## 1 Introduction
In general, none of these previously mentioned factors concerning nutrients could eliminate mould growth as the nutritional requirements are usually minimal and satisfied either by the material constituents or by a minor contamination of the surface by dust or other deposit [13]. Therefore, environmental conditions--relative humidity (RH) and temperature--and time are the limiting factors influencing mould growth. The combination of them determines the probability and the extent of the mould development. Indoors, the environmental factors can be relatively easily controlled, and prevention of persistent dampness, elimination of moisture intrusion and temporary leaks prevent mould proliferation [2]. A special situation is when wood is subjected to water disaster such as domestic accidents, floods or firefighting activities resulting in high RH and prolonged periods of increased moisture content. In the literature, it is intensively discussed that the RH of the ambient air cannot represent the real situation near the surface where the actual biological activity happens [7]. Therefore, in microbiology, \(\mathrm{a_{w}}\) is adapted, which characterises the amount of free water available to microorganisms, RH of the air and also the moisture content of the material [6]. The \(\mathrm{a_{w}}\) is undoubtedly the most important factor in determining whether or not mould growth is initiated [3]. RH (expressed as a fraction) and \(\mathrm{a_{w}}\) are equal in the situations when a system reaches the state of moisture equilibrium [13]. For simplicity, the RH and \(\mathrm{a_{w}}\) at equilibrium conditions will be both referred as equilibrium relative humidity (ERH) and in the case of non-equilibrium conditions, the terms RH and \(\mathrm{a_{w}}\) will be used. Mould growth requires specific moisture condition. Depending on ERH and \(\mathrm{a_{w}}\), different outcomes are possible: no mould growth, mould germination (observed only with microscope) and visible mould growth. It has been found that the limits for each outcome are as follows: for ERH below 80% mould cannot develop; for ERH above 80%, spore germination takes place; and for ERH above 85%, visible mould growth can develop [7; 23; 24]. According to other sources, mould does not grow at ERH \(<\) 75% at 20 \({}^{\circ}\)C [25]. It is also believed that the lower limit of \(\mathrm{a_{w}}\) for fungal growth on wood and wood composites is 0.78 at 20-25 \({}^{\circ}\)C, and the germination process requires a slightly (approx. 0.02) higher \(\mathrm{a_{w}}\) than the critical \(\mathrm{a_{w}}\) for growth [3]. The differences in the results can be explained by a different experimental design. In addition, it should be noted that each fungal species has a specific minimum \(\mathrm{a_{w}}\) to grow. Moderately xerophilic fungi begin to grow at a \(\mathrm{a_{w}}\) of 0.75-0.79, but the minimum \(\mathrm{a_{w}}\) for hydrophilic fungi is 0.90 [26]. Indoors, the RH can significantly fluctuate. Even in such a well-controlled environments as museums, RH can fluctuate in relatively broad range: 10-80% [27]. It seems that the mould growth is almost impossible; however, as mentioned above, the main parameter is \(\mathrm{a_{w}}\), which can be much higher due to surface condensation or accidental moistening. The studies have shown that in the case of surface wetting, favourable conditions of mould growth can be maintained for a relatively long period, but it may only take a few days for a fungus to germinate and begin to grow [7; 13]. These aspects have had only minor attention in the literature concerning wood. Water is undoubtedly the most important factor for initiation of mould growth; however, temperature also has a major effect since biochemical processes are highly dependent on the metabolic activity of mould growth [1].