It is recommended to keep the ventilation filters dry. However there are manyventilating systems that cannot fulfil this requirement all the time. It is then interestingto know the state of filters in use, and how they perform during extreme wetconditions, especially at the end of their lifetime.This investigation includes used filters collected at the time of normal change offilters.The used filters absorbed a different amount of humidity. The particle filtrationefficiency was nearly the same as for unused filters (comparing only filter classF7/EU7). The efficiency of the filters was slightly decreased during the wetconditions compared with the dry conditions. The amount of contaminants in thefilters was probably a result of the external conditions.There was found thermotolerant moulds in all the filters. This is very different fromthe natural composition of moulds in this region.