## 3 Results and Discussion  
Effect of Constant RH and Temperature on Mould Growth Development at 'Clean' Conditions (Experiment I)  
Moulds are present nearly anywhere [13]. Therefore, objects are exposed to spores also indoors regardless of any locally noticeable mould contamination. At certain conditions, spores can germinate causing mould growth. The investigation of the effect of RH and temperature at such 'clean' conditions has been rarely reported due to ambiguity caused by unknown variety and number of spores in the air, which can influence reproducibility [36]. However, the situation without artificial mould inoculation is more natural and possibly can even provide a more reliable estimate of the period required for the mould development indoors. In the study, a scenario of sudden change in RH and temperature to values, which are favourable to mould growth, is investigated for 'clean' indoor conditions. The results are compared with the widely used VTT model developed by [PERSON] and [PERSON] as a reference [24]. The obtained results presented in Figure 2 show the days necessary to reach mould germination (Figure 2a) and visible mould growth (Figure 2b) on wood specimens depending on different ambient air conditions for both situations (experiment I and calculation of VTT model). Figure 2: Effect of temperature (10, 20 and 30 \({}^{\circ}\)C) and RH (91% and 97%) on the time necessary to reach (**a**) mould germination and (**b**) visible mould growth stages in the experiment I (‘Dry’) and calculated according to the VTT model [24]; light blue dots on the T and RH plane are the corresponding projections of the results. The results show that both RH and temperature are important factors, which significantly influence the rate of mould growth. By decreasing temperature and RH, a longer period is necessary for mould to develop. This agrees with the overall knowledge [30]. The results showing the difference between 'clean' conditions and the prediction of the VTT model are quite interesting. At lower temperature, the difference is large; however, at higher temperature, it is relatively small or even insignificant. At 10 \({}^{\circ}\)C, it takes on average 108 days at RH 91% and 75 days at RH 97%, to reach the mould germination stage in the case of the 'clean' conditions, which is 2.7 and 4.4 times longer than it is predicted by the VTT model. Concerning the stage of visible mould growth, the difference is smaller; however, still significant: 1.6 (at RH 91%) and 2.7 (at RH 97%) times longer period is necessary at the 'clean' conditions. In contrast, for the highest temperature (30 \({}^{\circ}\)C), the differences between the 'clean' conditions and the VTT model in most cases are insignificant, except for the mould germination at RH 91%. This indicates that in the case of higher temperatures (30 \({}^{\circ}\)C), the spores present in the air can develop on the wood as effective as the ones which are inoculated onto the wood surface in a relatively large amount prior to the test. This could be explained by the increase in the probability of the spore germination, which at optimal conditions can be close to 100%; therefore, the effect of spore concentration is becoming less decisive [37]. Furthermore, the results suggest that the initial spore concentration (spores in the ambient air versus spore suspension of high concentration) for lower temperatures (10 and 20 \({}^{\circ}\)C) has a significant influence only on the mould germination stage. Further mould development up to visible mould growth is more influenced by other factors. In addition, the results show that the period form mould germination to visible mould growth is even shorter in the case of 'clean' conditions. These results could be explained by the potential reciprocal influence of the mould fungi among each other, which can interfere with the mould development in the case of mixed mould monocultures [38]. Further discussion of the subject will be continued in Section 3.3.