The aim of this research work was determination of humidity impact on yeast and moulds survival in fibrous filters. It was revealed that water content of about - 53-113% stimulated growth of fungi, especially in case of moulds. In stable filters humidity conditions (50% of weight), a number of fungi reached I 04 CFU/cm2 after 84 days, with the most intensive growth during first 7 days of the experiment. In the case of very low humidity ( 13% and less), the growth of fungi was not observed.