This project aims to study and present the findings obtained from the reviewed literature materials regarding 13 unregulated substances, which are Acetaldehyde, Acrolein, Benzene, 1,3-Butadiene, Ethylbenzene, Formaldehyde, Naphthalene, Toluene, Xylene, Acetone, Benzo[a]pyrene, Phenanthrene and Propionaldehyde. These substances are either aromatic hydrocarbons or carbonyl compounds, and are found to be toxic, mutagenic, genotoxic, carcinogenic and/or teratogenic to humans. This report will present the statistics done by gathering emission data from other experiment-based journal articles and compare the emissions of these 13 substances when a diesel engine is running on biodiesel blends (with petroleum diesel as base fuel) and that of pure petroleum diesel. The results are inconsistent most of the time and the reason of this is considered to be the varying experimental setups and testing fuel chosen. Although the data are as such of no statistical significance due to the above reason, the conclusion of B20 and B100 would be giving the least types of these unregulated substances is still presented as the result of the data analysis of within this report. Other than that, this report will also be presenting some relevant information regarding these unregulated substances as well as the formation theories of them in a diesel engine. It is summarized that switching from petroleum diesel to biodiesel fuel is an imperative action and in order to be fully prepared for such change, it is necessary to have repeated and extended researches done for assessing the use of biodiesel fuels.