# Characteristics of Mould Growth in Pine and Spruce Sapwood and Heartwood under Fluctuating Humidity
## 1 Introduction  
Mould is a small organism that occurs on many types of surfaces and is widely disseminated through spores. The mould is important in the biochemical cycle of elements, the movement and transformation of chemical elements, and compounds between living organisms, and it enables the decomposition of materials and therefore is necessary in life. For humans, they can cause several health problems, especially immunodeficiency ones [1; 2; 3; 4; 5]. The most dangerous mould for human health is species such as _Apergillus_ spp. or _Stachylotris_ spp. [6]. In buildings, they can lead to the deterioration of interior and exterior finishes as well as other structural components. This deterioration can have adverse effects on both the physical condition of the building and human health, resulting in economic and social costs. Additionally, the presence of mould is indicative of high humidity, which can accelerate the degradation of structures and lead to an uncomfortably humid environment for occupants. Because of the above, knowledge of the probability of occurrence of mould in a building is very important. The mould requires for its life available nutrients and humidity as well as a sufficient pH [7; 8]. The amount of humidity required for the germination and subsequent development of mycelium and the dissemination of mould vary depending on the species [9]. The amount of humidity on finishing materials is dependent on the ratio of environmental humidity and temperature. Generally, the building materials in conditions below 80% relative humidity (RH) do not support mould growth [10]. In order to assess the parameters that contribute to mould growth on wood, it is necessary to consider the effect of temperature and RH. To accurately simulate the conditionsthat occur in real building conditions where mould growth occurs, it is necessary to employ a dynamic prediction model of mould growth that includes changing conditions over time. Furthermore, it is crucial to consider the specific species of wood, the portion of the tree from which the samples were obtained, and the direction of the cut [11; 12; 13; 14]. Furthermore, the species of wood, the part of the tree, and the method of cutting the sample are important information [15; 16]. The more important factor for mould germination is the moisture of the substrate rather than the environment RH [17]. In general, a higher humidity is required for germination and then for subsequent growth [18]. The minimum humidity requirements to growth are dependent on the species, which are divided into three groups. Hydrophilic fungi require relative humidity above 90%, moderately xerophilic fungi begin to grow at RH 75-79% and slightly xerophilic fungi need RH in the range of 80-89% [19; 20; 21]. For example, _Aspergillus_ spp., _Penicillium_ spp. and _Alternaria_ spp. begin germination around 85% RH [22; 23; 24; 25]. These moisture ranges for mould growth were calculated from the experiments based on laboratory experiments on nutrient agar, where the nutrient condition is optimal [26; 27; 25]. For building materials, where nutrient availability is not optimal but rather limited, the requirement for available moisture is probably slightly higher than mentioned above [28; 29]. Moisture requirements are also related to temperature; at lower temperatures, the mould requires more available humidity to germinate and grow [14; 30]. The growth of mould is also dependent on temperature even when other conditions remain constant [14; 31]. A reduction in temperature from 20 to 5 \({}^{\circ}\)C has been observed to result in a fourfold reduction in the rate of mould growth. According to Ref. [14], the other requirements for mould grow are nutrients such as sources of carbon, nitrogen, etc. Especially when glucose is available as a carbon source, germination is faster [32]. The majority of published results pertaining to mould growth are focused on pine wood [11; 15; 33; 34].