APOE and dementia: Because APOE4 frequencies vary widely by populations,57 their potential interactions with spatially varying ambient PM exposure to accelerate pathological brain aging may explain the geographic disparities in dementia incidence.58 Possible interactions of air pollution exposure and APOE ɛ4 on accelerated brain aging are also suggested by postmortem findings from a study in Mexico,59 which lacked comparison with APOE ɛ4 carriers residing in cleaner air.