A three-dimensional mathematical model is developed for heat and mass transfer in local non-uniform hydrogen-air mixtures that are formed during continuous release of hydrogen in a room. A comparison between simulation and experimental results is performed. Numerical investigation of characteristic parameters' fields is made. Influence of vent opening and local burning upon mass concentration fields of hydrogen is investigated. The formula for calculation of the height of the local explosion danger zone that is formed above the source of hydrogen is obtained. The influence of mechanical and natural ventilation on parameters of hydrogen-air mixture is discussed. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.