Biological contaminants include viable and non-viable microbiological matter such as viruses, bacteria, fungi, protozoa, house dust mite and insect feces and pollens. Their presence can result in infectious disease, toxics effects or allergic reaction. Much is known about the occurrence and health effects of microbial growth associated with water damage but there has been little investigation on the impact of other microbial contaminants on indoor air quality.
1.1 Infectious contaminants
While all types of microorganisms can cause problems indoors, bacteria and fungi are most commonly associated with indoor air quality complaints. In any indoor environment a variety of species will be present at different times and in different micro-environments.
In order for airborne disease transmission to occur from microbes in buildings, there must be a source or reservoir for the microbes, some means for the microbes to multiply (amplifiers) and a mechanism for their release and dispersion into indoor air. The major indoor reservoir is stagnant water or moist interior surfaces. These can accumulate microbes that enter the building in outdoor air and act as amplifiers for bacteria. Fungi can grow in relatively dry environments (eg at relative humidities above 75%). Airborne dispersion is relatively easy for microbes found in building ventilation systems (eg fungal and bacterial spores) or contaminated carpet.
Bioaerosols (which include bacteria, fungi, dust mites and other biological particles) are recognised as an important subgroup involved in building-related illness. Microbes, particularly fungi, which can contain mycotoxins, and VOCs, are now thought to be more involved as causative agents in mechanically ventilated indoor work environments. Several studies suggest that indoor air microbials may have a more common role in the sick building syndrome than previously thought.
Several companies routinely screen buildings for airborne microbial levels; standardised test methods, which provide reproducible test results, are now readily available. Some caution is required when evaluating any one-off microbial test results, because airborne microorganisms are ubiquitous in the environment, with ‘typical’ levels ranging from 50 to 1500 CFUs per cubic metre in outside city air. An external sample taken on the day of test can provide a useful baseline figure for the evaluation of results.
Measurement of microorganisms in excess of about 1000 CFUs per cubic metre of air indicates that the indoor environment may need to be investigated for microbiological contamination. However, the report also stated that exceeding this level does not mean the air is unsafe or hazardous. Merely using a number to represent CFUs per cubic metre is an unreliable indicator of the actual hazard posed by airborne micro-organisms.
Because of the universal presence of microorganisms, it is critically important to obtain an indication of the ratios of organism groups present. This information will be of greatest value with periodic testing to provide data for a trend analysis of the microbial groups in a particular site. By differentiating these groups of organisms, the likely source, the risk potential and the need for any action can be established.
Determining the groups of microorganisms present in a sample is significantly different from identifying all organisms present. The full identification of isolates from a sample would not generally provide any useful information and is certainly not cost-effective. A number of techniques have been recommended for sampling for microbial contamination in indoor environments.
Bacteria are ubiquitous in the air and general environment. They can cause adverse human health effects and deterioration of building materials when they proliferate in indoor environments. The health effects of bacterial exposure in indoor air will depend on the species and the route of exposure.
The bacteria in building air can come from airborne sources from the wind’s action on soil and vegetation, compost, municipal landfills, sewage sludge, etc. They can also be a direct result of human activity, such as breathing, coughing and sneezing, or they may colonise the ductwork of the cooling systems, the water cooling towers (eg Legionella spp) or interior building materials and furnishings such as wallboard, wallpaper, flooring and furnishings.
In indoor environments, bacteria usually grow in areas with standing water such as water spray and condensation areas of air conditioning systems. Dirty or poorly maintained air handling systems can become contaminated over time by bacterial populations that thrive on moisture-laden surfaces caused by water condensation. Legionella is probably the most common group of bacteria mentioned in association with airconditioning systems (see below).
Viruses are important airborne organisms and a significant contributor to occupational absenteeism. Examples of important viruses include the causative agent of the common cold (rhinoviruses) and the flu (influenza viruses types A, B, C, etc). The spread of these illnesses can be aided by inadequate ventilation levels within a building. Viruses cannot multiply outside the human host, but can survive and remain infective for extended periods in the warm recirculating airspace of the modern air-conditioned building.
Routine testing for airborne viruses is expensive and not generally recommended. However a useful correlation between the levels of some airborne bacteria, particularly the Micrococcus group, and poor ventilation levels has been noted, and these can thus be used as an indicator of potential cross-infection problems.
Allergens are substances that cause an allergic reaction. Many substances that are found indoors may act as allergens and have a significant impact on the health and quality of life of the 30–40% of individuals in our population who are atopic. Domestic allergen exposures may represent significant risks for the development of asthma and of acute asthma attack.
The prevalence of allergy and asthma has increased considerably in recent years. The emergence of allergies has been attributed to a number of factors, in particular, the changes in landscape from forest to pasture, and the trend for most people to lead an indoor, climate-controlled lifestyle.
A wide range of trigger allergens have been identified, many of which are found in the indoor environment. Findings suggest that house dust mite, cat and cockroach allergens may cause the onset of asthma while pollen and fungal allergens may exacerbate asthma or trigger episodes of asthma.
The National Academy of Sciences of the United States has found sufficient evidence to conclude that there is a causal relationship between exposure to cat, cockroach and house dust mite allergens and exacerbations of asthma in sensitised individuals. It also found that there was evidence of a causal relationship between environmental tobacco smoke exposure and exacerbations of asthma in preschool-aged children. An association (rather than causal relationship) was found for exposure to allergens of dogs, fungi and high-level exposures to nitrogen dioxide and oxides of nitrogen (at concentrations that may occur when gas appliances are used in poorly ventilated kitchens).
House dust mite allergen appears to have the widest public health impact, with insect allergens (primarily cockroach) also important. Pets, particularly dogs and cats and to a lesser extent birds, impact on a smaller part of the community.
Bedding, soft furnishings and carpets have been shown to act as reservoirs for allergens and it has been suggested that strategies to reduce asthma risk should be targeted at these sources.
House dust mites are microscopic organisms that live mainly in mattresses and carpets within the home, where they feed off human skin flakes and other products. These mites are a major source of allergens that can sensitise various organs, leading to inflammation at the next exposure. It is suggested that house dust mites were more likely to cause sensitisation than pollens or fungi since house dust mite exposure is perennial rather than seasonal and the level of allergen exposure can be 20 ng/hour for house dust mites compared to 1 mg/season for pollen and fungi. The sources of dust mites are well known and general health effects are understood, yet the risks of developing allergy to house dust mites is a more complicated issue relating to exposure and genetic predisposition.
House dust mite faeces, which contain the allergen, may become airborne if disturbed and build up in carpets, sofas, cushions, bedding etc. Allergic reaction in the lung leads to asthma; in the nose, to hay fever or allergic rhinitis; in the skin, to dermatitis or a form of eczema.
Most households have pets, but there is little information on pet allergens. Allergen sources include animal dander, saliva, faeces and urine. A review found that the levels of pet allergens were high in homes with pets (especially if the standard of cleaning was poor and ventilation inefficient) and that they were greatest during seasons when heating was likely to be used.
The potential for exposure to pet allergens can be seen from the estimates of households with pets (45% dogs, 30% cats, 20% birds). Anecdotal evidence suggests that dogs with woolly coats and short-haired cats have less effect on sensitised individuals. Dog and cat allergens are readily transported by air currents and attachment to objects like clothes which then are moved.
Allergens from pets, particularly dogs and cats, appear to be ubiquitous, even in houses that do not have pets. A study of houses of allergy clinic patients in Baltimore (United States) showed that there were significant differences in the levels of pet allergens between houses with and without pets, but that many of the houses without resident pets still had high concentrations of pet allergens (Wood et al 1988).
Allergic reactivity to cats appears to be stronger than that seen with other animals; exposure to cat allergen in the first year of life is an important factor in the development of sensitisation and asthma (Dr Connie Katelaris, Westmead Hospital, presentation on ‘Indoor allergens – risks and management’). Allergen in cat saliva is transferred to the fur in grooming. The salvia then dries, detaches from the fur and commonly becomes airborne with the particle it is attached to. As with dust mites, allergens build up in carpets, furnishings and beddings.
The major allergen of domestic cats is Fel d found very high levels of Fel d 1 in homes of preschool children. The levels found in the beds of asthmatic children were significantly higher than in non-asthmatic (2.60 mg/g versus 0.89 mg/g). For asthmatics, bed levels exceeded bedroom and living room levels, but for non-asthmatics no differences were seen in samples from the three locations.
This study concluded that the very high level of cat allergen exceeded proposed levels of risk for allergy and asthma and represented a significant risk of asthma in Adelaide. The results suggested that it is likely that pet cats are permitted to sleep on beds and that this practice should be discouraged.
The primary allergens in dogs are from tongue tissues and tongue glands. Dog fur rarely causes an allergic response.
Cockroach allergen (present in cockroach saliva, gut and faeces) is found throughout the home, but kitchens usually have the highest levels. The allergen has also been found in schools. It has been found to be associated with particles greater in size than 10 µm and is found in the air only after vigorous disturbance.
Although the health risks are less than those associated with house dust mites, the allergen has a significant consequences for some individuals.
Pollen is the popular term for the microspores (male spores) of seed-producing plants. The symptoms associated with pollen allergy, hayfever and allergic rhinitis can be either acute or chronic. It is not unusual for 10–20% of a population to be pollen sensitised.
As part of the ambient aerosol, pollens are readily transported into the indoor environment through windows, doors, air intakes and even humans. There has been little or no systematic investigation of indoor/outdoor pollen concentrations across the United States.
The major source of pollen exposure is the outdoor environment, so exposure would be expected to be low in mechanically ventilated buildings with well maintained systems. Indoor plants may produce allergenic pollen. Carpets are a reservoir for particles but regular cleaning will reduce pollen load provided the equipment used captures the particles.
Humans release a variety of odorous gaseous bioeffluents (eg body odours) that influence the perceived acceptability of indoor air. Carbon dioxide, which is odourless, is one of the gaseous human bioeffluents in exhaled air. Humans are normally the main indoor source of carbon dioxide. The outdoor carbon dioxide concentration is approximately 350 ppm, whereas indoor concentrations are usually in the range of 500 ppm to a few thousand parts per million. At these concentrations, carbon dioxide is not thought to be a direct cause of adverse health effects; however, carbon dioxide is an easily measured surrogate for other occupant-generated pollutants, such as body odours.
Human breath emissions may be a significant source of VOCs. Fenske and Paulson (1999) used data from previous studies to calculate the proportion of VOCs attributable to human emissions in places where people congregate indoors, such as schools and offices. They calculated that human emissions are likely to be the source of at least 10–20%, and sometimes more than 50%, of VOCs in these places.