MOULD GROWTH PREDICTION BY COMPUTATIONAL SIMULATION

In buildings growing conditions for mould fungi can occur and cause fungus infestation. The possible danger for the occupants of dwellings lies in the production and spreading of pathogens (disease causing agents). Therefore, consequent measures have to be taken to avoid health dangers that result from mould fungi in buildings. A strategy has to be set up that focuses on the growth conditions for mould fungi and also considers the complex transient processes of building physics. The boundary conditions for the growth of fungi are temperature, humidity and substrate conditions which have to be simultaneously favourable over a certain period of time. In the Fraunhofer-Institute of Building Physics (IBP) in Germany a biohygrothermal procedure is developed, which allows the prediction of mould growth under transient boundary conditions. In order to differentiate the mould fungi according to the health dangers they may cause, so called hazardous classes were defined. This new method will be described with its new features and its limitations. Typical results of this model are shown within an example. Furthermore the significance of the results are discussed. INDEX TERMS Mould; Health effects; Modeling INTRODUCTION The application of biocides is always accompanied by additional health risks, especially when used indoors, and moreover can prevent the formation of mould fungus only over a limited period of time. A prerequisite for preventing mould fungus without the use of biocides is the knowledge of the boundary conditions under which fungus growth takes place. In reference to the boundary conditions for the growth of fungus it turns out that the decisive parameters of influence like humidity (Grant, 1989) and temperature (Smith,1982) as well as the substrate (Ritschkoff, 2000) have to be available over a certain period of time simultaneously in order to enable the formation of mould fungi. Therefore, the main focus of this scientific paper on hand is to develop a planning instrument from the point of view of an engineer that aims at predicting the formation of mould fungus. This procedure consists of two consecutive predictive models, i.e. the Isopleth model and the transient Biohygrothermal model. HEALTH ASPECTS OF MOULD People are exposed to mould spores in the air they breathe daily; however, sometimes moulds grow excessively in certain areas and can cause different types of illnesses (Pasanen, 2001). The most prevalent affect of mould on human health is caused by the allergenic impact of its spores (Horner, 1995). Some moulds are more hazardous than others. Different people have different responses to mould exposure. In particular, those with allergies, existing respiratory conditions or suppressed immune systems are especially susceptible to health problems. In addition, some moulds produce chemicals called mycotoxins, which can cause flu-like symptoms. It should be noted that the causes and effects of mould exposure on people are not very well understood. For this reason mould growth should be restricted as far as possible. GROWTH CONDITIONS FOR MOULD German literature often states a relative humidity of 80% at wall surfaces as decisive criterion for mould growth, independent of temperature. Sometimes it is mentioned that many types of mould can also thrive at lower humidities (see for example the new draft of DIN 4108-X, Mould (Deutsches Institut für Normung, 1999)). Other growth conditions, namely a suitable nutrient substrate and a temperature within the growth range are taken for granted on all types of building elements usually. The growth conditions for mould may be described in so-called isopleth diagrams (Ayerst, 1969). These diagrams describe the germination times or growth rates. Beyond the lowest line every mould activity ceases, under these unfavorable temperature and humidity conditions spore germination or growth can be ruled out. The isopleths are determined under steady state conditions, i.e. constant temperature and relative humidity. The three factors required for growth – nutrients, temperature and humidity – must exist simultaneously for a certain period of time; this is the reason why time is one of the most important influence factors. It is assumed that germinable spores are present in most cases. This means that mould growth will occur when hygrothermal growth conditions are fulfilled. ISOPLETH SYSTEMS Significant differences exist among the various fungus species. Therefore, when developing common Isopleth systems all fungi were regarded that can be detected in buildings. Quantitative statements on the growth prerequisites temperature and humidity will be set up for these more than 150 species that fulfil both features, as far as they are given in literature. Within the Isopleth model the prerequisites for the growth of mould fungi in dependence of temperature and relative humidity are stated for the above mentioned hazardous classes at first for the optimal culture medium. The Isopleth systems are based on measured biological data and also consider the growth prerequisites of all fungi of one hazardous class. The resulting lowest boundary lines of possible fungus activity are called LIM (Lowest Isopleth for Mould). In order to regard the influence of the substrate, that is the building materials or possible soiling, on the formation of mould fungus, Isopleth systems for 4 categories of substrates were suggested that could be derived from experimental examinations: Substrate category 0: Optimal culture medium; Substrate category I: Biologically recyclable building materials like wall paper, plaster cardboard, building materials made of biologically degradable raw materials, material for permanent elastic joints; Substrate category II: Biologically adverse recyclable building materials such as renderings, mineral building material, certain wood as well as insulation material not covered by Ι; Substrate category III: Building materials that are neither degradable nor contain nutrients. For the substrate category III no Isopleth system is given since it can be assumed that formation of mould fungi is not possible without soiling. In case of considerable soiling, substrate category I always has to be assumed (Fig. 1 left). Persistent building materials with high open porosity mostly belong to substrate category II. The basic principle of the new method and of defining the building material categories is to assume a worst case scenario, therefore always being on the safe side in respect of preventing the formation of mould fungi. To what extent correcting the Isopleth systems for individual building material categories towards increased relative humidity can still be done with a clear conscience, has to be proved by further measurements. Figure 1. Isopleth systems for 3 categories of substrates (left), in order to regard the influence of the substrate on the formation of mould fungus (Sedlbauer, 2001) and Isopleth systems for the so called critical fungus species (right). In order to differentiate the mould fungi according to the health dangers they may cause, a so called hazardous class K will be defined as follows (Krus and Sedlbauer, 2002): The isopleth system K applies to mould fungi, which are discussed in the literature because of their possible health effect (Fig. 1 right). For these species (Aspergillus fumigatus, Apergillus flavus and Stachybotrys chartarum) growth data from (Sedlbauer, 2001) are available. The isopleth system for the fungi estimated as critical is based on the available data on optimum culture medium. To make a adequate substrate specific isopleth precise measurements are missing. To move the isopleth to a higher humidity analogically to the development of the imagined building material isopleth, is too risky especially for these fungi according to