AbstractBackgroundLong‐term ambient air pollution has known to affect cognitive function. However, limited studies investigated the relationship between co‐exposure to ambient air pollutants and cognitive functions in non‐demented older adults. Therefore, this study aimed to explore this association in a community‐based prospective cohort study.MethodThis four‐year prospective cohort study is part of the ongoing Taiwan Initiative Geriatric Epidemiological Research (TIGER, 2011‐present) and included 286 non‐demented older adults between 2015‐2019. Ambient air pollution data were collected from 29 monitoring stations from Taiwan Environmental Protection Administration (TEPA) between 1994 and 2019. Global and domain‐specific (memory, attention, executive function, and language) cognitive functions were assessed using the Taiwanese version of Montreal Cognitive Assessment (MoCA‐T) and a battery of neuropsychological tests. The generalized linear mixed models were used to evaluate the association of co‐exposure to air pollutants with cognitive function adjusting for important covariates. Furthermore, we conducted stratified analyses by olfaction, known as an early marker of dementia.ResultExcept for PM2.5, the annual average exposure of the other five pollutants was below the TEPA standards. A 25‐year high‐level exposure of NO2 (≥ 27.7 ppb) was associated with good performance executive function (β = 0.25) over four years. For joint effect, high‐level (the highest tertile) PM2.5 with low to medium concentration level of NO2, CO, or SO2 were significantly associated with the poor performance of memory (β = ‐0.39 to ‐0.35). Similarly, high‐level PM2.5 with low to medium concentration level of NO2, PM10 with O3 were significantly associated with poor executive function (β = ‐0.28 to ‐0.19). In participants with normal olfaction, high co‐exposure level to PM2.5 and PM10, with CO, SO2, O3, and NO2 was significantly associated with poor memory and executive function (β = ‐0.55 to ‐0.30). No significant associations were found in participants with abnormal olfaction.ConclusionLong‐term low to medium‐level co‐exposure to NO2, CO, or SO2 and high‐level PM2.5 was associated with poor memory; co‐exposure to O3 with PM10 and co‐exposure to NO2 with PM2.5 was associated with poor executive function. A revision on exposure standards shall be considered to protect older adults at the preclinical phase of dementia.