This paper reports a critical analysis of the assessment of contaminant removal efficiency. Measurements have been carried out in a ventilated room equipped with a pine wood floor, which emits Volatile Organic Compounds (VOCs), considered as air pollutants in this paper. Thereafter, Computational Fluid Dynamics (CFD) simulations are performed. CFD results are compared to measurements to check their accuracy. Moreover, air quality within the ventilated room is numerically analysed via indices. From this, it is shown that the currently used contaminant removal efficiency index, ec, can especially reveal the characteristics of contaminant distribution. Unfortunately, ec is not suitable for assessing the contaminant removal efficiency of ventilation scenarios with different air flow rates. Indeed, its definition does not take into account absolute contaminant concentration levels. Furthermore, since the VOC concentrations found in this study were very low compared to the Lowest Concentration of Interest, no health risks could occur, and so, it was inappropriate to use the Indoor Air Quality index IIAQ. Therefore, a new formulation of ec is proposed. Its values are analysed taking into account the conformity degree of the ventilation rate with the nominal air flow rate of the ventilation systems handled. This gives a good tool to compare the contaminant removal efficiency whatever the ventilation rate.