In this study the effects of temporal variations in individual work steps for the DAPI total cell count approach were investigated. It was shown that 24-hour storage (4 degrees C) of the original bio-aerosol samples (loaded polycarbonate filters) did not result in quantification differences. Therefore, a period of 24 hours (e.g. time of transport) before further processing in the laboratory is tolerable. The reaction time of formaldehyde for cell fixation should be at least 24 hours. Subsequent to the fixation procedure, the samples can be stored at 4 degrees C for up to two years without any considerable changes in the detection efficiency. Staining time with DAPI should be at least 30 minutes. The DAPI-stained bioaerosol samples, however, must then be analysed microscopically within several hours because storage at room temperature, even in the dark, will lead to substantial detection losses. The results of this study provide sufficient evidence to describe a uniform standardized procedure for the quantification of a work-related microbial exposure level using the DAPI total cell count approach.
