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. It is crucial to study the specific sizes of fog droplets, which optimally collect dangerous substances. We present a computational fluid dynamic model for estimating droplet number flow rate through a laser beam. It shows, that we can make a quantitative assessment of both the number flow rate and the droplet diameter distribution. After we have confirmed that it is possible to monitor these parameters, we proceed and investigate how they are varied, when the laser beam’s distance from the nozzle is changed. Thus, by selecting the distance between the laser beam and the nozzle, it will be possible to measure the diameters and the number of fog droplets. Different nozzles can be simulated easily by entering their output parameters (mass flow rate, diameter distribution, orifice diameter) in the setup.