INSTRODUCTION: Air pollution is linked to dementia, but evidence from low-exposure settings is limited. We examined sex-specific associations between long-term PM2.5 and dementia risk in older adults living in Australia. METHODS: In 16,145 dementia-free ASPREE participants (≥70 years; median follow-up 10.3 years), Cox models assessed associations between 1-year mean PM2.5 (continuous and guideline-based categories) and incident dementia, adjusting for demographic, lifestyle, environmental, and genetic factors. Subgroups analyses by sex, APOE ε4, and age were conducted. RESULTS: Overall associations were null, however with a trend for increased risk at exposure > 10 vs ≤ 5 µg/m³. In subgroup analyses, positive associations were observed among females, with larger effect estimates at exposure > 10 µg/m³, whereas associations remained null among males. No differences were observed across APOE ε4 or age groups. DISCUSSION: Findings suggest a threshold of > 10 µg/m³ and heightened susceptibility in females. Further research in low-exposure settings is warranted.