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Acute ischemic strokes (AIS) are closely linked with air pollution, and there is some evidence that traditional cardiovascular risk factors may alter the relationship between air pollution and strokes. We investigated the effect of atrial fibrillation (AF) on the association of AIS with air pollutants. This was a nationwide, population-based, case-only study that included all AIS treated in public healthcare institutions in Singapore from 2009 to 2018. Using multivariable logistic regression, adjusted for time-varying meteorological effects, we examined how AF modified the association between AIS and air pollutant exposure. A total of 51,673 episodes of AIS were included, with 10,722 (20.7%) having AF. The odds of AIS in patients with AF is higher than those without AF for every 1 µg/m3 increase in O3 concentration (adjusted OR [aOR]: 1.005, 95% CI 1.003-1.007) and every 1 mg/m3 increase in CO concentration (aOR: 1.193, 95% CI 1.050-1.356). However, the odds of AIS in patients with AF is lower than those without AF for every 1 µg/m3 increase in SO2 concentration (aOR: 0.993, 95% CI 0.990-0.997). Higher odds of AIS among AF patients as O3- and CO concentrations increase are also observed in patients aged ≥65 years and non-smokers. The results suggest that AF plays an important role in exacerbating the risk of AIS as the levels of O3 and CO increase.