This study investigates the translocation of inhaled ultrafine manganese oxide particles (UFPs) to the central nervous system (CNS) in rats. The researchers exposed groups of rats to manganese oxide UFPs (30 nm, ~500 μg/m³) with either both nostrils open or the right nostril occluded. They found that Mn concentrations in the olfactory bulb increased 3.5-fold after 12 days of exposure, while lung Mn concentrations doubled. Mn accumulation was also observed in the striatum, frontal cortex, and cerebellum. Lung lavage analysis showed no signs of lung inflammation, but increases in tumor necrosis factor-α (TNF-α) and protein levels were detected in the olfactory bulb. Macrophage inflammatory protein-2, glial fibrillary acidic protein, and neuronal cell adhesion molecule mRNA levels were also elevated in the olfactory bulb. When the right nostril was occluded for 2 days, Mn accumulated only in the left olfactory bulb. The study concludes that the olfactory neuronal pathway is an efficient route for translocating inhaled Mn oxide UFPs to the CNS, leading to potential inflammatory changes. Despite differences between human and rodent olfactory systems, this pathway is likely relevant in humans.This study investigates the translocation of inhaled ultrafine manganese oxide particles (UFPs) to the central nervous system (CNS) in rats. The researchers exposed groups of rats to manganese oxide UFPs (30 nm, ~500 μg/m³) with either both nostrils open or the right nostril occluded. They found that Mn concentrations in the olfactory bulb increased 3.5-fold after 12 days of exposure, while lung Mn concentrations doubled. Mn accumulation was also observed in the striatum, frontal cortex, and cerebellum. Lung lavage analysis showed no signs of lung inflammation, but increases in tumor necrosis factor-α (TNF-α) and protein levels were detected in the olfactory bulb. Macrophage inflammatory protein-2, glial fibrillary acidic protein, and neuronal cell adhesion molecule mRNA levels were also elevated in the olfactory bulb. When the right nostril was occluded for 2 days, Mn accumulated only in the left olfactory bulb. The study concludes that the olfactory neuronal pathway is an efficient route for translocating inhaled Mn oxide UFPs to the CNS, leading to potential inflammatory changes. Despite differences between human and rodent olfactory systems, this pathway is likely relevant in humans.