In the field of industrial production and processing, a large number of hazardous substances are used and handled. For this reason, there is a great risk of leakage to the surrounding environment. The aim of the diploma thesis is to evaluate the impact of the selected environment due to leakage of hazardous substances and to observe and compare the leakage and spreading of the hazardous substance in a major industrial accident using the ALOHA simulation geographic information system. The diploma thesis compares the different situations in the accident with carbon monoxide of November 16, 2006, in the company U.Steel Košice, s.r.o, and the simulated accident with parameters from 2006, only under current meteorological conditions. Through simulations, it has been found that the greatest impact on the size of the affected area, and hence the negative impacts, are the wind direction and humidity the main factors.