For many years, there has been debate, both in the scientific and user communities, on the influence of the flow pattern on the adsorption behaviour of a filter filled with activated carbon. Some authors reported significant differences in the breakthrough times of gases and vapours, whereas others did not. Some laboratories are switching to testing with a pulse flow, but this is not without problems. Hence, the goal of this work is twofold: to understand the influence of a real flow pattern on the breakthrough time for organic vapours of a gas mask filter and to ascertain if this influence can be modelled. The work demonstrates that the difference in breakthrough time is due to the influence of the flow pattern on the width of the mass transfer zone, via its impact on adsorption kinetics. Within the validity domain of the Wheeler-Jonas equation, this effect can be easily and accurately modelled, giving excellent estimates of the experimental breakthrough times. There are still some issues with using this model in the presence of ambient water vapour, as the water uptake from a pulsed air flow cannot be estimated correctly yet.