply air diffusers should be examined in both complaint and non-complaint areas to see if they are functional and clean. Is moisture present indicating that the dew point of the supply air is lower than that of the room air? Does the number of supply and return air vents reflect the usage pattern of the building? The investigator should then examine the complaint area specifically for signs of microbial growth. Adjacent areas, particularly those above and upstream of the complaint area, should also be examined.

How does the information gained from the site inspection relate to the types of complaints registered by the occupants? How do the complaints relate to diurnal patterns of building use? A thorough inspection and evaluation of the gained information in light of the complaints which have been registered will help the investigator to collect meaningful, cost-effective samples.

Sample Collection

Unlike the technology that exists for some chemicals, no direct reading instruments for microbials exist. However, there are several types of bioaerosol sampling techniques, including gravitational, impactors, centrifugation, and filtering and precipitation all of which are designed to recover microorganisms from indoor air.

Impactors are the most common bioaerosol sampling units and include several different technologies. Examples include cascade impactors (such as the 6 Stage-Andersen sampler which collects progressively finer particles on each petri dish), slit samplers (for example the Burkard Spore Trap which has a rectangular opening above a greased glass slide), sieve samplers (e.g., the single stage Andersen N6, Spiral System, and BioTest, units which achieve particle collection via many small holes in the sieve cover over media) and Impingers (Ace Glass Inc., which employ impaction into a liquid rather than onto a solid media or glass slide).

All impactors have some vacuum device to pull air through the sampler at a known rate. Particle inertia causes impaction onto the collection surface as the airstream is deflected sideways inside the sampling unit. The flow rate of air through the sampling unit is critical, and the manufacturer's instructions should be followed exactly. Because inertia is the mechanism for particulate deposition, the flow rate should be measured during sample collection, or the pump calibrated before and after sampling. This is an important part of proper sampling protocol, which is often overlooked.

Recommended sampling durations vary with types of units; however, it is important to start with the manufacturer's recommendations and make adjustments it according to the expected microbial load to avoid missing microbes present in low concentrations or to avoid overloading the plate. A prudent practice is to bracket the exposures times as in photography; and collect a one-minute and a three-minute sample, rather than, for instance, just to avoid a single two-minute sample. Recording the sampling duration allows the reporting of data quantitatively, typically in colony forming units per cubic meter (CFU/m³).

In many situations, determining the extent of the microbial contamination present on surfaces is also important. These organisms may become aerosolized during the workday as a result of air currents. Surface sampling methods include Rodac or contact plates and swab culturettes. Surface sampling data need to be interpreted with caution, however. A single fungi conidial head may contain 3x10⁴ spores. If swabs are used for sample collection and then subsequently streaked on a petri dish for incubation, those spores may be spread all over the plate and result in multiple colony forming units. In addition, there is no way to assure 100 percent efficiency of the swab sampling procedure or the extraction of the swab at the laboratory. For these reasons, swabs should not be used to generate quantitative data. Contact plates do generate quantitative data, although some caution regarding the efficiency of the sampling procedure may be warranted, particularly if the surface is porous or fibrous.

Bulk samples (for example, insulation, wallboard, carpet padding) may also be collected. These samples are critical to assess potential microbial reservoirs. For example, interior fiberglass insulation in ductwork may be contaminated with fungal spores which serve as a continual source of downstream contamination. This often occurs when an air filter fits poorly, is installed incorrectly, or is otherwise nonfunctional and particularly when these conditions are combined with an improperly installed or non-draining condensate drip pan which contains standing water.

Factors which need to be considered when developing a sampling plan include diurnal patterns of building use (the most appropriate time of day for sampling and the number repeat sampling periods during the day), the distribution of complaint and non-complaint areas relative to the HVAC system layout, whether passive or aggressive strategies are more appropriate for the site, the appropriate location for background sample collection, and the types of sampling equipment and media.

Samples should be sent to the laboratory by overnight delivery, with clear instructions regarding desired analyses. The laboratory should be able to assist you in determining whether to request fungi enumeration and identification, or just enumeration.

Laboratory Analysis

After samples are collected, a variety of analytical methods can be employed, including culturing, direct microscopy, bioassay, biochemical assay and immunological assay. The most commonly used method for viable microbes is the cultural assay. In this procedure, samples are collected onto the most appropriate media and then incubated to optimize growth.

Fungal identification is performed by visual examination and microscopic examination of reproductive structures. Bacterial identification is performed by using one of several semi-automated identification systems. One of the most robust systems for environmental bacteria is the MIDI Sherlock system, which uses gas chromatography to develop a cellular fatty acid (CFA) profile of the isolate, and statistical pattern recognition software to compare the profile to those in the databases. The need to identify all isolates recovered during sampling is something you should discuss with your laboratory; in many situations, this is unnecessary.

Data Interpretation

No regulatory guidelines exist regarding permissible levels of bioaerosols in indoor air. Thus data interpretation depends heavily upon the expertise of the investigator. Research indicates that, in the absence of known pathogens and under a given threshold, it may be the relative levels of microbial contamination, rather than the absolute concentrations, which are most important. Thus, collection of a background sample to serve as a reference level is critical to data interpretation.

By comparing the microbial data collected at complaint sites versus non-complaint or background sites, it may be possible to determine whether amplification of microorganisms is occurring within the building. For example, is the concentration of fungi or bacteria substantially higher indoors that outdoors? Are the counts from samples taken late in the day higher than those taken earlier? Are specific types of fungi present in higher concentrations indoors than out? Bacteria? If the total bacterial count is elevated indoors, are the types of bacteria human associated? How do the data change between morning, afternoon and evening sampling times? Are there known pathogens present? By evaluating these and other related questions, the investigator should be able to prepare a report which summarizes the investigation protocol, the types of samples collected at different sites, the concentrations and types of microorganisms present, and data interpretation section.

Remediation

A thorough discussion of remediation techniques is beyond the scope of this article, but typical procedures include cleaning the affected areas, decontaminating or sanitizing the HVAC system, removing contaminated materials, replacing damaged structures, and improving the indoor environment.

Building owners should look for a qualified contractor who can provide references from similar projects. Issues to be addressed prior to remediation include the technique to be employed, project containment to prevent exposure of the building occupants to dust, bioaerosols, and particulates, the safety of the remediation workers themselves, the work schedule, and final inspection procedures. Due to the inherent difficulty in thoroughly inspecting most HVAC systems, it is recommended that a routine monitoring program be implemented for at least a brief period to help verify the effectiveness of the remediation procedures.

As with most issues, in indoor air quality an ounce of prevention is worth a pound of cure. Remediation is not only time consuming and expensive; indirect costs of decreased employee productivity and lost worker confidence may be even more costly to the business.

At the time of publication, Paul Warden was the manager of client services and technical sales at Analytical Services, Inc., a microbiology laboratory firm located in Williston, VT.

Published in the January 1997 issue of Indoor Environment Review. Reprinted by permission of Indoor Environment Review, a division of IAQ Publications.