<p>Despite the global transition toward electrification, diesel engines continue to play a critical role in heavy-duty transportation, maritime operations, and decentralized power generation. Consequently, nitrogen oxides (NOₓ) emissions from diesel combustion remain a major contributor to urban air pollution and associated public health risks. However, research on diesel emissions, atmospheric chemistry, and pollution-related health effects has largely progressed in separate disciplines, limiting a comprehensive understanding of the pathway linking engine emissions to human health outcomes. This review addresses this gap using a structured Source–Pathway–Receptor framework that connects diesel combustion processes, atmospheric transformation of NOₓ, urban exposure dynamics, and biological health effects. The review synthesizes findings from combustion science, atmospheric chemistry, exposure assessment, epidemiology, and environmental health research to explain how diesel-derived NOₓ evolves from an engine-level pollutant into a complex urban health concern. The analysis highlights major factors influencing population exposure, including near-road pollution gradients, street-canyon effects, and discrepancies between laboratory certification and real-world driving emissions. Quantitative epidemiological evidence linking short-term and long-term NOₓ exposure with respiratory and cardiovascular outcomes is also discussed. In addition, the review evaluates current mitigation approaches, including in-cylinder combustion strategies, exhaust after-treatment technologies, fuel-based solutions, urban planning measures, and exposure-reduction policies. Future research priorities are identified to support stronger integration between emission-control technologies, air-quality management, and public health protection. Overall, the review provides an interdisciplinary perspective for understanding and mitigating diesel-related NOₓ emissions in modern urban environments.</p>