Regression models of the time factor for the growth of mould fungi in pine and spruce sapwood are studied. The first group of models are based on the results of exposures of mould fungi in static humidity and temperature conditions indicating the time for the start of mould growth in wood. The second group of models are based on the exposures of mould fungi in fluctuating or alternating humidity and temperature conditions when limited models are presented. An aspect of surface quality (nutrient rich, nutrient poor) and wood species (pine, spruce) are included in the models. The growth of mould fungi on wooden materials depends mainly on the water activity (the air relative humidity), temperature, exposure time, presence of mould fungi and other organisms and the nutrient content of wood surface. In humidity exposures at RH above 80% (water activity above 0,8) of several weeks - months, the risk for the mould growth in pine and spruce sapwood exists when the temperature is between +5 and +50°C. Between -5 and +5°C, the growth of mould fungi is slow and possible only when the water activity is above 0,9. At water activity above 0.95 (RH 95%), the critical time for mould growth in wood is only some days, when temperature is between +25 and +45°C, and some weeks, when temperature is between +10 and +20°C. Within the same temperature and humidity conditions, the critical time for mould growth on nutrient rich wood is shorter than that of wood with low nutrient content In fluctuating humidity conditions when favourable and unfavourable conditions are alternating, the development of mould fungi is slower and the final rate of mould growth is lower as compared to the constant favourable condition

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