<p>This study numerically investigates the effect of exhaust gas recirculation (EGR) and fuel-premixing ratio on the PAH emissions (naphthalene, benzo[a]pyrene, phenanthrene, acenaphthene, pyrene, benzo perylene, chrysene, and benzo [g,h,i]perylene), its precursors (such as C₂H₃, C₂H₂, C₄H₅, C₄H₃, C₆H₅, C₆H₆), and their toxicity potential from conventional diesel and hydrogen/diesel dual-fuel (HDDF) engine. The aim of this investigation is to understand the effect of EGR mass fraction and hydrogen energy share (HES) on the toxicity potential of PAHs and total PAH mass emission in conventional diesel and dual-fuel combustion under different loading conditions. This study is focused on the thermal, chemical, and dilution effects of EGR with and without HES on PAH formation. The simulations are performed on ANSYS Forte using a detailed chemical mechanism of diesel surrogate (66.8% n-decane/33.2% alpha-methylnaphthalene). The reaction mechanism used for simulation consists of 189 species and 1392 reactions. Results demonstrate that as the EGR increases from 10 to 30% in conventional diesel combustion, the toxicity equivalent potential of PAHs increases by 25% at lower engine load. However, at a fixed engine load and a constant EGR level of 30%, increasing the hydrogen energy share (HES) to 30% results in a 33% reduction in the toxicity equivalent potential of PAHs. This reduction highlights the potential benefits of hydrogen addition alongside EGR in future hydrogen–diesel dual-fuel engines. Additionally, the incorporation of hydrogen significantly reduces the mass emission of PAHs.</p>