Modelling propagation of hazardous substances released in road accidents is important issue from the point of view of safety in road transport. The paper presents use of the Geographic Information Systems to analyse the impact of the emissions of hazardous compounds in the built-up area. In the paper dispersion of pollutants released in a road accident, involving a chlorine tanker truck, followed by chlorine spillage has been analysed. It is assumed that the accident took place in a mountainous area on a bypass within a built-up area. Concentrations of pollutant have been calculated in the own program. The calculation of field of wind speed was taking into account the terrain elevation and type of land cover. The package Idrisi Taiga has been applied in the pre-processing stage. It was used to generate a site map with buildings and aerodynamic roughness map. Meteorological conditions were also taken into account: wind speed and direction, temperature, atmospheric pressure and atmospheric stability class. The computer program consist two basic modules: diagnostic model of air velocity field and Lagrangian model of particles, being the original implementation of the Lagrange model of particles. In both models, some elements of mathematical modelling were applied. The models have been successfully validated and verified. The results of the calculations of concentrations of chlorine have been analysed in the Geographic Information System. The following analyses have been made, among others: designation of hazardous zones AEGL 1-3, determination spatial size of built-up areas within the zone, analysis of changes in pollution concentrations over time. The proposed analyses allows to determine areas exposed to deadly and highly dangerous concentration of chlorine.