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)