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While accumulating evidence shows that air pollution exposure is an important risk factor to influenza prevalence, their association has been inadequately investigated in mountainous regions with dense populations and high humidity. We aim to estimate the association and exposure-outcome effects between exposure to nitrogen dioxide (NO 2) and influenza prevalence in a mountainous region with a dense population and high humidity. We investigated 14,993 patients with confirmed influenza cases from January 2013 to December 2017 in Chongqing, a mountainous city in southwest China. We developed distributed lag non-linear models with quasi-Poisson link to take into account the lag and non-linear effects of NO 2 exposure on influenza prevalence. We estimated that the cumulative effect of a 10 μg/m3 increase in NO 2 with seven-day lag (i.e., summing all the contributions up to seven days) corresponded to relative risk of 1.24 (95% CI: 1.17–1.31) in daily influenza prevalence. Comparing to annual mean of the World Health Organization air quality guidelines of 40 μg/m3 for NO 2 , we estimated that 14.01% (95% CI: 10.69–17.08%) of the influenza cases were attributable to excessive NO 2 exposure. Our results suggest that NO 2 exposure could worsen the risk of influenza infection in this mountainous city, filling the gap of relevant researches in densely populated and mountainous cities. Our findings provide evidence for developing influenza surveillance and early warning systems. Image 1 • Influenza prevalence is associated with NO 2 exposure in a mountainous city. • Exposure-response effect is delineated with distributed lag nonlinear regression. • Distributed lag nonlinear effects over seven-day lag are modeled. • Cumulative effect of a 10 μg/m3 NO 2 increase corresponds to relative risk of 1.24 • Attributable fraction due to NO 2 exposure exceeding WHO AQG is 14.01%. [ABSTRACT FROM AUTHOR]