Despite the augmented safety offered by wearing a cyclist crash helmet, many cyclists still refuse to wear one because of the thermal discomfort that comes along with wearing it. In this paper, a method is described that quantifies the ventilation characteristics of a helmet using tracer gas experiments. A Data-Based Mechanistic model was applied to provide a physically meaningful description of the dominant internal dynamics of mass transfer in the imperfectly mixed fluid under the helmet. By using a physical mass balance, the local ventilation efficiency could be described by using a single input-single output system. Using this approach, ventilation efficiency ranging from 0.06 volume refreshments per second (s-1) at the side of the helmet to 0.22 s-1 at the rear ventilation opening were found on the investigated helmet. The zones at the side were poorly ventilated. The influence of the angle of inclination on ventilation efficiency was dependent on the position between head and helmet. General comfort of the helmet can be improved by increasing the ventilation efficiency of fresh air at the problem zones.