A general criterion to foresee whether the presence of an obstacle on the cloud pattern can influence significantly the hazardous distance resulting from the dispersion of a heavy hazardous gas has been proposed. This criterion requires comparing the characteristic dimensions of the obstacles with those of the cloud in open field conditions on a suitable dimensionless space. It can be useful to discriminate between accidental scenarios that can be conservatively simulated using an integral model to that requiring CFD models, to identify which obstacles can be safely neglected when representing the real environment's geometry in a computational domain without compromising the simulation reliability, and to estimate at a first glance the minimum size of a mitigation barriers able to induce a significant change in the hazardous distance.