Microbial growth in stored dust samples as a function of the relative humidity

House dusts, collected for detection of microorganisms in buildings, sometimes have to be stored before measurement. It is possible that continued growth of microorganisms in the stored samples will increase the levels of CFU, chemical marker, biomass etc. The aim of this investigation was to assess the influence of relative humidity on microbial growth in dust samples. We have exposed dust samples to different RH and measured their thermal activity with isothermal microcalorimetry. We also measured concentrations of the three microbial markers ergosterol (fungi), 3-OH fatty acids (Gram-negative bacterial), and muramic acid (Gram-positive bacteria) by GC-MS-MS. The calorimetric results showed that there were a significant biological activity in samples kept at 75 and 94% RH, but not at 54% RH. The marker for Gram-positive bacteria only showed an increase at the highest RH (94%). No changes were seen in the marker for Gram-negative bacteria. We conclude that at least fungi will continue to grow in dust which is kept at 75% RH and higher. Significantly higher amounts of ergosterol and other measures of fungal growth may be found in such samples after storage with higher humidity.