The fine filter (EU7) was included in the ventilation system which operated at full capacity only during working days. The first three months the filter was located in the ventilation system of the office building and it was later moved for one month period into the laboratory chamber. The aim of the study was to investigate whether particles or microbes are released from the filter during the turn off and start up of the fan. The released particle (size range of 0.3 m) concentration was below 10 #/dm 3 in normal field conditions.