This study presents two methodologies which can be used to determine the classification and extended area of hazardous zones caused by gas, vapours and mists. The first is based on UNE 60079/10/1: Electrical apparatus for explosive gas atmospheres - Part 10: Classification of hazardous areas, whilst the second is developed on the basis of Computational Fluid Dynamics (CFD) using the FLUENT software application. Both methodologies were applied in the same case study of differing leakage components from a dairy farm steam boiler room fuelled by liquid natural gas (LNG). The results obtained show that CFD methodology is a powerful tool with a significant capacity for determining the size of an explosive atmosphere for a broad range of exhaust sources. This methodology offers more conservative results than, those obtained from the analytical methodology recommended in Standard UNE 60079/10/1. Results obtained using CFD are more useful in enabling the study of turbulence phenomena, boundaries, and diverse initial and contour conditions. In contrast, the Standard UNE 60079/10/1 methodology is less conservative and aims at determining the hypothetical volume V-z of the explosive atmosphere. This volume is a measurement of the ventilation efficiency which is in turn proportional to a massive gas release through an exhaust source divided by the number of air changes in the enclosed area. From the results obtained, it can be confirmed that Standard UNE 60079/10/1 should be revised. (C) 2013 Elsevier' Ltd. All rights reserved.