Mould growth on building materials under low water activities: Influence of humidity and temperature on fungal growth and secondary metabolism

The influence of relative humidity (RH) and temperature on growth and metabolism of eight microfungi on 21 different types of building material was investigated. The fungi were applied as a dry mixture to the materials, which were incubated at 5degreesC, 10degreesC, 20degreesC and 25degreesC at three humidity levels in the range 69-95% RH over 4-7 months. The lower limit for fungal growth on wood, wood composites and starch-containing materials was 78% RH at 20-25degreesC and increased to 90% RH at 5degreesC. An RH of 86% was necessary for growth on gypsum board. Ceramic materials supported growth at RH > 90%, although 95% RH was needed to yield chemically detectable quantities of biomass. Almost exclusively only Penicillium, Aspergillus and Eurotium (contaminant) species grew on the materials. Production of secondary metabolites and mycotoxins decreased with humidity and the quantities of metabolites were insignificant compared with those produced at high RH (RH > 95%), except in the case of Eurotiurn.