Photo: Canada Mortgage and Housing Corporation (CMHC)
The potential impacts of indoor air contamination on human health have received considerable public attention in recent years. This is especially true in Canada, and other countries with cold weather, where people spend most of their time indoors. Indoor air can be contaminated by pollutants released from carpets and building materials, cleaning chemicals, tobacco smoke, cooking and heating, as well as biological contaminants such as dust mite and animal allergens (derived from skin, saliva and urine) and molds. This report discusses only one aspect of this complex array of contaminants: mold. Much of what is known about the population health effects of biological contaminants in indoor environments comes from studies of people living in damp homes.
Living in damp houses is associated with increased rates of disease, and the cause is believed to be exposure to biological contaminants (Institute of Medicine 2000). Occupants in houses that have dampness problems are at greater risk of exposure to mold, dust mites and bacterial endotoxins. Lower socio-economic status has been associated with higher prevalence of respiratory disease (Dales et al. 2002). In most countries, poverty translates into living in substandard housing that leaks water and air, and is difficult to heat. When houses are difficult or expensive to heat or cool, the air in some rooms is often not conditioned. This leads to moisture accumulation (condensation) on cold surfaces. Depending on the surfaces and degree of house cleaning, contaminants accumulate and airborne particulate concentrations will vary accordingly. Due to complex exposures, however, the attributable risk to each of the biological contaminants discussed here remains unknown. This alone makes it difficult to assign tolerable exposure values.
In addition, outdoor-source fine particles (PM2.5) can be higher indoors than outdoors. Houses near sources of outdoor air pollution (e.g. vehicular traffic) are at greater risk of increased indoor concentrations of particulate matter and volatile organic compounds (VOCs).
Among indoor air contaminants, mold is a cause of increasing concern, with many epidemiological studies and case reports linking mold to a wide range of adverse effects on respiratory health.
The fungus kingdom consists of eukaryotic organisms. Fungi are subdivided into four different phyla based on their reproduction mode: ascomycetes, basidiomycetes, zygomycetes and mitosporic fungi.
The word "mold" is a non-scientific term that in popular parlance generally refers to members of a few dozen filamentous fungi. Such fungi are often visible as colonies on food and building materials, appearing on close inspection as multicellular filaments called hyphae. Mold growth on building material surfaces can influence air quality because both spores and mycelial fragments are dispersed into the air and can be inhaled, depending on their size.
The spores of fungi have a large size range: 1 to 50 µm. Furthermore, the degree of hydration of spores, a consequence of the prevailing relative humidity, affects this range (Madelin and Johnson 1992). Particles at the lower end of the size range (less than 10 µm) can reach the alveoli; others may be swallowed. There is some variation with age: lower airway deposition for 5 µm particles is six times higher in newborns than in adults (Phalen and Oldham 2001). The average aerodynamic diameter of a number of spore types is listed in Table 1. The average sizes of some spores are well within the respirable range (<10 µm); others such as Stachybotrys chartarum appear to be too large to penetrate into the lungs. However, there is considerable variation not represented by the average. For example, even though the average aerodynamic diameter of Stachybotrys spores is too large to penetrate into the lungs, approximately one third of the spores are within the respirable range (Sorenson et al. 1996). Similar data for some strains of Cladosporium cladosporioides, Penicillium viridicatum and P. chrysogenum showed a large range in spore sizes whereas most spores of P. commune, Aspergillus versicolor, A. ustus, A. niger and A. sydowii were of similar dimensions (Miller and Young 1997). As noted, mycelial fragments are also typically present in indoor air. These are usually of respirable size. The number of fragments compared to the number of spores present is highly variable, but typically represent a few percent of the fungal particles present. It is known that mycelial fragments of some species contain different allergens than those present in spores of the same species (Gˇrny et al. 2002).
|Cladosporium cladosporioides||2.3||(2.0-3.5) x (2.0-2.5)|
|Paecilomyces variotii||2.7||2.9 x 1.3|
|Penicillium chrysogenum||2.6||2.5 x 2.5|
(After Madelin and Johnson 1992; Sorenson et al. 1996).
Three features of mold biochemistry are of special interest in terms of human health. First, mold cell wall contains (1->3)-ß-D-glucan, a compound with inflammatory properties. Second, spores and mycelial fragments contain allergens (Gˇrny et al. 2002), few of which have been chemically characterized. Many of the known fungal allergens are serine proteases, or proteins, which are present in fairly high concentrations in the spores. These have been described mainly from work done in phylloplane species and Aspergillus fumigatus (Horner et al. 1995). Third, the spores of some species contain low molecular weight chemicals that are cytotoxic or have other toxic properties (e.g. satratoxins produced by Stachybotrys chartarum). Some molds, such as Aspergillus fumigatus, can cause opportunistic infection in immunocompromised individuals and severe allergic diseases in people with underlying respiratory conditions, such as asthma or cystic fibrosis (Burge 2000).Fungi commonly found in moldy building materials are shown in
Table 2. Common fungi from mold-damaged building materials
Photos : Centraalbureau voor Schimmelcultures, Koninklijke Nederlandse Akademie van Wetenschappen, Pays-Bas