Early risk detection and management are essential for cognitive preservation. While particulate matter with a diameter smaller than 2.5 μm (PM_2.5) is considered harmful to cognition, the effect of smaller, more penetrative particulate matter with a diameter smaller than 1 μm (PM₁) requires further evidence and explicit safety thresholds. In this study, we explored the effects of long-term PM₁ exposure on early cognitive impairment and longitudinal cognitive changes in middle-aged and older populations. This study assessed data from two large-scale longitudinal surveys: the China Health and Retirement Longitudinal Study (CHARLS) and UK Biobank (UKB). Cross-sectional, longitudinal, and trajectory analyses were conducted to investigate the association between long-term PM₁ exposure and cognition. Additionally, the exposure–response curves were fitted to determine the customized thresholds. The findings indicated that sustained PM₁ exposure may lead to mild cognitive impairment, particularly at concentrations exceeding 30 and 5.6 μg·m⁻³ in CHARLS and UKB participants, respectively. Furthermore, we found that long-term PM₁ exposure can contribute to rapid cognitive decline at concentrations exceeding 23 and 5.5 μg·m⁻³ in CHARLS and UKB participants, respectively. In conclusion, reducing PM₁ exposure can improve the cognitive health of middle-aged and older adults.