Toxic metals and unregulated emissions from IC engines (ICEs), often overlooked in conventional regulatory frameworks, demand attention due to their potential health risks, even at trace levels.The present study conducts a comparative assessment of the toxicity of metal, unregulated, and particulate matter (PM) emissions from both conventional diesel (CDC) and gasoline/methanol-diesel (GD/MD) reactivity-controlled compression ignition (RCCI) engines.Instrumentation and modification are done to a single-cylinder automotive diesel engine to enable it to operate in RCCI mode, which involves integrating a port fuel injection system and installing pressure sensors.Gasoline and methanol are injected into the port during the intake stroke, and early direct injection of diesel is done to facilitate the transition to RCCI mode.The operational load of the engine and the premixed fuel ratio are systematically varied to investigate their effects on emissions and the associated toxicity levels.An in-vitro cytotoxicity assessment is performed using a human lung epithelial cell line (BEAS-2B) to evaluate the cytotoxic potential of PM emissions produced by both diesel and RCCI combustion modes.To evaluate the toxicity associated with metal and unregulated emissions, the potential inhalation cancer risk is assessed utilizing risk assessment formulations prescribed by regulatory bodies.Furthermore, the environmental risk associated with unregulated emissions is also assessed.The findings reveal that methanol-diesel RCCI shows promise in mitigating the cytotoxic effects of PM, particularly at medium load conditions, compared to both CDC and gasoline-diesel RCCI.Moreover, at lower engine loads, RCCI combustion exhibits a decrease in the cancer risk potential associated with metals compared to CDC combustion.Compared to CDC under medium load conditions, the RCCI combustion demonstrates a reduction in emissions equivalent to global warming, acidification, and eutrophication potentials.Despite its advantages, RCCI combustion engines exhibit higher levels of ozone-forming potential and cancer risk potential in their non-regulated emissions than CDC engines.However, as engine load increases, these potentials decrease in RCCI engines.Increasing the fuel premixing ratio under constant load conditions leads to a rise in both ozone-forming potential and cancer risk potential associated with non-unregulated emissions from RCCI engines.