Release of toxic substances may cause comprising and lethal casualties. Dispersion models of different types are often used to estimate the injury outcome as well as the risk area following these events. Some gases may, either by high molecular weight or through leakage from a pressurized container, act as dense gases which means that they possess higher density than the surroundings. This will, in turn, result in a gas plume that is horizontally widened and located close to the ground. PUMA is a real-time model for atmospheric dispersion of hazardous substances which utilizes Gaussian puffs to represent the resulting plumes. A detailed description of the dense gas implementation into PUMA is given in this report. Physical processes such as buoyant effects, turbulence modifications, thermodynamics and puff deformation are included. Quantitative comparisons with both large scale field experiments and detailed CFDsimulations serve as validation.