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Fungi and fungal growth
From the World Health Organization in its report WHO Guidelines for
Indoor Air Quality: Dampness and Mould, published July 16, 2009
Fungi are ubiquitous eukaryotic organisms, comprising an abundance of
species. They may be transported into buildings on the surface of new
materials or on clothing. They may also penetrate buildings through active
or passive ventilation.
Fungi are therefore found in the dust and surfaces of every house,
including those with no problems with damp. Once fungi are indoors, fungal
growth can occur only in the presence of moisture, and many fungi grow
readily on any surface that becomes wet or moistened; that is, virtually
all fungi readily germinate and grow on substrates in equilibrium with a
relative humidity below saturation (i.e. below 100%).
The species that grow on a given substrate depends largely on the water
activity of the substrate. Water activity is a measure of water
availability and is defined as the ratio of the vapour pressure above a
substrate relative to that above pure water measured at the same
temperature and pressure. The minimum water activity required for fungal
growth on building surfaces varies from less than 0.80 to greater than
0.98 (Grant et al., 1989).
On the basis o their water requirements, indoor fungi can be divided into:
(1) primary colonizers, which can grow at a water activity less than or
equal to 0.80; (2) secondary colonizers, which grow at a water activity
level of 0.80–0.90; and (3) tertiary colonizers, which require a water
activity greater than 0.90 to germinate and start mycelial growth (Grant
et al., 1989).
Although high levels of humidity and some surface and interstitial
condensation may be sufficient for most primary and secondary colonizers,
tertiary colonizers generally require serious condensation problems. These
problems may be due to construction faults, including inadequate
insulation, in combination with poor ventilation, or water damage from
leaks, flooding and groundwater intrusion.
Fungi also need nutrients, which may include carbohydrates, proteins and
lipids. The sources are diverse and plentiful, ranging from plant or
animal matter in house dust to surface and construction materials (such as
wallpaper and textiles), condensation or deposition of cooking oils, paint
and glue, wood, stored products (such as food), and books and other paper
products. Nutrients are therefore generally not a limiting factor for
indoor fungal growth. In fact, fungi are known to grow even on inert
materials such as ceramic tiles and can obtain sufficient nutrients from
dust particles and soluble components of water. As most indoor fungi grow
at 10–35 °C, common indoor temperatures are also not a limiting factor;
however, although temperature and nutrients are not critical, they may
affect the rate of growth and the production of certain allergens and
metabolites (Nielsen et al., 1999; Institute of Medicine, 2000).
Thus, water remains the most critical factor in indoor fungal growth, as
also indicated in field studies, which show elevated numbers of fungi and
fungal spores in damp houses (Gallup et al., 1987; Waegemaekers et al.,
1989; Douwes et al., 1999). House dampness therefore significantly
contributes to fungal spores, fragments and allergens.
Fungi not only have adverse effects on health but also cause considerable
damage to buildings, the wood-rotting fungi being particularly destructive
to (wooden) building structures. The commonest and possibly the most
destructive wood decay fungus found in buildings in temperate regions,
including Australia, Europe and Japan, is the dry-rot fungus Serpula
lacrymans (previously known as Merulius lacrymans) (Singh,
1999). This fungus can grow quickly and may spread throughout a building
from one timber to another, potentially causing devastating effects in the
whole building. There are many other dry- and wet-rot fungi that can cause
wood decay and subsequent damage to the built environment (reviewed by
Singh, 1999). They have also been implicated in the causation of
hypersensitivity pneumonitis (extrinsic allergic alveolitis).
Table 1. Moisture levels required for growth of selected microorganisms in
construction, finishing and furnishing materials
Moisture level
High (aw, > 0.90; ERH, > 90%)
Intermediate (aw, 0.80–0.90; ERH, 80–90%)
Low (aw, < 0.80; ERH, < 80%)
Category of microorganism
Tertiary colonizers (hydrophilic)
Alternaria alternata
Aspergillus fumigatus
Epicoccum
spp.
Exophiala
spp.
Fusarium moniliforme
Mucor plumbeus
Phoma herbarum
Phialophora
spp.
Rhizopus
spp.
Stachybotrys chartarum
(S. atra)
Trichoderma
spp.
Ulocladium consortiale
Rhodotorula
spp.
Sporobolomyces
spp.
Actinobacteria (or Actinomycetes)
Secondary colonizers
Aspergillus flavus
Aspergillus versicolora
Cladosporium cladosporioides
Cladosporium herbarum
Cladosporium sphaerospermum
Mucor circinelloides
Rhizopus oryzae
Primary colonizers (xerophilic)
Alternaria citri
Aspergillus (Eurotium) amstelodami
Aspergillus candidus
Aspergillus (Eurotium) glaucus
Aspergillus niger
Aspergillus penicillioides
Aspergillus (Eurotium) repens
Aspergillus restrictus
Aspergillus versicolorb
Paecilomyces variotii
Penicillium aurantiogriseum
Penicillium brevicompactum
Penicillium chrysogenum
Penicillium commune
Penicillium expansum
Penicillium griseofulvum
Wallemia sebi
Sources:
Grant et al. (1989); Gravesen, Frisvad, Samson (1994); ISIAQ (1996)
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