The current work is related to improving security by developing a new type of fog sensors and devices. We investigate fogs and their ability to absorb and clean various pollutants from air, including chemical, biological, radiological and nuclear (CBRN) agents. Fog can be used very effectively for counteraction to terrorist attacks and weapons of mass destruction, as well as for prevention of industrial accidents and disasters. We present a device developed by us – Fog Detector 3 (FD-3), which main purpose is to continuously monitor the atmospheric air and to produce analogue signal proportional to the density of any present aerosols, including water fog. The main components of the device, as well as their connections and way of operation are described.