Air pollution was recently found to be associated with increased risk of developing dementia. Zhang et al. investigated the relationship between PM2.5, an index of particle pollution, and the development of Lewy body dementia (LBD), the second most common form of dementia. Epidemiological data showed that PM2.5 exposure was associated with an increased risk of developing LBD. In mice, nasal administration of PM2.5 led to brain atrophy due to the accumulation of pathological α-synuclein, loss of dopaminergic neurons in the substantia nigra, and impaired cognition and motor function. The results identify a potential mechanistic link between air pollution and the development of dementia.