M. D’Orazio,     Università Politecnica delle Marche, Italy

12.1 Introduction

It is impossible to contemplate life on Earth without the presence of fungi, algae, bacteria and other microorganisms. Some of these are in fact responsible for the rapid decomposition of dead organic matter and for its subdivision into components which are then involved in starting a new life cycle (Heseltine and Rosen, 2009). However, despite their utility for the ecosystem, the presence of these organisms on the inside or outside surfaces of building components is not welcome, both because of the well-known implications for human health when they infest the indoor environment and because of their contribution to the defacement of paint and finishes. Moreover, in recent years the presence of moulds and algae inside buildings has considerably increased, despite the fact that better quality is required for living spaces. The main cause of this is the need to limit energy consumption, which has led to a deterioration in the indoor environment and the working conditions of the external finishes, favouring, more than in the past, the growth of these organisms.

12.2 Mould fungi in construction materials

The problem of mould growth inside buildings has been observed in different geographical areas and various types of building (Rousseau, 1983; Hud, 2004). The main characteristic of mould fungi is that they do not have chloroplasts and therefore they are not able to carry out photosynthesis. Their life cycle is divided into four phases: sporulation, germination, hyphal growth (vegetative growth) and reproduction (Moon, 2005).

During the germination phase, the spores settle on the surfaces and remain inactive until they can absorb moisture and nutrients from the substrate. If the substrate does not provide adequate nourishment and moisture, the spores do not germinate. The growth of hyphae (pluricellular filaments) occurs immediately after germination and as these thicken they form a mass called mycelium. From this time onwards the fungi metabolize the substrate material by extracting the necessary nutrients and retaining the moisture needed for growth. In the final phase the fungi constitute a reproductive organism producing spores.

The development and proliferation of organisms of biological origin (moulds, bacteria, fungi) inside a building require high levels of humidity, associated with adequate nutrients in the substrate (Heseltine and Rosen, 2009). Mould fungi specifically need oxygen, a temperature of between 22 °C and 35 °C, indoor relative humidity ranging between 71% and 95% (Baughman and Arens, 1996; Ayerst, 1969), and an adequate substrate to provide the nutrients (Hens, 1999). Other secondary factors are the pH value and the roughness of the substrate on which the moulds grow, light, biotic interactions between different cultures, exposure time, and indoor air velocity (Górny, 2004; Krus et al., 2001; Adan, 1994).

Grant et al. (1989) showed that, with adequate nourishment, fungal growth on substrates already occurs at values of a_w = 0.65 and the colonizing strains may be subdivided into three groups according to the minimum value of a_w at which growth begins (Table 12.1). Clarke et al. (1999) carried out experiments on six different categories of fungi which can be found in buildings on the basis of temperature and indoor relative humidity.

Most fungi are saprophytes, which means that they can feed on carbohydrates, proteins and lipids. Sources are varied and abundant: plants, pets, dust and building materials (such as wallpaper and fabrics), condensation/deposit of cooking fat, paint and glue, timber, packed products (such as food), books and other items made of paper. Therefore, in indoor environments, fungi have numerous sources of nutrients. Some building materials which are particularly rich in carbon, like cellulose or carbonates (wallpaper, wood-based building materials), are more propitious to the development of moulds compared with others that have a lower carbon content (for example, plaster and glass wool) (Górny, 2004; Vacher et al., 2010). The relationship between construction materials and mould growth has been studied by several authors. However, the results available are not easily summarized because of the great variety of materials and other factors which intervene to condition the development of moulds. Sedlbauer (2001) provided a classification based on the relationship between temperature, RH% and type of substrate (Table 12.2). On the basis of these categories the same author also developed isopleth systems which correlate the temperature with the relative humidity (Sedlbauer, 2001) (Table 12.3). Isacsson et al. (2010) have recently proposed a model on the basis of the ‘dose–response’ ratio.

Ritschkoff et al. (2000) conducted experiments on wood-based composite materials (chipboard planks, wood wool and plywood boards), plaster, concrete, and insulating materials (glass wool and rock wool) in different temperature and relative humidity conditions. The results show that all the materials in a building may contribute to the growth of moulds if their relative humidity reaches 90%. Other authors (Rowan et al., 2003; Viitanen and Ritschkoff, 1991) have indicated that mould fungi do not grow with RH < 80% (Adan, 1994) or with RH < 75% when the temperature is between 5 and 40 °C. Johansson et al. (2005), after analysing the literature, summarized the critical values of RH for different materials which, if exceeded, encourage microbial growth (Table 12.4).

Some inorganic materials, such as metals and plastic, are not in themselves nutrients which are suitable for mould fungi, although the dust which deposits on top of them may represent a source of nourishment. For this reason some studies show that in existing buildings the rate of deterioration due to the action of fungi is similar for both organic and inorganic materials (Kowalski and Bahnfleth, 1999). D’Orazio et al. (2008) showed that there is a direct correlation between the growth rate of some fungal species (S. chartarum, P. chrisogenum, A. versicolor) and the content in organic matter which various coatings and indoor finishes are able to provide as nutrients for the spores (Fig. 12.1). Moreover, D’Orazio et al. (2008) showed that although various types of plaster and finish (experimentally analysed) belong to the same class of substrate (II) according to the classification of Sedlbauer (2001), in reality there are sometimes quite remarkable differences in the results for the various substrates. Experimental results showed that the species S. chartarum (the most highly toxic for human health) had the greatest development on the various types of support surface used.

The exposure time needed for the formation of moulds varies considerably according to the environmental conditions in which the building material is located (Pasanen et al., 2000; Sedlbauer et al., 2003). Inorganic building materials have higher minimum humidity requirements and a longer exposure time compared with the optimal medium for fungi germination. In favourable environmental conditions (surface temperature and relative humidity) a shorter exposure time is required in order for germination to occur. However, it is necessary to consider that surface conditions may vary considerably inside the same building as a result of thermal bridges and cracks in the walls. Some authors point out that the rate of microbial growth increases in proportion to the amount of nutrient available in the substrate (D’Orazio et al, 2008; Vacher et al, 2010).

12.3 Algae in construction materials

Unlike moulds, algae are ‘pioneer organisms’ of outdoor environments and are widespread on the external surfaces of buildings (Escadeillas et al., 2007; Künzel et al., 2006a) since they are able to survive through frequent freeze-thaw and dehydration cycles. Algae are classified in two main groups: green algae and blue algae (cyanobacteria), according to their bacterial structure. Green algae develop with RH = 70–80% (Zillig et al., 2003), while blue algae develop with RH = 100% (Zillig et al., 2003) and a temperature of between 15 and 50 °C (Karsten et al., 2005), even if the range 20–25 °C is the ideal temperature (Zillig et al., 2003). Algae exploit photosynthesis, moisture, carbon and other elements which may be present as a result of either run-offrainwater or pollution. Green algae and cyanobacteria (Fig. 12.2) (mistakenly called blue algae, blue-green algae or cyanophyta) are a phylum of photosynthetic bacteria (Nay and Raschle, 2003); they are unicellular or pluricellular organisms which may be eukaryotic (green algae) or prokaryotic (cyanobacteria). One important characteristic which distinguishes algae from (heterotrophic) mould fungi is their autotrophy, which is the ability of an organism to synthesize its own organic molecules starting from inorganic substances, using energy which is not derived from assimilated organic substances. Therefore the algae, by means of chlorophyllian photosynthesis, transform light energy into chemical energy (Johansson, 2005) managing to synthesize inorganic compounds such as carbon dioxide, water and some elements, and thereby obtain organic substances which guarantee their long-term survival (Hofbauer et al., 2003):

These phototrophic organisms which develop outdoors are particularly resistant to the wind and rainfall, although they are not well protected against evaporation. For this reason they require more moisture than moulds (present in the surrounding environments), but manage to survive very easily even with few nutrients. The characteristic feature which distinguishes algae from moulds is their resistance to drying (also called anhydrobiosis), which allows them to survive even if greatly dehydrated. In fact, in these circumstances, the algae manage to survive by accumulating saccharose and trehalose, thereby permitting the cell to maintain its integrity during dehydration (Johansson, 2005). The algal cells reactivate their biological processes when moisture in both vapour and liquid state (Johansson et al., 2010) again becomes available.

The growth of algal species is favoured by an optimal combination of biotic and abiotic factors, in particular nutrients coming from the substrate, type of organisms present, moisture (Sedlbauer, 2001) and other environmental factors (Nay and Raschle, 2003; Gaylarde and Gaylarde, 2000) (Fig. 12.2).

If any one of these three macro-conditions is missing, vegetative growth becomes unlikely. Algae do not have roots and they absorb moisture through their cell walls by osmosis (Lengsfeld and Krus, 2004). Hofbauer et al. (2003) studied and classified the types of organisms that contribute to the deterioration of the external façades of buildings and reported the presence of four groups of autotrophic plants: algae, lichens, moss and ferns, and (non-autotrophic) fungi.

Various authors have highlighted the fundamental role of moisture, and the way in which it runs off the surface layer of the external vertical walls, in the formation of organisms. It has been demonstrated that porosity, roughness and the composition of the external finishes all participate in retaining rain (Barberousse et al., 2007), while even the shape of the building may create preferential routes where the rainwater stagnates, creating the ideal conditions for the growth of microorganisms (Nay and Raschle, 2003). Once the algae have grown, the run-off rainwater contributes to replacing the old cells with new cells and favours the spread of spots of biofilm to other non-contaminated building components (Künzel, 2007). Even the tendency to heavily insulate buildings encourages the presence of algae (Kastien, 2003) on the external surfaces of buildings. High insulation determines a lowering in the surface temperatures of the construction components during the night-time compared with the temperature of the air (‘undercooling’) and consequently favours condensation on these surfaces (Künzel, 2007; Adan, 1994; Blaich et al., 2000). The effects of moisture may be amplified by exposure, the season and ventilation. Deterioration due to algal species is found mainly on north and north-west facing surfaces (Nay and Raschle, 2003), since these are hardly ever irradiated by the sun throughout the daytime, and remain damp for a longer time. Walls which have good exposure to the sun receive more light and more heat and therefore dry out in a short time; consequently they are not suitable for microbial growth. Direct sunlight may also damage the photosynthetic pigments of the algae: the optimal light intensity for algae is about 1000 lux, usually corresponding to north-facing walls (Karsten et al., 2005; Nay and Raschle, 2003).

Even the season is important for the formation of algae on the façade. In fact, during the hot season the external surfaces tend to dry out more quickly, hindering the development of these organisms. However, if algae develop when conditions are more favourable, the season is decisive for their survival: in the summer the external surface temperature of buildings may reach temperatures close to 60 °C, causing the death of the algal cells (Karsten et al., 2005), while in the winter temperatures lower than 15 °C inhibit their development (Künzel et al., 2001, 2006b).

Studies conducted on the relationship between building material components and the development of algal species show the importance of the physical (porosity, absorption, etc.) and morphological (roughness, etc.) rather than the compositional characteristics of the materials.

Barberousse et al. (2007) evaluated susceptibility to the growth of green algae and cyanobacteria (blue algae) on various types of external façades. The final results indicate that porosity and roughness are the parameters which most influence the growth of algae on building materials. The most porous materials tend to shorten the periods of algal dehydration: for this reason the evolutionary capacity of the algae grows in proportion to the increase in the porosity of the underlying material. On the contrary, roughness facilitates the conservation and adhesion of the algae to the material. Venzmer et al. (2008) showed how the propensity for algal growth depends mainly on the hydration and desiccation of the microorganisms present on the infected surfaces. Algae appear earlier on hydrophobic than on hydrophilic substrates, since the moisture is completely available for the organism. The more frequent appearance of algae nowadays is also related to greater thermal insulation (Aelenei and Henriques, 2008), above all in buildings with low thermal inertia (Kehrer and Schmidt, 2008; Johansson, 2005; Johansson et al., 2010).

Various studies have been carried out regarding the use of antifungal chemicals to reduce the growth of algae and lichens. The results show that biocide treatment has an impact for about 3–4 years, that no significant differences have been recognized as regards the level of biodiversity (Shirakawa et al., 2002; Tretiach et al., 2007) and that the formation of biofilm is conditioned more by the environmental conditions (irradiance, humidity and temperature) than by the use of the biocide. Moreover most antifungal chemicals are non-specific to the organism affected and can have detrimental effects on the environment, including toxicity for plants and animals. Photocatalysis has recently been discussed as a solution to this problem. The antifungal activity of titanium dioxide (anatase) has been examined intensively on several fungal species (Markowska-Szczupak et al., 2011; Pacheco-Torgal and Jalali, 2011), and in comparison with other bio-cides (Fonseca et al., 2010). When TiO₂-based coatings are applied to exterior surfaces, such coatings also allow the microorganisms to be washed away by rainfall.

12.4 Potential toxic effects and ways they can be monitored

The organisms described so far are harmful for humans when they develop inside buildings, since people spend 90% of their time indoors. Numerous epidemiological studies have demonstrated that long-term exposure in unhealthy environments, subject to the proliferation of moulds and fungi, is one of the main causes of allergies and irritative reactions. This phenomenon is mainly due to the facility with which spores and their metabolic waste can be inhaled.

Moulds may cause allergy-based illnesses (rhinitis and conjunctivitis, bronchial asthma, extrinsic allergic alveolitis) and infectious diseases (Legionnaire’s disease, Pontiac fever) (Hoffman et al., 1993; Górny, 2004; Baughman and Arens, 1996). High concentrations which develop in very damp environments may be responsible for bronchial asthma and the onset of extrinsic allergic alveolitis. The same organisms present in surrounding environments may cause infectious diseases which are easily transmitted in closed and overcrowded spaces. Epidemics of ‘Legionnaire’s disease’ are particularly serious; this often fatal pneumonia is caused by the development and spread of Legionella pneumophila through air-conditioning systems (Fraser et al., 1977).

Studies conducted by Johanning et al. (1996), Górny (2004) and the World Health Organization (Heseltine and Rosen, 2009) show that the toxins produced by some types of mycetes (mycotoxins) may cause serious illnesses. In particular an association was found between the exposure to mycotoxins (satratoxins) produced by the metabolism of Stachybotrys chartarum and irritative symptoms (skin, eyes and mucous), asthenia and immunological alterations (Johanning et al., 1996). Subsequent experiments (Beyer et al., 1997) showed that damp environments favour not only the growth of spores but also the formation of harmful bacteria (mycotoxins). Among these, actinomycetes, which provoke granulomatous tissue infections, are very common. Mould fungi are found in the environment as mycelial filaments and spores, which are easily airborne (being between 0.5 and 50 |im in size) and are highly resistant to environmental stress (Górny, 2004). The micro-fungi most frequently encountered in closed environments belong to the genera Aspergillus, Alternaria, Cladosporium and Penicillium (Górny, 2004). In general it is possible to divide the illnesses provoked by moulds into three categories: mycoses, mycotoxicoses and allergies. Mycosis refers to fungal growth on human organs. In medicine the names of the various forms of this illness derive from the type of fungus it is caused by, followed by the suffix-osis (from mycosis). The most widespread diseases are aspergillosis and penicilliosis. Mycosis does not usually represent a threat for human health, although in the presence of immunodeficiency it can become a serious danger. The organs which are most subject to this illness are the skin, the respiratory organs, the eyes, heart, liver and kidneys and above all the digestive tract. The most important fungal species which can cause mycosis are (Senkpiel et al., 2000) Absidia sp., Aspergillus sp., Basidiobolus ranarum, Cephalosporium sp., Cladosporium sp., Fusarium sp., Mortierella sp., Mucor sp., Penicillium sp., Rhizopus sp., Scopulariopsis sp. and Verticillium sp.

Mycotoxicosis is an intoxication of the human organism caused by toxic substances produced by the fungi metabolism. Fungi are assumed to release these toxins to defend themselves against rival species. Reiß (1998) and Senkpiel et al. (2000) have indicated the main toxins produced by moulds: aflatoxin and ochratoxin A, patulin, citrinin, citreoviridin, sterigmatocystin and mycophenol acid. Aflatoxins, for example, may be created in the blood after the inhalation of dust or spores. Generally these toxins are not harmful for individuals, but if the intoxication becomes chronic the human body is no longer able to break down and expel these compounds. The most widespread disease of this type is cancer of the liver (hepatocellular carcinoma) (Reiß, 1998). Toxic responses by the organism, as a result of the inhalation of spores, generally occur with high concentrations, > 108 CFU/m³, which occur only in workplaces with high dust loads. Particular attention must be paid to the species Stachybotrys chartarum which has a high toxin content and may have toxic effects on the immune system even at concentrations as low as 10 CFU/m³ (Senkpiel et al., 2000). Moulds which produce these types of toxins are Aspergillus sp., Penicillium sp., Fusarium sp., Cladosporium sp. and Stachybotrys sp.

Allergies involve an exaggerated response of the immune system. The various types of allergy can be divided into four groups, according to the reaction which they provoke in the individual (Reiß, 1998; Kahle, 2000). Allergies in the first group are caused by the inhalation of allergens produced by fungi and lead to rhinitis and asthma; allergies in the second group are due to the reaction of antibodies towards the allergens that have been introduced into the organism. Unlike the first group, in this case the allergens are not inhaled but enter the body via the mouth. The allergies in the third group are caused by a complex combination of antigens and antibodies. This occurs approximately 4–6 hours after exposure and is therefore a delayed reaction. The allergies in group four come about when some kinds of lymphocytes develop sensitivity to an allergen.

The moulds which develop on the indoor surfaces of houses may cause allergies due to the inhalation of spores. In many types of fungi the spores are only 2 μm in size and can therefore easily penetrate the bronchial tubes, causing asthma. Spores which have a diameter of greater than 10 μm cannot arrive at the bronchial cavities but are retained in the mucous membranes of the pharynx and may give rise to allergic rhinitis. Moulds may also be responsible for the pathologies grouped together under the acronyms SBS (Sick Building Syndrome) and SHS (Sick House Syndrome). Redlich et al. (1997) and Mizoue et al. (2004) have conducted studies on a vast sample of subjects and have highlighted and classified the characteristic symptoms of SBS/SHS. Koskinen et al. (1999) showed that an increased risk of the onset of SHS is closely related to the level of damp and to the growth of moulds inside buildings. Engvall et al. (2002) showed that SHS is most frequent in places with a high level of humidity together with pungent odours and moulds. Kishi et al. (2009) have studied the possible causal relationship between SHS and indoor air quality and describe how the pathology is found more often in environments characterized by the presence of moulds. A survey of sampling methods for analysing the concentration of spores in the air, which is useful for assessing the risks connected with the presence of these organisms in indoor environments, is reported in Cabral (2010).

12.5 Remedial action and future trends

Moulds develop on surfaces as a result of both a critical level of RH and indoor temperature and the specific property of the building material to retain water (a_w) together with its capacity to be a nutritive substrate. To hinder the development of moulds different courses of action are required: control of the indoor climate conditions, avoiding fluctuations in temperature and RH% (Cabral, 2010) which favour an increase in the surface moisture content, choice of low-carbon, basic finishes and coatings with a limited adsorption capacity. It is also possible to resort to antifungal chemicals, although it should be borne in mind that they are effective for only about 3–4 years. Moreover most antifungal chemicals are non-specific to the organism affected and can have detrimental effects on the environment, including toxicity for plants and animals (Markowska-Szczupak et al., 2011). On the contrary, algae develop according to the temperature of the substrate, the availability of moisture on the surface, and the capacity of the substrate material to make the moisture immediately available. Even in this case, to avoid these problems, the most common course of action is to use biocides in paints. Photocatalysis has recently been discussed as a valid alternative to traditional antifungal chemicals (Markowska-Szczupak et al., 2011; Fonseca et al., 2010; Makowski and Wardas, 2001). Studies highlight that since the availability of moisture is the fundamental element for the development of algae, it is necessary to reduce the surface absorption capacity and to avoid materials which are characterized by surface roughness since they are able to increase moisture retention. When TiO₂-based coatings are applied to exterior surfaces, such coatings also allow the microorganisms to be washed away by rainfall (Pacheco-Torgal and Jalali, 2011).

Moreover, an adequate thickness of the exterior façade must be guaranteed so as to avoid the effects of surface undercooling, which is, in turn, a source of condensation on external surfaces. Considering the current tendency to insulate buildings, studies should therefore be concentrated on identifying external building façades which are able to minimize these potential negative effects.

12.6 Sources of further information and advice

This information is in two main parts. The first part gives some suggestions for further reading. The second part lists a number of institutions and agencies which can offer information.

12.7 References