Aims:The dynamics of bioaerosol generation in specific occupational environments where mail is manually unpacked and sorted was investigated. Methods and Results:Total number of airborne particles was determined in four different size classes (0.3-0.5, 0.5-1, 1-5 and > 5 mu m) by laser particle counting. Time dependent formation of bioaerosols was monitored by culturing methods and by specific staining followed by flow cytometry. Besides handling of regular mail, specially prepared letters ('spiked letters') were added to the mailbags to deliberately release powdered materials from letters and to simulate high impact loads. These letters contained various dry powdered biological and nonbiological materials such as milk powder, mushrooms, herbs and cat litter. Regarding the four size classes, particulate aerosol composition before mail handling was determined as 83.2 +/- 1.0, 15.2 +/- 0.7, 1.7 +/- 0.4 and 0.04 +/- 0.02%, respectively, whereas the composition changed during sorting to 66.8 +/- 7.9, 22.3 +/- 3.6, 10.4 +/- 4.0 and 0.57 +/- 0.27%, respectively. Mail processing resulted in an increase in culturable airborne bacteria and fungi. Maximum concentrations of bacteria reached 450 CFU m(-3), whereas 270 CFU of fungi were detected. Conclusions:Indoor particle concentrations steadily increased during mail handling mostly associated with particles of diameters > 1 mu m. However, it was not possible to distinguish spiked letters from nonspiked by simple particle counting and CFU determinations. Significance and Impact of Study:The dynamics of bioaerosol generation have to be addressed when monitoring specific occupational environments (such as mail sorting facilities) regarding the occurrence of biological particles.