a method for fire detection in enclosed environments subject to explosion protection regulations out of which an air - dust mixture is filtered while the concentration of dust of the air - dust mixture is lowered to under the explosion limit by a filter and the aspirated air - dust mixture is checked for burning characteristics and if appropriate a fire alarm is triggered , in which the function of the filter is monitored , the fire detection is carried out by use of an aspirating smoke detector system not approved for use in potentially explosive areas but instead is deployed outside the ex - zone and if the filter is destroyed a signal processing unit of a malfunction monitoring sensor generates a signal which initiates a subsequent action preventing aspirated air - dust mixture with an elevated concentration of dust from entering the aspirating smoke detector system .