Characteristics and Origins of Biological Indoor Air Contamination

Although there is a diverse range of particles of biological origin (bioparticles) in indoor air, in most indoor work environments micro-organisms (microbes) are of the greatest significance for health. As well as micro-organisms, which include viruses, bacteria, fungi and protozoa, indoor air can also contain pollen grains, animal dander and fragments of insects and mites and their excretory products (Wanner et al. 1993). In addition to bioaerosols of these particles, there may also be volatile organic compounds which emanate from living organisms such as indoor plants and micro-organisms.

Pollen

Pollen grains contain substances (allergens) which may cause in susceptible, or atopic, individuals allergic responses usually manifested as “hay fever”, or rhinitis. Such allergy is associated primarily with the outdoor environment; in indoor air, pollen concentrations are usually considerably lower than in outdoor air. The difference in pollen concentration between outdoor and indoor air is greatest for buildings where heating, ventilation and air-conditioning (HVAC) systems have efficient filtration at the intake of external air. Window air-conditioning units also give lower indoor pollen levels than those found in naturally ventilated buildings. The air of some indoor work environments may be expected to have high pollen counts, for example, in premises where large numbers of flowering plants are present for aesthetic reasons, or in commercial glasshouses.

Dander

Dander consists of fine skin and hair/feather particles (and associated dried saliva and urine) and is a source of potent allergens which can cause bouts of rhinitis or asthma in susceptible individuals. The main sources of dander in indoor environments are usually cats and dogs, but rats and mice (whether as pets, experimental animals or vermin), hamsters, gerbils (a species of desert-rat), guinea pigs and cage-birds may be additional sources. Dander from these and from farm and recreational animals (e.g., horses) can be brought in on clothes, but in work environments the greatest exposure to dander is likely to be in animal-rearing facilities and laboratories or in vermin-infested buildings.

Insects

These organisms and their excretory products may also cause respiratory and other allergies, but do not appear to contribute significantly to the airborne bioburden in most situations. Particles from cockroaches (especially Blatella germanica and Periplaneta americana) may be significant in unsanitary, hot and humid work environments. Exposures to particles from cockroaches and other insects, including locusts, weevils, flour beetles and fruit flies, can be the cause of ill health among employees in rearing facilities and laboratories.

Mites

These arachnids are associated particularly with dust, but fragments of these microscopic relatives of spiders and their excretory products (faeces) may be present in indoor air. The house dust mite, Dermatophagoides pteronyssinus, is the most important species. With its close relatives, it is a major cause of respiratory allergy. It is associated primarily with homes, being particularly abundant in bedding but also present in upholstered furniture. There is limited evidence indicating that such furniture may provide a niche in offices. Storage mites associated with stored foods and animal feedstuffs, for example, Acarus, Glyciphagus and Tyrophagus, may also contribute allergenic fragments to indoor air. Although they are most likely to affect farmers and workers handling bulk food commodities, like D. pteronyssinus, storage mites can exist in dust in buildings, particularly under warm humid conditions.

Viruses

Viruses are very important micro-organisms in terms of the total amount of ill health they cause, but they cannot lead an independent existence outside living cells and tissues. Although there is evidence indicating that some are spread in recirculating air of HVAC systems, the principal means of transmission is by person-to-person contact. Inhalation at short range of aerosols generated by coughing or sneezing, for example, common cold and influenza viruses, is also important. Rates of infection are therefore likely to be higher in crowded premises. There are no obvious changes in building design or management which can alter this state of affairs.

Bacteria

These micro-organisms are divided into two major categories according to their Gram’s stain reaction. The most common Gram-positive types originate from the mouth, nose, nasopharynx and skin, namely, Staphylococcus epidermidis, S. aureus and species of Aerococcus, Micrococcus and Streptococcus. Gram-negative bacteria are generally not abundant, but occasionally Actinetobacter, Aeromonas, Flavobacterium and especially Pseudomonas species may be prominent. The cause of Legionnaire’s disease, Legionella pneumophila, may be present in hot water supplies and air-conditioning humidifiers, as well as in respiratory therapy equipment, jacuzzis, spas and shower stalls. It is spread from such installations in aqueous aerosols, but also may enter buildings in air from nearby cooling towers. The survival time for L. pneumophila in indoor air appears to be no greater than 15 minutes.

In addition to the unicellular bacteria mentioned above, there are also filamentous types which produce aerially dispersed spores, that is, the Actinomycetes. They appear to be associated with damp structural materials, and may give off a characteristic earthy odour. Two of these bacteria that are able to grow at 60°C, Faenia rectivirgula (formerly Micropolyspora faeni) and Thermoactinomyces vulgaris, may be found in humidifiers and other HVAC equipment.

Fungi

Fungi comprise two groups: first, the microscopic yeasts and moulds known as microfungi, and, second, plaster and wood-rotting fungi, which are referred to as macrofungi as they produce macroscopic sporing bodies visible to the naked eye. Apart from unicellular yeasts, fungi colonize substrates as a network (mycelium) of filaments (hyphae). These filamentous fungi produce numerous aerially dispersed spores, from microscopic sporing structures in moulds and from large sporing structures in macrofungi.

There are spores of many different moulds in the air of houses and nonindustrial workplaces, but the most common are likely to be species of Cladosporium, Penicillium, Aspergillus and Eurotium. Some moulds in indoor air, such as Cladosporium spp., are abundant on leaf surfaces and other plant parts outdoors, particularly in summer. However, although spores in indoor air may originate outdoors, Cladosporium is also able to grow and produce spores on damp surfaces indoors and thus add to the indoor air bioburden. The various species of Penicillium are generally regarded as originating indoors, as are Aspergillus and Eurotium. Yeasts are found in most indoor air samples, and occasionally may be present in large numbers. The pink yeasts Rhodotorula or Sporobolomyces are prominent in the airborne flora and can also be isolated from mould-affected surfaces.

Buildings provide a broad range of niches in which the dead organic material which serves as nutriment that can be utilized by most fungi and bacteria for growth and spore production is present. The nutrients are present in materials such as: wood; paper, paint and other surface coatings; soft furnishings such as carpets and upholstered furniture; soil in plant pots; dust; skin scales and secretions of human beings and other animals; and cooked foods and their raw ingredients. Whether any growth occurs or not depends on moisture availability. Bacteria are able to grow only on saturated surfaces, or in water in HVAC drain pans, reservoirs and the like. Some moulds also require conditions of near saturation, but others are less demanding and may proliferate on materials that are damp rather than fully saturated. Dust can be a repository and, also, if it is sufficiently moist, an amplifier for moulds. It is therefore an important source of spores which become airborne when dust is disturbed.

Protozoa

Protozoa such as Acanthamoeba and Naegleri are microscopic unicellular animals which feed on bacteria and other organic particles in humidifiers, reservoirs and drain pans in HVAC systems. Particles of these protozoa may be aerosolized and have been cited as possible causes of humidifier fever.

Microbial volatile organic compounds

Microbial volatile organic compounds (MVOCs) vary considerably in chemical composition and odour. Some are produced by a wide range of micro-organisms, but others are associated with particular species. The so-called mushroom alcohol, 1-octen-3-ol (which has a smell of fresh mushrooms) is among those produced by many different moulds. Other less common mould volatiles include 3,5-dimethyl-1,2,4-trithiolone (described as “foetid”); geosmin, or 1,10-dimethyl-trans-9-decalol (“earthy”); and 6-pentyl-α-pyrone (“coconut”, “musty”). Among bacteria, species of Pseudomonas produce pyrazines with a “musty potato” odour. The odour of any individual micro-organism is the product of a complex mixture of MVOCs.

History of Microbiological Indoor Air Quality Problems

Microbiological investigations of air in homes, schools and other buildings have been made for over a century. Early investigations were sometimes concerned with the relative microbiological “purity” of the air in different types of building and any relation it might have to the death rate among occupants. Allied to a long-time interest in the spread of pathogens in hospitals, the development of modern volumetric microbiological air samplers in the 1940s and 1950s led to systematic investigations of airborne micro-organisms in hospitals, and subsequently of known allergenic moulds in air in homes and public buildings and outdoors. Other work was directed in the 1950s and 1960s to investigation of occupational respiratory diseases like farmer’s lung, malt worker’s lung and byssinosis (among cotton workers). Although influenza-like humidifier fever in a group of workers was first described in 1959, it was another ten to fifteen years before other cases were reported. However, even now, the specific cause is not known, although micro-organisms have been implicated. They have also been invoked as a possible cause of “sick building syndrome”, but as yet the evidence for such a link is very limited.

Although the allergic properties of fungi are well recognized, the first report of ill health due to inhalation of fungal toxins in a non-industrial workplace, a Quebec hospital, did not appear until 1988 (Mainville et al. 1988). Symptoms of extreme fatigue among staff were attributed to trichothecene mycotoxins in spores of Stachybotrys atra and Trichoderma viride, and since then “chronic fatigue syndrome” caused by exposure to mycotoxic dust has been recorded among teachers and other employees at a college. The first has been the cause of illness in office workers, with some health effects being of an allergic nature and others of a type more often associated with a toxicosis (Johanning et al. 1993). Elsewhere, epidemiological research has indicated that there may be some non-allergic factor or factors associated with fungi affecting respiratory health. Mycotoxins produced by individual species of mould may have an important role here, but there is also the possibility that some more general attribute of inhaled fungi is detrimental to respiratory well-being.

Micro-organisms Associated with Poor Indoor Air Quality and their Health Effects

Although pathogens are relatively uncommon in indoor air, there have been numerous reports linking airborne micro-organisms with a number of allergic conditions, including: (1) atopic allergic dermatitis; (2) rhinitis; (3) asthma; (4) humidifier fever; and (5) extrinsic allergic alveolitis (EAA), also known as hypersensitivity pneumonitis (HP).

Fungi are perceived as being more important than bacteria as components of bioaerosols in indoor air. Because they grow on damp surfaces as obvious mould patches, fungi often give a clear visible indication of moisture problems and potential health hazards in a building. Mould growth contributes both numbers and species to the indoor air mould flora which would otherwise not be present. Like Gram-negative bacteria and Actinomycetales, hydrophilic (“moisture-loving”) fungi are indicators of extremely wet sites of amplification (visible or hidden), and therefore of poor indoor air quality. They include Fusarium, Phoma, Stachybotrys, Trichoderma, Ulocladium, yeasts and more rarely the opportunistic pathogens Aspergillus fumigatus and Exophiala jeanselmei. High levels of moulds which show varying degrees of xerophily (“love of dryness”), in having a lower requirement for water, can indicate the existence of amplification sites which are less wet, but nevertheless significant for growth. Moulds are also abundant in house dust, so that large numbers can also be a marker of a dusty atmosphere. They range from slightly xerophilic (able to withstand dry conditions) Cladosporium species to moderately xerophilic Aspergillus versicolor, Penicillium (for example, P. aurantiogriseum and P. chrysogenum) and the extremely xerophilic Aspergillus penicillioides, Eurotium and Wallemia.

Fungal pathogens are rarely abundant in indoor air, but A. fumigatus and some other opportunistic aspergilli which can invade human tissue may grow in the soil of potted plants. Exophiala jeanselmei is able to grow in drains. Although the spores of these and other opportunistic pathogens such as Fusarium solani and Pseudallescheria boydii are unlikely to be hazardous to the healthy, they may be so to immunologically compromised individuals.

Airborne fungi are much more important than bacteria as causes of allergic disease, although it appears that, at least in Europe, fungal allergens are less important than those of pollen, house dust mites and animal dander. Many types of fungus have been shown to be allergenic. Some of the fungi in indoor air which are most commonly cited as causes of rhinitis and asthma are given in table 1. Species of Eurotium and other extremely xerophilic moulds in house dust are probably more important as causes of rhinitis and asthma than has been previously recognized. Allergic dermatitis due to fungi is much less common than rhinitis/asthma, with Alternaria, Aspergillus and Cladosporium being implicated. Cases of EAA, which are relatively rare, have been attributed to a range of different fungi, from the yeast Sporobolomyces to the wood-rotting macrofungus Serpula (table 2). It is generally considered that development of symptoms of EAA in an individual requires exposure to at least one million and more, probably one hundred million or so allergen-containing spores per cubic meter of air. Such levels of contamination are only likely to occur where there is profuse fungal growth in a building.

Table 1. Examples of types of fungus in indoor air, which can cause rhinitis and/or asthma

Table 2. Micro-organisms in indoor air reported as causes of building-related extrinsic allergic alveolitis

As indicated earlier, inhalation of spores of toxicogenic species presents a potential hazard (Sorenson 1989; Miller 1993). It is not just the spores of Stachybotrys which contain high concentrations of mycotoxins. Although the spores of this mould, which grows on wallpaper and other cellulosic substrates in damp buildings and is also allergenic, contain extremely potent mycotoxins, other toxicogenic moulds which are more often present in indoor air include Aspergillus (especially A. versicolor) and Penicillium (for example, P. aurantiogriseum and P. viridicatum) and Trichoderma. Experimental evidence indicates that a range of mycotoxins in the spores of these moulds are immunosuppressive and strongly inhibit scavenging and other functions of the pulmonary macrophage cells essential to respiratory health (Sorenson 1989).

Little is known about the health effects of the MVOCs produced during the growth and sporulation of moulds, or of their bacterial counterparts. Although many MVOCs appear to have relatively low toxicity (Sorenson 1989), anecdotal evidence indicates that they can provoke headache, discomfort and perhaps acute respiratory responses in humans.

Bacteria in indoor air do not generally present a health hazard as the flora is usually dominated by the Gram-positive inhabitants of the skin and upper respiratory passages. However, high counts of these bacteria indicate overcrowding and poor ventilation. The presence of large numbers of Gram-negative types and/or Actinomycetales in air indicate that there are very wet surfaces or materials, drains or particularly humidifiers in HVAC systems in which they are proliferating. Some Gram-negative bacteria (or endotoxin extracted from their walls) have been shown to provoke symptoms of humidifier fever. Occasionally, growth in humidifiers has been great enough for aerosols to be generated which contained sufficient allergenic cells to have caused the acute pneumonia-like symptoms of EAA (see Table 15).

On rare occasions, pathogenic bacteria such as Mycobacterium tuberculosis in droplet nuclei from infected individuals can be dispersed by recirculation systems to all parts of an enclosed environment. Although the pathogen, Legionella pneumophila, has been isolated from humidifiers and air-conditioners, most outbreaks of Legionellosis have been associated with aerosols from cooling towers or showers.

Influence of Changes in Building Design

Over the years, the increase in the size of buildings concomitantly with the development of air-handling systems which have culminated in modern HVAC systems has resulted in quantitative and qualitative changes in the bioburden of air in indoor work environments. In the last two decades, the move to the design of buildings with minimum energy usage has led to the development of buildings with greatly reduced infiltration and exfiltration of air, which allows a build-up of airborne micro-organisms and other contaminants. In such “tight” buildings, water vapor, which would previously have been vented to the outdoors, condenses on cool surfaces, creating conditions for microbial growth. In addition, HVAC systems designed only for economic efficiency often promote microbial growth and pose a health risk to occupants of large buildings. For example, humidifiers which utilize recirculated water rapidly become contaminated and act as generators of micro-organisms, humidification water-sprays aerosolize micro-organisms, and siting of filters upstream and not downstream of such areas of microbial generation and aerosolization allows onward transmission of microbial aerosols to the workplace. Siting of air intakes close to cooling towers or other sources of micro-organisms, and difficulty of access to the HVAC system for maintenance and cleaning/disinfection, are also among the design, operation and maintenance defects which may endanger health. They do so by exposing occupants to high counts of particular airborne micro-organisms, rather than to the low counts of a mixture of species reflective of outdoor air that should be the norm.

Methods of Evaluating Indoor Air Quality

Air sampling of micro-organisms

In investigating the microbial flora of air in a building, for example, in order to try to establish the cause of ill health among its occupants, the need is to gather objective data which are both detailed and reliable. As the general perception is that the microbiological status of indoor air should reflect that of outdoor air (ACGIH 1989), organisms must be accurately identified and compared with those in outdoor air at that time.

Air samplers

Sampling methods which allow, directly or indirectly, the culture of viable airborne bacteria and fungi on nutritive agar gel offer the best chance of identification of species, and are therefore most frequently used. The agar medium is incubated until colonies develop from the trapped bioparticles and can be counted and identified, or are subcultured onto other media for further examination. The agar media needed for bacteria are different from those for fungi, and some bacteria, for example, Legionella pneumophila, can be isolated only on special selective media. For fungi, the use of two media is recommended: a general-purpose medium as well as a medium that is more selective for isolation of xerophilic fungi. Identification is based on the gross characteristics of the colonies, and/or their microscopical or biochemical characteristics, and requires considerable skill and experience.

The range of sampling methods available has been adequately reviewed (e.g., Flannigan 1992; Wanner et al. 1993), and only the most commonly used systems are mentioned here. It is possible to make a rough-and-ready assessment by passively collecting micro-organisms gravitating out of the air into open Petri dishes containing agar medium. The results obtained using these settlement plates are non-volumetric, are strongly affected by atmospheric turbulence and favour collection of large (heavy) spores or clumps of spores/cells. It is therefore preferable to use a volumetric air sampler. Impaction samplers in which the airborne particles impact on an agar surface are widely used. Air is either drawn through a slit above a rotating agar plate (slit-type impaction sampler) or through a perforated disc above the agar plate (sieve-type impaction sampler). Although single-stage sieve samplers are widely used, the six-stage Andersen sampler is preferred by some investigators. As air cascades through successively finer holes in its six stacked aluminium sections, the particles are sorted out onto different agar plates according to their aerodynamic size. The sampler therefore reveals the size of particles from which colonies develop when the agar plates are subsequently incubated, and indicates where in the respiratory system the different organisms would most likely be deposited. A popular sampler which works on a different principle is the Reuter centrifugal sampler. Centrifugal acceleration of air drawn in by an impeller fan causes particles to impact at high velocity onto agar in a plastic strip lining the sampling cylinder.

Another approach to sampling is to collect micro-organisms on a membrane filter in a filter cassette connected to a low-volume rechargeable pump. The whole assembly can be clipped to a belt or harness and used to collect a personal sample over a normal working day. After sampling, small portions of washings from the filter and dilutions of the washings can then be spread out on a range of agar media, incubated and counts of viable micro-organisms made. An alternative to the filter sampler is the liquid impinger, in which particles in air drawn in through capillary jets impinge on and collect in liquid. Portions of the collection liquid and dilutions prepared from it are treated in the same way as those from filter samplers.

A serious deficiency in these “viable” sampling methods is that what they assess is only organisms which are actually culturable, and these may only be one or two per cent of the total air spora. However, total counts (viable plus non-viable) can be made using impaction samplers in which particles are collected on the sticky surfaces of rotating rods (rotating-arm impaction sampler) or on the plastic tape or glass microscope slide of different models of slit-type impaction sampler. The counts are made under the microscope, but only relatively few fungi can be identified in this way, namely, those that have distinctive spores. Filtration sampling has been mentioned in relation to the assessment of viable micro-organisms, but it is also a means of obtaining a total count. A portion of the same washings that are plated out on agar medium can be stained and the micro-organisms counted under a microscope. Total counts can be also made in the same way from the collection fluid in liquid impingers.

Choice of air sampler and sampling strategy

Which sampler is used is largely determined by the experience of the investigator, but the choice is important for both quantitative and qualitative reasons. For example, the agar plates of single-stage impaction samplers are much more easily “overloaded” with spores during sampling than those of a six-stage sampler, resulting in overgrowth of the incubated plates and serious quantitative and qualitative errors in assessment of the airborne population. The way in which different samplers operate, their sampling times and the efficiency with which they remove different sizes of particle from the ambient air, extract them from the airstream and collect them on a surface or in liquid all differ considerably. Because of these differences, it is not possible to make valid comparisons between data obtained using one type of sampler in one investigation with those from another type of sampler in a different investigation.

The sampling strategy as well as the choice of sampler, is very important. No general sampling strategy can be set down; each case demands its own approach (Wanner et al. 1993). A major problem is that the distribution of micro-organisms in indoor air is not uniform, either in space or time. It is profoundly affected by the degree of activity in a room, particularly any cleaning or construction work which throws up settled dust. Consequently, there are considerable fluctuations in numbers over relatively short time intervals. Apart from filter samplers and liquid impingers, which are used for several hours, most air samplers are used to obtain a “grab” sample over only a few minutes. Samples should therefore be taken under all conditions of occupation and usage, including both times when HVAC systems are functioning and when not. Although extensive sampling may reveal the range of concentrations of viable spores encountered in an indoor environment, it is not possible to assess satisfactorily the exposure of individuals to micro-organisms in the environment. Even samples taken over a working day with a personal filter sampler do not give an adequate picture, as they give only an average value and do not reveal peak exposures.

In addition to the clearly recognized effects of particular allergens, epidemiological research indicates that there may be some non-allergic factor associated with fungi which affects respiratory health. Mycotoxins produced by individual species of mould may have an important role, but there is also the possibility that some more general factor is involved. In the future, the overall approach to investigating the fungal burden in indoor air is therefore likely to be: (1) to assess which allergenic and toxicogenic species are present by sampling for viable fungi; and (2) to obtain a measure of the total amount of fungal material to which individuals are exposed in a work environment. As noted above, to obtain the latter information, total counts could be taken over a working day. However, in the near future, methods which have recently been developed for the assay of 1,3-β-glucan or ergosterol (Miller 1993) may be more widely adopted. Both substances are structural components of fungi, and therefore give a measure of the amount of fungal material (i.e., its biomass). A link has been reported between levels of 1,3-β-glucan in indoor air and symptoms of sick building syndrome (Miller 1993).

Standards and Guidelines

While some organizations have categorized levels of contamination of indoor air and dust (table 3), because of air sampling problems there has been a justifiable reluctance to set numerical standards or guideline values. It has been noted that the airborne microbial load in air-conditioned buildings should be markedly lower than in outdoor air, with the differential between naturally ventilated buildings and outdoor air being less. The ACGIH (1989) recommends that the rank order of fungal species in indoor and outdoor air be used in interpreting air sampling data. The presence or preponderance of some moulds in indoor air, but not outdoors, may identify a problem inside a building. For example, abundance in indoor air of such hydrophilic moulds as Stachybotrys atra almost invariably indicates a very damp amplification site within a building.

Table 3. Observed levels of micro-organisms in air and dust of nonindustrial indoor environments

^a CFU, colony-forming units.

Source: adapted from Wanner et al. 1993.

Although influential bodies such as the ACGIH Bioaerosols Committee have not established numerical guidelines, a Canadian guide on office buildings (Nathanson 1993), based on some five years of investigation of around 50 air-conditioned federal government buildings, includes some guidance on numbers. The following are among the main points made:

1. The “normal” air flora should be quantitatively lower than, but qualitatively similar to, that of outdoor air.
2. The presence of one or more fungal species at significant levels in indoor but not outdoor samples is evidence of an indoor amplifier.
3. Pathogenic fungi such as Aspergillus fumigatus, Histoplasma and Cryptococcus should not be present in significant numbers.
4. The persistence of toxicogenic moulds such as Stachybotrys atra and Aspergillus versicolor in significant numbers requires investigation and action.
5. More than 50 colony-forming units per cubic meter (CFU/m³) may be of concern if there is only one species present (other than certain common outdoor leaf-inhabiting fungi); up to 150 CFU/m³ is acceptable if the species present reflect the flora outdoors; up to 500 CFU/m³ is acceptable in summer if outdoor leaf-inhabiting fungi are the main components.

These numerical values are based on four-minute air samples collected with a Reuter centrifugal sampler. It must be emphasized that they cannot be translated to other sampling procedures, other types of building or other climatic/geographical regions. What is the norm or is acceptable can only be based on extensive investigations of a range of buildings in a particular region using well-defined procedures. No threshold limit values can be set for exposure to moulds in general or to particular species.

Control of Micro-organisms in Indoor Environments

The key determinant of microbial growth and production of cells and spores which can become aerosolized in indoor environments is water, and by reducing moisture availability, rather than by using biocides, control should be achieved. Control involves proper maintenance and repair of a building, including prompt drying and elimination of causes of leakage/flood damage (Morey 1993a). Although maintaining the relative humidity of rooms at a level less than 70% is often cited as a control measure, this is effective only if the temperature of the walls and other surfaces are close to that of the air temperature. At the surface of poorly insulated walls, the temperature may be below the dew point, with the result that condensation develops and hydrophilic fungi, and even bacteria, grow (Flannigan 1993). A similar situation can arise in humid tropical or subtropical climates where the moisture in the air permeating the building envelope of an air-conditioned building condenses at the cooler inner surface (Morey 1993b). In such cases, control lies in the design and correct use of insulation and vapor barriers. In conjunction with rigorous moisture control measures, maintenance and cleaning programmes should ensure removal of dust and other detritus that supply nutrients for growth, and also act as reservoirs of micro-organisms.

In HVAC systems (Nathanson 1993), accumulation of stagnant water should be prevented, for example, in drain pans or under cooling coils. Where sprays, wicks or heated water tanks are integral to humidification in HVAC systems, regular cleaning and disinfection are necessary to limit microbial growth. Humidification by dry steam is likely to reduce greatly the risk of microbial growth. As filters can accumulate dirt and moisture and therefore provide amplification sites for microbial growth, they should be replaced regularly. Micro-organisms can also grow in porous acoustical insulation used to line ducts if it becomes moist. The solution to this problem is to apply such insulation to the exterior rather than the interior; internal surfaces should be smooth and should not provide an environment conducive to growth. Such general control measures will control growth of Legionella in HVAC systems, but additional features, such as the installation of a high-efficiency particulate air (HEPA) filter at the intake have been recommended (Feeley 1988). In addition, water systems should ensure that hot water is heated uniformly to 60°C, that there are no areas in which water stagnates and that no fittings contain materials that promote growth of Legionella.

Where controls have been inadequate and mould growth occurs, remedial action is necessary. It is essential to remove and discard all porous organic materials, such as carpets and other soft furnishings, ceiling tiles and insulation, on and in which there is growth. Smooth surfaces should be washed down with sodium hypochlorite bleach or suitable disinfectant. Biocides which can be aerosolized should not be used in operating HVAC systems.

During remediation, care must always be taken that micro-organisms on or in contaminated materials are not aerosolized. In cases where large areas of mould growth (ten square meters or more) are being dealt with it may be necessary to contain the potential hazard, maintaining negative pressure in the containment area during remediation and having air locks/decontamination areas between the contained area and the remainder of the building (Morey 1993a, 1993b; New York City Department of Health 1993). Dusts present before or generated during removal of contaminated material into sealed containers should be collected using a vacuum cleaner with a HEPA filter. Throughout operations, the specialist remediation personnel must wear full-face HEPA respiratory protection and disposable protective clothing, footwear and gloves (New York City Department of Health 1993). Where smaller areas of mould growth are being dealt with, regular maintenance staff may be employed after appropriate training. In such cases, containment is not considered necessary, but the staff must wear full respiratory protection and gloves. In all cases, both regular occupants and personnel to be employed in remediation should be made aware of the hazard. The latter should not have pre-existing asthma, allergy or immunosuppressive disorders (New York City Department of Health 1993).

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