Emissions from dust collected in air filters have been investigated using in situ and chambermeasurements. Two air filters (class F6 and F8/9) were exposed to outdoor air for a period ofsix months, after which measurements were carried out during continuous and intermittentoperation. Air samples were taken upstream and downstream of the filters and analysed forseveral substances, including VOCS, formaldehyde, microbial VOCS (MVOC), vital microorganisms,ergosterol and endotoxin. Particles and fungus spores released while starting thefan were monitored using an optical particle counter and slit samplers.No significant change of VOCS, aldehydes or MVOCS could be measured across the filterswhile in use. Particulate matter, including endotoxins, ergosterol and spores, was reduced. Inthe intermittent mode, a moderate and short-lived increase of particles was briefly measurabledownstream of the filters after starting the fan. In the chamber experiments, the filters werefound to be a source of various VOCS, such as aldehydes and MVOCS. Nevertheless, mycologicalexamination of the filters did not show any active microbial growth.This investigation does not indicate any deterioration of air quality due to outdoor dust in theair filters, even though they were occasionally dampened. The large amount of air passingthrough a filter in use obviously dilutes the gaseous emissions found in the chamber experimentsto very low concentrations of minor importance. Finally, it should be stated that thesefilters were only moderately dirty and that this study does not include odour or other perceivedair quality parameters.