Lattice-gas cellular automata are often considered as a particular case of cellular automata in which additional constraints apply, such as conservation or spatial exclusion of particles. But what about their updating? How to deal with non-perfect synchrony? Novel definitions of asynchronism are proposed that respect the specificity of lattice-gas models. These definitions are then applied to a well-known swarming rule in order to explore the robustness of the model to perturbations of its updating.