Nowadays, Volatile Organic Compounds (VOC) are a class of air pollutants increasingly studied in the scientific literature, due to the well-known impact on human health and the environment. They can be emitted from different sources, including various types of industrial plants, such as oil refinery and petrochemical plants. For these reasons, the attention of control agencies on VOC emissions has increased and with it the request to quantify the total amount of VOC emitted by a plant, to monitor and to reduce this emission. However, several critical aspects still exist related to the classification of VOCs and their quantification. Indeed, different definitions of VOC are reported in scientific and technical literature. These diversities result in multiple strategies for quantifying the VOCs emitted by an industrial plant. Therefore, this paper reviews the available regulation about the quantification of VOC emitted from industrial plants, focusing on refineries and petrochemical plants, to underline some formal inconsistencies in VOC estimation procedures. From this comparison, it appears clear that VOC quantification can be addressed in different manners, obtaining a substantial non-uniformity of the VOC emission data. The data difference can conduct to a misleading estimation of emission rate.