<p>A growing body of epidemiological evidence links maternal exposure to air pollution with an increased risk of adverse pregnancy outcomes, such as preterm birth and low birth weight. Cerium dioxide nanoparticles (CeO₂ NPs or nanoceria) are emerging pollutants, used as additives in diesel fuels and cigarettes for their catalytic properties, and released into the environment. Due to their high surface-to-volume ratio and reactivity, CeO₂ NPs develop a surface coating during combustion, which may incorporate other released fuel-borne chemicals, such as benzo[a]pyrene (BaP), a known carcinogen, mutagen and reprotoxicant, raising concerns about their combined impacts on human health. To better reflect environmental reality, we produced BaP-coated CeO₂ NPs and exposed primary human trophoblasts and chorionic villi. Our findings show that BaP-coated CeO₂ NPs activate the aryl hydrocarbon receptor (AhR) pathway, enhancing trophoblast differentiation and syncytium formation, with effects distinct from those of BaP or CeO₂ NPs alone, or their unbound mixture. Additionally, exposure to CeO₂ NPs alone altered homeostasis of mitochondria, affecting their phenotype and function. While individual exposures or BaP-coated CeO₂ NPs had no detectable impact, parallel co-exposure resulted in a slight but significant reduction in basal respiration. Finally, uncoated CeO₂ NPs altered placental steroidogenesis, increasing estrone level while decreasing dehydroepiandrosterone level, with sex-specific effects. These findings suggest that CeO₂ NPs can influence the biological effects of BaP in the human placenta, including modulating trophoblast differentiation, as well as disrupting mitochondria homeostasis and steroid production, with potential implications for pregnancy outcomes in polluted environments.</p>